INTRO:
  3/4
   Am            G              Em             Am
E|-------------|--------------|--------------|--------------|
B|-1-------1---|----0---------|----0---------|--------------|
G|-----2-------|---0----------|---0--2-0-----|---2----------|
D|-------------|--0-----------|--2-----------|--2-----------|
A|-0-----------|--------------|--------------|-0------------|
E|-------------|-3--------2---|-0------------|--------------|

   Am             G               Em              Am
E|--------------|---------------|---------------|-----------|
B|---0h1--0h1---|----0----------|----0--------0-|-----------|
G|--2---2-------|---0--------2--|---0--2-0-0h2--|---2-------|
D|--------------|--0--------4---|--2------------|--2--------|
A|-0------------|---------------|---------------|-0---------|
E|--------------|-3--------2----|-0-------------|-----------|

                       4/4
   G     Am    G        C       Am        Em
E|--------------------|-0---------------|-------------|
B|-----0-----1-----3--|-------3-1---3---|---1--0------|
G|---0-----2-----0----|-----0-----------|-------------|  Pause...
D|--------------------|-----------------|-2-----------|
A|-------0-----2------|-3-------0-------|-------------|
E|-3------------------|-----------------|-------------|

PART I:
                                                         ...................
    4/4                                                   1,2,3.           |
    Am                  G                Em               Am
E||--------------------|----------------|----------------|-----------------||
B||--0h1p0---0---1---0-|----------------|0---------0-----|-----------------||
G||*-------2-----2---0-|----0-----0-----|--2-0-2-------2-|----2-----2-----*||
D||*-------------------|--0-----0-----0-|----------------|--2-----2-----2-*||
A||--0---------0---0---|----------------|----------------|0-----0-----0----||
E||--------------------|3-----2-----0---|0-------0---0---|-----------------||
                          
                              
 ........................
  4.                                                  2/4     4/4
   Am      G         Am      G       C       Am        Em      F    Am
E|-----------------|---------------|-0---------------|-------|-------------|
B|-------0-----0-1-|-----1-----3---|---1---3-1---3---|---1-0-|-------------|
G|-----2-----0-----|---2-----0-----|-----0-----2-----|-0-----|-2--0-2-2--0-|
D|---2-------------|---------------|---------------2-|-------|-3-----------|
A|-0---------------|-0-----2-----2-|-3-------0-------|-------|------0------|
E|---------3-------|---------------|-----------------|-------|-------------|

                                   6/4                       3/4
   Am      G         Am    G        C     F     D7    G       E
E|-----------------|--------------|-0-----1-----2-----3-----|-4---5---7---|
B|-------0-----0-1-|-----1-----3--|---1-----1-----1-----3---|---0---0---0-|
G|-2---2-----0-----|---2-----0----|-----0-----2-----2-----0-|-------------|
D|---2-------------|--------------|-------------0-----------|-------------|
A|-0---------------|-0-----2------|-3-----0-----------2-----|-------------|
E|---------3-------|--------------|-------------------------|-0---0---0---|

  4/4
   Am                D                 Am                D
E|-8-7-5-----5-----|---5-----5-------|-8-7-5-----5-----|-------5---------|
B|-5-----8-------7-|-------7---5-----|-------8-------7-|---8-----8---7---|
G|-----------------|-----------------|-----------------|-----------------|
D|-----------------|-0---0---0---0---|-----------------|-0---0---0---0---|
A|-0-------0---0---|-----------------|-0-------0---0---|-----------------|
E|-----------------|-----------------|-----------------|-----------------|

   Am                D             G     C   F   Bb  A7sus4
E|-8-7-5-----5-----|---8---5-----5-3-|---3---1---1-----|-0---0---0---0---|
B|-------8-------7-|---------8-----3-|---5---1---3-----|-3---3---3---3---|
G|-----------------|---------------4-|---5---2---3-----|-0---0---0---0---|
D|-----------------|-0---0---0---0-5-|---5---3---3-----|-2---2---2---2---|
A|-0-------0---0---|---------------5-|---3---3---1---0-|---0---0---0---0-|
E|-----------------|---------------3-|-------1---------|-----------------|

   Dm9
E|--------0-----|
B|-----3--------|
G|---2----------|  Pause...
D|-3------------|
A|--------------|
E|--------------|

PART II:
  4/4
   Db
E|-----------------|-----------------|
B|-----------------|-----------------|
G|-----------------|-----------------|
D|-----------------|-----------------|
A|-4---4---4---4---|-4---4---4---4---|
E|-----------------|-----------------|

   Db                Gb/Ab   Db
E|---------------2-|---2-------------|-----------------|-----------------|
B|-6-6---6-------2-|---2---6-6-------|-----------------|-----------------|
G|-6-6---6-------3-|---3---6-6-------|-----------------|-----------------|
D|-6-6---6---------|-------6-6-------|-----------------|-----------------|
A|-4---4---4---4---|---------4---4---|-4---4---4---4---|-4---4-----------|
E|-----------------|-4---4-----------|-----------------|---------4---4---|

   E                 A/B     E
E|---------------5-|---5-------------|-----------------|-----------------|
B|-9-9---9-------5-|---5---9-9-------|-----------------|-----------------|
G|-9-9---9-------6-|---6---9-9-------|-----------------|-----------------|
D|-9-9---9---------|-------9-9-------|-----------------|-----------------|
A|-7---7---7---7---|---------7---7---|-7---7---7---7---|-7---7---7---7---|
E|-----------------|-7---7-----------|-----------------|-----------------|

   Db                Gb/Ab   Db
E|---------------2-|---2-------------|-4---------------|-----------------|
B|-6-6---6-------2-|---2---6-6-------|-------------7-6-|-7-6-------------|
G|-6-6---6-------3-|---3---6-6-------|-----------------|-------6---------|
D|-6-6---6---------|-------6-6---6---|-----------------|-----------------|
A|-4---4---4---4---|---------4---4---|-4---4---4---4---|-4---4-----------|
E|-----------------|-4---4-----------|-----------------|---------4---4---|

   E                 A/B     E
E|---------------5-|---5-------------|-7----------------|------------------|
B|-9-9---9-------5-|---5---9-9-------|-------------10-9-|-10-9-------------|
G|-9-9---9-------6-|---6---9-9-------|------------------|--------9---------|
D|-9-9---9---------|-------9-9---9---|------------------|------------------|
A|-7---7---7---7---|---------7---7---|-7---7---7---7----|-7----7---7---7---|
E|-----------------|-7---7-----------|------------------|------------------|

   Db                Gb/Ab   Db        Db                Gb/Ab   Db
E|-----------------|-2---2-----------|-----------------|-2---2-----------|
B|-6-6---------6---|-2---2---6-------|-6-6---------6---|-2---2---6-------|
G|-6-6---6---6-6---|-3---3---6-------|-6-6---6---6-6---|-3---3---6-------|
D|-6-6---6-----6---|---------6-------|-6-6---6-----6---|---------6-------|
A|-4---4---4---4---|---------4---4---|-4---4---4---4---|---------4---4---|
E|-----------------|-4---4-----------|-----------------|-4---4-----------|

     E                 A/B     E       E                 A/B     E     A
E|-----------------|-5---5-----------|-----------------|-5---5---------5-|
B|-9-9---------9---|-5---5---9-------|-9-9---------9---|-5---5---9-----5-|
G|-9-9---9---9-9---|-6---6---9-------|-9-9---9---9-9---|-6---6---9-----6-|
D|-9-9---9-----9---|---------9-------|-9-9---9-----9---|---------9-----7-|
A|-7---7---7---7---|---------7---7---|-7---7---7---7---|---------7---7-7-|
E|-----------------|-7---7-----------|-----------------|-7---7---------5-|

  6/4                                                 4/4
     D   G   C   F   Bb  A7sus4                        Dm9     ad.lib.
E|---5---3---3---1---1-----|-0---0---0---0---0---0---|-------0-----------|
B|---7---3---5---1---3-----|-3---3---3---3---3---3---|-----3---3-------3-|
G|---7---4---5---2---3-----|-0---0---0---0---0---0---|---2-------2---2---|
D|---7---5---5---3---3-----|-2---2---2---2---2---2---|-3-----------3-----|
A|---5---5---3---3---1---0-|---0---0---0---0---0---0-|-------------------|
E|-------3-------1---------|-------------------------|-------------------|

   Am
E|-----------------------|-----------------|
B|-0h1p0h1-----0-----1---|---3-----1---0---|
G|---------2-----2-----2-|-----2-----2---2-|
D|-----------2-----2-----|-------2---------|
A|-0---------------------|-0---------------|
E|-----------------------|-----------------|

   Am
E|-----------------------|-----------------|
B|-0h1p0h1-----0---------|---0-----1---0---|
G|---------2-----2---2---|-----2-----2---2-|
D|-----------2-----2---2-|-------2---------|
A|-0---------------------|-0---------------|
E|-----------------------|-----------------|

   Am
E|-----------------------|-----------------|
B|-0h1p0h1-----0---------|---0-----1---3---|
G|---------2-----2---2---|-----2-----2---2-|
D|-----------2-----2---2-|-------2---------|
A|-0---------------------|-0---------------|
E|-----------------------|-----------------|

   Am                            G
E|-0---------------|-----------------|
B|-------3-----1---|---0-------0-1-3-|
G|---2-----2-----2-|-----2---2-------|
D|-----2-----2-----|-------2---------|
A|-0---------------|-0-------0---2---|
E|-----------------|-----------------|

   C       Am        G       G7        C       Am        G       G7
E|-0---0-----------|---------1-0-----|-0---0-----------|---------1-0-----|
B|---3---1-----1-0-|-----3-------3---|---3---1-----1-0-|-----3-------3---|
G|-----------2-----|---0---0-------0-|-----------2-----|---0---0-------0-|
D|-----------------|-----------------|-----------------|-----------------|
A|-3-------0-------|---------2-------|-3-------0-------|---------2-------|
E|-----------------|-3---------------|-----------------|-3---------------|

   C       Am        G       G7        C       Am        G
E|-0---0-----------|---------1-0-----|-0---0-----------|-----------------|
B|---3---1-----1-0-|-----3-------3---|---3---1-----1-0-|-----------------|
G|-----------2-----|---0---0-------0-|-----------2-----|-----0-----0-----|
D|-----------------|-----------------|-----------------|---0-----0-----0-|
A|-3-------0-------|---------2-------|-3-------0-------|-----------------|
E|-----------------|-3---------------|-----------------|-3-----2-----0---|

                                                        3/4
   Em                Am  G   Am  G     C   F   D7  G     E
E|-----------------|-----------------|-0---1---2---3---|-4---5---7---|
B|-0---------0-----|-----0---1---3---|-------1---1---0-|---0---0---0-|
G|---2-0-2-------0-|-2-----0---2---0-|---0-------------|-------------|
D|-----------------|---2-------------|---------0-------|-------------|
A|-----------------|-0-------0---2---|-3---0-------2---|-------------|
E|-0-------0---0---|-----3-----------|-----------------|-0---0---0---|

  4/4
   Am                D                 Am                D
E|-8-7-5-----5-----|---5-----5-------|-8-7-5-----5-----|-------5---------|
B|-5-----8-------7-|-------7---5-----|-------8-------7-|---8-----8---7---|
G|-----------------|-----------------|-----------------|-----------------|
D|-----------------|-0---0---0---0---|-----------------|-0---0---0---0---|
A|-0-------0---0---|-----------------|-0-------0---0---|-----------------|
E|-----------------|-----------------|-----------------|-----------------|

   Am                D             G     C   F   Bb  A7sus4
E|-8-7-5-----5-----|---8---5-----5-3-|---3---1---1-----|-0---0---0---0---|
B|-------8-------7-|---------8-----3-|---5---1---3-----|-3---3---3---3---|
G|-----------------|---------------4-|---5---2---3-----|-0---0---0---0---|
D|-----------------|-0---0---0---0-5-|---5---3---3-----|-2---2---2---2---|
A|-0-------0---0---|---------------5-|---3---3---1---0-|---0---0---0---0-|
E|-----------------|---------------3-|-------1---------|-----------------|

  Dm9       ad.lib.
E|-------0--------|
B|-----3---3------|
G|---2-------2----| Pause...
D|-3-----------3--|
A|----------------|
E|----------------|

CODA:
                                                         ...................
    4/4                                                   1.               |
    Am                  G                Em               Am
E||--------------------|----------------|----------------|-----------------||
B||--0h1p0---0---1---0-|----------------|0---------0-----|-----------------||
G||*-------2-----2---0-|----0-----0-----|--2-0-2-------2-|----2-----2-----*||
D||*-------------------|--0-----0-----0-|----------------|--2-----2-----2-*||
A||--0---------0---0---|----------------|----------------|0-----0-----0----||
E||--------------------|3-----2-----0---|0-------0---0---|-----------------||
                          
                              
 ........................
  2.                                                  2/4     4/4
   Am      G         Am      G       C       Am        Em      F    Am
E|-----------------|---------------|-0---------------|-------|-------------|
B|-------0-----0-1-|-----1-----3---|---1---3-1---3---|---1-0-|-------------|
G|-----2-----0-----|---2-----0-----|-----0-----2-----|-0-----|-2--0-2-2--0-|
D|---2-------------|---------------|---------------2-|-------|-3-----------|
A|-0---------------|-0-----2-----2-|-3-------0-------|-------|------0------|
E|---------3-------|---------------|-----------------|-------|-------------|

                                      3/4
   Am  G   Am  G     C   F   D7  G     E
E|-----------------|-0---1---2---3---|-4---5---7---|
B|-----0---1---3---|-------1---1---0-|---0---0---0-|
G|-2-----0---2---0-|---0-------------|-------------|
D|---2-------------|---------0-------|-------------|
A|-0-------0---2---|-3---0-------2---|-------------|
E|-----3-----------|-----------------|-0---0---0---|

                              4/4
   Am  D   G     C   F   Bb    Dm9  ad.lib.
E|-5---5---3---|-3---1---1---|---------0-------------|
B|-5---7---3---|-5---1---3---|-------3---3-------3---|
G|-5---7---4---|-5---2---3---|-----2-------2---2---2-|
D|-7---7---5---|-5---3---3---|---3-----------3-------|
A|-7---5---5---|-3---3---1---|-0---------------------|
E|-5-------3---|-----1-------|-----------------------|

   D9  ad.lib.                A
E|----------0---------------|----------------||
B|--------3---3-------3-----|----2-----------||
G|------2-------0---2---0---|---2------------|| Fine.
D|----4-----------4---------|--2-------------||
A|--0-----------------------|-0--------------||
E|--------------------------|----------------||
=-=-= end =-=-=

From: Paul Zimmerman 

                               CLASSICAL GAS
                             By Mason Williams
  Am       G       Em       Am
E----------------------------------|
B-1-----1--0-------0---------------|
G-2--2-----0-------0--2--0--2------|   Play twice
D---------------------------2------|
A-0-------------------------0------|
E----------3----2--0---------------|
  G     Am    G      C    Am   Em
E--------------------0----0---------------|
B-----0-----1-----3--1------1--3----1--0--|
G---0-----0-----0-------0---2-------------|
D---------------------------------2-------|
A-------0-----2------3--------------------|
E-3---------------------------------------|
  Am       G       Em       Am
E--------------------------------------|
B-1-----1--0---------0-----------0-----|
G----2------------------2--0--2-----2--|       Play 4 times
D--------------------------------------|
A-0---------------------------------0--|
E----------3--2--0---0-----------------|

  Am    G       Am    G   C   Am  Em    F   Am
E-------------------------0--------------------------|
B-----0-----0-1-----1-3-----3-1-3-0-1-0--------------|
G---2-----0-------2---------------------2-0-2-2-0-2--|  & - To Coda (2nd time)
D-2-------------------------------2-----3------------|
A---------------0-----2---3---0-------------0-----0--|
E-------3--------------------------------------------|

  Am    G       Am    G   C     F     D7    G     E7
E-------------------------0-----1-----2-----3-----4---5---7----|
B-----0-----0-1-----1-3-----1-----1-----1-----3-0---5---5---5--|
G---2-----0-------2-----------0-----2-----2--------------------|
D-2------------------------------------------------------------|
A---------------0-----2----------------------------------------|
E-------3------------------------------------------------------|

E-8-7-5---5-----5---5---8-7-5---5-----------8-7-5---5-----8-5---5--|
B-------8---7-----7---5-------8---7---8-7-8-------8---7-------8----|
G------------------------------------------------------------------|
D-------------0---------------------0-------------------0----------|
A-0---------------------0-------------------0----------------------|
E------------------------------------------------------------------|

Strum:   G    C    F   Bb    A7sus4
         /    /    /   /      /  /  /  /

    Dm9
E----------0--|
B-------3--3--|
G-----2----2--|
D---3------3--|
A-0-----------|
E-------------|

Part II
(This is what is written for the music that I have.  I noticed in playing
around with it that playing straight off the Db and Eb also sounds good.)
  Db                                      Eb
E-1-1-----1-2-2-1--1-1-----1-2-2-1-----1--4-4-4---4-5-5-4--4-4-4---|
B------------------------------------2----------7----------------7-|
G-----1-3--------------1-3---------1-------------------------------|
D------------------------------------------------------------------|
A------------------------------------------------------------------|
E------------------------------------------------------------------|

E-4-5-5-4---4-|
B---------5---|
G-------------|
D-------------|
A-------------|
E-------------|
Strum:   G    C    F   Bb    A7sus4
         /    /    /   /      /  /  /  /

  Dm9
E---------0-----------|
B-------3---3-------3-|
G-----2-------2---2---|
D---3-----------3-----|
A-0-------------------|
E---------------------|
This next section is strumming around  an  A-minor at the first position and
either lifting the first finger off to form Asus2 (002200) or adding the 4th
finger on the second string third  fret  to  form Asus4 (002230).  These are
all eighth notes with no rests, but  the  chord change comes on the 4th note
of each measure (the 'and' of the second beat) like:
Strum: Asus2 Am        Asus4 Am        Asus2 Am        Asus4 Am
       |     |
       |     |  etc.
       |     |
       v     v
      |/ / / / / / / /|/ / / / / / / /|/ / / / / / / /|/ / / / / / / /|
       Asus2 Am        Asus4 Am        Asus2 Am        Asus2
      |/ / / / / / / /|/ / / / / / / /|/ / / / / / / /|/   /
                                                       (1/4 notes)

E---------0-0-0-1-0-------0-------0-----------0---0-0-0-1-0-------0--------|
B---0-1-3-----------3-3-3---3-1-3---1---0-0-3---3-----------3-3-3---3------|
G-2-----------------------------------2------------------------------------|
D--------------------------------------------------------------------------|
A--------------------------------------------------------------------------|
E--------------------------------------------------------------------------|
                                                                   Am G Am
E-----0-----------0---0-0-0-1-0-------0-------0----------------------------|
B-1-3---3---0-0-3---3-----------3-3-3---3-1-3---1---0-----0-------0---0-1--|
G---------2---------------------------------------2---------2-0-2---2------|
D--------------------------------------------------------------------------|
A-------------------------------------------------------------------0---0--|
E-----------------------------------------------------3-2-0-----------3----|
  G C F D G
E---0-1-2-3-4-5-6-|
B-3---------------|
G-----------------|
D-------0---------|
A-2-3-0---2-------|
E-----------0-0-0-|

E-8-7-5---5-----5---5---8-7-5---5-----------8-7-5---5-----8-5---5--|
B-------8---7-----7---5-------8---7---8-7-8-------8---7-------8----|
G------------------------------------------------------------------|
D-------------0---------------------0-------------------0----------|
A-0---------------------0-------------------0----------------------|
E------------------------------------------------------------------|

Strum:   G    C    F   Bb    A7sus4
         /    /    /   /      /  /  /  /
  Dm9
E----------0--|
B-------3--3--|
G-----2----2--|              D.S al Coda &
D---3------3--|
A-0-----------|
E-------------|
Coda:
 Am G Am G C F D G
E----------0-1-2-3-4-5-6-|
B---0-1--3---------------|
G-2----------------------|
D--------------0---------|
A-0---0--2-3-0---2-------|
E---3--------------0-0-0-|

Strum:      Am   D   G   C   F   Bb

  Dm9                    D                            A
E---------0----------------------0-----------------------------------|
B-------3---3-------3----------3---3-------3----------2--------------|
G-----2-------2---2---2------2-------0---2---0--------2--------------|
D---3-----------3----------4-----------4--------------2--------------|
A-0----------------------0----------------------------0--------------|
E--------------------------------------------------------------------|

------------------------------------------------------------------------------
INTRO:
3/4 time rubato, that is, played slowly "with feeling".  Actually,
everyone wants to play it just like Mason does on the record...
                *            *            *
               / \          / \          / \
E|------------|------------|------------|------------|
B|1-------1---|--0---------|--0---------|--2---------|
G|----2-------|-0----------|-0--2-0-----|-2----------|
D|------------|------------|------------|------------|
A|0-----------|------------|------------|0-----------|
E|------------|3-------2---|0-----------|------------|

                *            *            *
    h   h      / \          / \     h    / \
E|------------|------------|------------|------------|
B|--01--0-1---|--0---------|--0-------0-|--2---------|
G|-2--2-------|-0----------|-0--2-0-02--|-2----------|
D|------------|------------|------------|------------|
A|0-----------|------------|------------|0-----------|
E|------------|3-------2---|0-----------|------------|
   **  **  **
  / \ / \ / \  Switches to 4/4 time here.
E|------------|0---------------|------------|
B|--0---1---3-|------3-1---3---|--1-0-------|
G|-0---2---0--|----0-----------|------------|  dramatic pause...
D|------------|----------------|2-----------|
A|----0---2---|3-------0-------|------------|
E|3-----------|----------------|------------|

* - arpeggio (The sheet music would denote this as three notes together
    with a squiggly line.  You are supposed to know that the notes are
    not played together.)
** - triplet (fill in the space that two notes usually take up with
     three evenly spaced notes.)
h - hammer on the next note.

THE MAIN PART:
Up-tempo 4/4 time.  The signature riff makes this song and the sheet
music doesn't give a clue about how to play it.
Thumb and two fingers are probably needed at a minimum.  In the first
bar, the thumb plays the A and D string notes. In the next two bars,
the thumb plays the E string notes, and in the fourth bar, it plays
the A and D string notes.
For the first measure, finger a normal Am chord.  I find it works
well in the third measure to alternate the 2-0-2 on the G string
between the index and middle finger (play the B string notes with
your middle finger, the first G string note with your index, the
second G string note with you middle finger again, and the next G
string note with your index finger again.  (just a suggestion).
   *
  / \
E|----------------|----------------|----------------|----------------|
B|010-1-----1---0-|----------------|0---------0-----|----------------|
G|------2---------|----0-----0-----|--2-0-2-------2-|----2-----2-----|
D|----------2---0-|--0-----0-----0-|----------------|--2-----2-----2-|
A|0-------0---0---|----------------|----------------|0-----0-----0---|
E|----------------|3-----2-----0---|0-------0---0---|----------------|
Repeat 4 times.
* - pick the first note and hammer on and pull off the second and
third notes. pick the fourth.  Thumb plays the A and D string notes
in the first measure.

I haven't had time to check the rest of the posted "sheet music"
for inaccuracies, but this first part is the important one.  You
can probably get most of the rest from sheet music.
Enjoy, and don't give up.  It is a very rewarding tune to play.

-----------------------------------------------------------------------------

Artist: Mason Williams
Title:  Classical Gas
Year:   1967

Chord voicings:
    A   Am A7sus  Bb  C   D   Db  Dm  D7  E   Em  E7  F   G   Gb  G7
e --0---0----3----1---0---2---1---1---2---0---0---0---1---3---2---1---
B --2---1----3----3---1---3---2---3---1---0---0---0---1---0---2---0---
G --2---2----2----3---0---2---1---2---2---1---0---1---2---0---3---0---
D --2---2----2----0---2---0---3---0---0---2---2---0---3---0---4---0---
A --0---0-------------3-------------------2---2---2-------2-------2---
E ----------------------------------------0---0---0-------3-------3---