Taken from HO Racing’s High Output newsletter Aug. 1976

TECHNICAL TOPIC: Correcting the Speedometer

Two very popular items for improving the performance of your Pontiac are low, wide tires and lower ratio ring and pinion gears. Both of these items change the engine speed at any given car speed, and the indicated speed on the speedometer will be off by an associated amount. This month's Tech Topic will discuss the way to figure out how to correct the speedometer reading.

1. Tires. The effective rolling diameter of the rear tires is part of the overall drive train gear ratio. The taller the tire, the less the effective gear ratio and vice versa.Listed below are the original equipment tire sizes with their respective revolutions per mile (rev/mile). On the right side are the popular replacement tire sizes (which do not correspond to original equipment size but are formatted for convenience).

Determine your speedometer correction factor due to tire size differences by the following:

Tire size ratio = new tire rev/mi

old tire rev/mi

2. Ring & Pinion. Changing the rear gear ratio obviously changes the engine speed and requires speedometer correction. The factory available and aftermarket ratios will not be listed here because the ratio numbers are commonly available. Just make certain that you know your exact rear gear ratio, both before and after. If in doubt, compute your gear ratio with this formula:

Ring & pinion ratio = number of ring gear teeth

number of pinion gear teeth

Determine your ring & pinion correction factor just like the tire size, i.e.,

Rear gear change = new ring & pinion ratio

old ring & pinion ratio

3.Drive and Driven Speedometer Gears. If the tire size change and/or rear gear change is not too large, then the speedometer may be corrected merely by changing the driven gear. However, you must know the speedometer drive gear (on the transmission output shaft and difficult to change). The chart below lists the speedometer drive gears (number of teeth) for the M-40 and M-38 transmissions. For other transmissions, consult Pontiac or H-O.

16 teeth (orange)

1967-71 code PS, PQ, PX, PF

1969-70 GP codePR, PW

1970 F & A code PD, PY

1971 F, A, & GP code PR, PY

1972-73 F, A, & GP codePG, PQ, PR, PX

1974 code PG, PQ, PR, PW, PX, PZ

18 teeth (yellow)

1965-71 B code PA, PC, PH

1967-70 A code PT, PV

1968-69 F code PY, PV

1969-70 GP code PT

1971-72 A, F code PT

1971 B code PH

1973 code PA, PB, PC, PD, PF, PH, PT, PZ

1974 code PA, PB, PC, PD, PF, PH, PL, PT

1975 code PG, PR, PS, PT, PX, PW

1970-75 all M-38

19 teeth (white)

1971 B all code except PH

1972 all B

1975 code PA, PB, PC, PD, PE, PF, PH

20 teeth (green)

1965 B code PB, PE

1967-68 B code PD, PG

1966 B code PB, PD, PG

1976 all

21 teeth (black)

1967-70B code PB

The speedometer driven gears are available in one-tooth increments from 34 (M -38) or 35 (M-40) to 45 teeth. You may have to change driven gear sleeve assemblies (the housing that holds the gear in the trans case) depending upon the driven gear you end up with. The following are the available driven gear sleeves.

M-40sleeve for 35 through 39 teeth gear#1362294

M-40sleeve for 40 through 45 teeth gear#1362293

M-38sleeve for 34 through 39 teeth gear#1362284

M-38sleeve for 40 through 45 teeth gear#1362285

4.Speedometer Correction.

If speedometer had read correctly prior to tire or ratio change:

New driven gear # teeth = (old driven gear # teeth)(tire size ratio)(rear gear change)

To compute correct speedometer gearing from scratch:

New driven gear # teeth = (rev/mi)(ring & pinion ratio)(# drive gear teeth)

1000

In both cases, if the number of driven gear teeth is outside the range of available gears, then the drive gear may have to be changed or an adapter fitted

For example if the desired number of driven teeth is 47 with an 18-tooth drive gear, then changing to a 16-tooth drive gear will allow the use of a 42-tooth driven gear with the same results.

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Taken from HO Racing’s High Output newsletter Aug. 1976