Speedo Gear Selection

Many of you have changed tire size or axle ratio on your car and now the speedo
reads too high or low. While this can be corrected with an aftermarket ratio adapter,
I like to change the speedometer driven gear at the rear of the tranny to correct for error.
If you want "correct", this is the way to go.

Determining Speedo Error

Checking the accuracy of your speedometer is easy. Look for a stretch of interstate
highway with mile markers. Use a stop watch (or a watch with a sweep second hand)
to determine the time to travel one mile with your speedo indicating exactly 60 MPH.
Let's say that you came up with 69.2 sec.

Now find your actual MPH using this formula:
Actual MPH = 3600/Time (Time in seconds to travel one mile at 60 MPH indicated)
Example: 3600/69.2 = 52 MPH actual, an error of 8 MPH
Percent Error = MPH error/actual MPH (8/52 = 15.4%)

Correcting Error

Turbo HydraMatic 400:
Speedo driven gears have from 34 to 45 teeth.
Each additional tooth lowers the speedo reading about 2.5%.
Each tooth removed raises the speedo reading about 2.5%.
In our example, we are reading 15.4% high so we would need to add teeth to spin the
speedo cable slower. Remember, one tooth = 2.5% so we need to add about six teeth.
(15.4%/2.5% = 6.2 teeth).

Turbo HydraMatic 350, Powerglide & 4 Speed:
Speedo driven gears have from 18 to 25 teeth.
Each additional tooth lowers the speedo reading about 5%.
Each tooth removed raises the speedo reading about 5%.
In our example, we are reading 15.4% high so we would need to add teeth to spin the
speedo cable slower. Remember, one tooth = 5% so we need to add about three teeth.
(15.4%/5% = 3.1 teeth).

At this point, pull the speedo gear out of your tranny by removing the small
bolt and clamp that retains the speedo gear sleeve and pulling on the speedo cable.
Then count the teeth, subtract or add to the number of teeth and select a new gear
from the chart below. Use color only as a check.
Gears immersed in tranny fluid for many years will be stained and off color.

Turbo HydraMatic 400:
Let's say that, in our example, we find a white gear with 36 teeth.
Adding six teeth to correct a speedo reading 15% low gives us a 42 teeth.
We need to buy a 42 tooth (green) 1362049 gear.

Turbo HydraMatic 350, Powerglide & 4 Speed:
Let's say that, in our example, we find a blue gear with 20 teeth.
Adding three teeth to correct a speedo reading 15% low gives us a 23 teeth.
We need to buy a 23 tooth (black) 3860346 gear.

If you find that your "correct" gear falls between two tooth counts as it
does in this case, it's usually best to pick the smaller gear so your speedo
will read slightly high. As I recall, GM practice was to pick speedo gears so
that the speedo would read between 60 and 63 MPH at a true 60 MPH.

Most of these parts are still available from GM so see your local dealer for parts.
They'll have the best prices and you'll find most very cooperative if you
know which Part Numbers you need. Most won't be very helpful, however, if you
ask them to solve your problem. Takes a lot of effort and risk on their part as
they're not usually very familiar with the 60's and 70's products and, of course,
you'll want them to take the parts back if they're the wrong stuff and
then they'll be stuck with parts they'll probably never sell.