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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.

TH400 Driven Gears
Part Number Teeth Color Code Sleeve
9774413 34 Lt. Green 1362294
9780628 35 Pink 1362294
1359270 36 White 1362294
1359271 37 Red 1362294
1359272 38 Blue 1362294
1359273 39 Brown 1362294
1362048 40 Black 1362293
1362195 41 Yellow 1362293
1362049 42 Green 1362293
1362196 43 Purple 1362293
9780470 44 Dk. Gray 1362293
9775187 45 Lt. Blue 1362293
 

TH350, 4 Speed & Powerglide
Driven Gears

Part Number Teeth Color Code PG Sleeve 350/4SP Sleeve
3987917 17 Purple 3969914 345215
3987918 18 Brown 3969914 345215
3987919 19 Natural 3969914 345215
3987920 20 Blue 3969914 345215
3987921 21 Red 3969914 345215
3987922 22 Silver 3969914 345215
3860345 22 Green 3969914 345215
3860346 23 Black 3969914 345215
3860347 24 Yellow 3969914 345215
3860348 25 Orange 3969914 345215

Speedo Sleeves

GM calls the part that mounts the speedo driven gear to the tranny a Sleeve -
Speedo Driven Gear. The THM400 uses two sizes which take care of the different
diameters of the various driven gears. They are marked with the driven gear
tooth numbers they will accept. In our example, our white gear used a PN 1362294
sleeve so we have to buy a new sleeve (PN 1362293) to match our new 42 tooth greengear.

Off the Chart or Can't Buy the Correct Gear

Ahh, back where we started. We'll have to use a ratio adapter and perhaps a
speedo gear change as well to tune in the speedo. GM used #3932220 ratio adapter
on many Chevelles. This part has a ratio of about 1.35/1 and retails for about
$45 from your friendly GM dealer.

By the Numbers

If you are into formulas, you can calculate the correct gear as follows:
- Count the teeth on the speedo drive gear,
   the one mounted on the transmission output shaft
- Determine your axle ratio
- Compute the number of teeth on the correct driven gear with this formula

N(driven) = N(drive) x (Axle Ratio) x (Rev per Mile) /1001

If this comes out as something other than a whole number (as it usually will),
then go on the small side to keep speedo reading slightly higher than actual.

Principal here is that a correctly calibrated GM mechanical speedo
turns 1001 rpms at 60 mph.


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