The Chevelle Engineer

Speedo Gear Selection

Many of you have changed tire size or axle ratio on your Chevelle 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 a "correct" car, 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.

Now find your actual MPH using this formula.

Actual MPH = 3600/Time (Time in seconds to travel one mile at exactly 60 MPH)

Example: It takes 69.2sec. to go a mile so 3600/69.2sec. = 52 MPH actual  The speedo is indicating 8 mph too high.

Percent Error = MPH error/actual MPH (8/52 = 15.4% high)

Correcting Error

TH400, TH350BOP, 700-R4: Speedo driven gears have from 34 to 45 teeth. Each tooth added lowers the speedo reading about 2.5%. Each tooth removed raises the speedo reading about 2.5%. In our example, we want to lower the speedo reading from 60mph indicated to 52mph indicated.  We need to add teeth to lower the reading by spinning the speedo cable slower. Remember, one tooth = 2.5% so we need to add about six teeth. (15.4%/2.5% = 6.2 teeth).

TH350C, Powerglide and Muncie 4 Speed: Speedo driven gears have from 17 to 25 teeth. Each tooth added 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 tail housing 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 cross check. Plastic gears immersed in tranny fluid for many years will be stained and off color.  You often see the speedo driven gear referred to as the  "pencil" gear and the speedo drive gear the "circle" gear.

TH400, TH350 BOP, 700-R4: Let's say that, in our example, we find a white gear with 36 teeth. Adding six teeth to correct a speedo reading 15% high gives us a 42 teeth. We need to buy a 42 tooth (green) 1362049 gear.

TH350C, Powerglide & Muncie 4 Speed: Let's say that, in our example, we find a brown gear with 18 teeth. Adding three teeth to correct a speedo reading 15% high gives us a 21 teeth. We need to buy a 21 tooth (red) 3987921 gear.

The GM TH350C, 4 speed and Powerglide transmissions all use the 345215 sleeve but they used three different drive gear diameters, 1.92", 1.84" & 1.76".  The charts show which drive & driven gears are compatible.  If you use incompatible gears, they will destroy themselves and possibly fill your tranny with plastic chips.

If you find that your "correct" gear falls between two tooth counts as it does in most cases, 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. 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.  Prices and availability shown are those at https://www.partszoneonline.com/index.php on 8/1/2004.  Your local dealers will probably not match these prices.

THM350BOC, THM400 &
700-R4 Speedo Driven Gears
Part
Number
Teeth Color THM400 Sleeve 700-R4,350BOP Sleeve Price
9774413 34 Lt.Green

1362294

25512340 $3.61
9780628 35 Orange

1362294

25512340 $3.61
1359270 36 White 1362294 25512340 $3.61
1359271 37 Red 1362294 25512340 $3.61
1359272 38 Blue 1362294 25512340 $3.61
1359273 39 Brown 1362294 25512340 $3.61
1362048 40 Black 1362293 25512339 $3.61
1362195 41 Yellow 1362293 25512339 $3.61
1362049 42 Green 1362293 25512339 $3.61
1362196 43 Purple 1362293 25512339 $3.61
9780470 44 Dk Gray 1362293 25512339 $3.61
9775187 45 Lt Blue 1362293 25512339 $3.61

350C, Powerglide & Muncie 4 Speed Speedo Driven Gears

Part
Number
Teeth Color Sleeve Price Drive
Gear
OD
3987917 17 Purple 345215 $3.61 1.84"
3987918 18 Brown 345215 $3.01 1.84"
3987919 19 Natural 345215 $3.01 1.84"
3987920 20 Blue 345215 $3.01 1.84"
3860329 20 Steel 345215   1.92"
3987921 21 Red 345215 $3.01 1.84"
3987922 22 Silver 345215 $3.01 1.84"
           
3860341 18 Brown 345215   1.76"
3860342 19 Natural 345215   1.76"
3860343 20 Blue 345215   1.76"
3860344 21 Red 345215   1.76"
3860345 22 Green 345215 $3.01 1.76"
3860346 23 Black 345215 $3.61 1.76"
3860347 24 Yellow 345215 $3.61 1.76"
3860348 25 Orange 345215 $3.61 1.76"

200-R4 Speedo Driven Gears
Part
Number
Teeth Color Sleeve Price
25502669 26 Brown 25519848 $3.61
410731 27 Black 25519848 $3.61
416765 28 Yellow 25519848 $3.61
403999 29 Green 25519848 $3.61
561035 30 Blue 25519848 $3.61
403417 31 White 25519848 $3.61

Speedo Sleeves

GM calls the part that mounts the speedo driven gear to the tranny a Sleeve - Speedo Driven Gear. The THM400, TH350BOC & 700-R4 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 TH400 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 green gear.

The charts below show all the drive gear information I've been able to collect.  If no price is shown, the part is no longer available from GM.

THM400
Speedo Drive Gears

Part
Number
Teeth Color Clip Price
8629547 15 Brown/Gray None $3.61
8623311 16 Orange None $14.34
8629548 17 Red None $3.61
8629549 18 Blue None ***
8629550 19 White/Yellow None ***
8629551 20 Green/Brown None ***
8623316 21 Black None ***

THM350C
Speedo Drive Gears

Part
Number
Teeth Color Clip Price OD
6271537 7 Pink 6261781 $3.61 1.84"
6261783 8 Black 6261781 $7.52 1.76"
6261784 8 Orange 6261781 *** 1.84"
6261782 9 Green 6261781 $3.01 1.84"
6261785 10 Purple 6261781 $3.01 1.84"
THM350BOP
Speedo Drive Gears
Part
Number
Teeth Color Clip Price
6260038 17 Green 8628557 $3.61
6260037 18 Yellow 8628557 $3.61
8640188 19 White 8628557 $14.43
700-R4
Speedo Drive Gears
Part
Number
Teeth Color Clip Price
8642620 15 Gray 6261781 $3.61
8640517 17 Red 6261781 $3.61
8640518 18 Blue 6261781 $3.61
8640519 19 Yellow 6261781 $3.61
8640520 20 Brown 6261781  
Muncie 4 Speed
Speedo Drive Gears
Part
Number
Teeth Color Spline Price Size
3845079 6 Steel 27T   1.92"
  7 Plastic 27T @@ 1.84"
3708145 8 Steel 27T @@ 1.76"
3708144 8 Steel 27T @@ 1.84"
6261794 8 Natural 27T $3.61 1.84"
3978758 8 Steel 32T @@ 1.84"
3805494 9 Steel     1.84"
6261796 9 Red 27T $3.61 1.84"

200-R4
Speedo Drive Gears

Part
Number
Teeth Color Clip Price
8639906 10 Green 8632589 $24.05
8634965 11 Orange 8632589 .
8634935 12 Red 8632589 $14.43
8634934 13 White 8632589 $14.43
*** These gears are available at: http://www.transmissioncenter.net/speedometer_calibration_______va.htm

@@ These gears are available at: http://www.dandltransmission.com/muncie.html

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 0.7333/1 and retails for about $75 list from your friendly GM dealer.  The effect of this adapter is the same as lowering the number of teeth on the speedo driven gear by 26.7%.  For example, if you needed a 48 tooth gear for your THM400, none is available but you can use this adapter with a 35 tooth gear to get the same result.

These adapters were used by GM for high (numerically like 4.11 or 4.56) axle ratios where the "correct" gear was larger than the largest available.  The adapter screws on the speedo connector on the tranny and the speedo cable attaches to the adapter.

Another GM ratio adapter is #368022 at $117 list.  It has a ratio of 0.9444/1 and lowers the speedo output by 5.6%.  *** above also offers several ratio adapters.

 

By the Numbers

If you are into formulas, you can calculate the correct gear as follows:
- Get your tire rev/mile from your tire manufacturer. Try www.BFGoodrichTires.com or approximate it from your Rear Tire Diameter.

Rev/Mile=20168/Rear Tire Diameter (in inches)

- Count the teeth on the speedo drive gear, the one mounted on the transmission output shaft
- Determine your axle ratio
- Compute the correct number of teeth on the driven gear with this formula

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

Principal here is that a correctly calibrated GM mechanical speedo turns exactly 1001 revolutions in 1 mile.  That's 1001 rpm at 60mph.

I've recently expanded the charts to include drive gears and coverage for the 200-R4 and 700-R4 transmissions.  I've made every effort to verify the data using a dozen or more sources including six different original GM documents.  Some of these documents are no longer available to me so I'm not able to recheck everything.  If you find errors or inconsistencies, please drop me an email.
Fred Aldrich
Aug. 2, 2004
Apr. 21, 2005

New info on THM400 Drive Gears added thanks to Charlie Brecheisen at Fletchs.com
July 17, 2006

Email: Fred Aldrich

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