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.
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)
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.
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.
|*** These gears are available at: http://www.transmissioncenter.net/speedometer_calibration_______va.htm
@@ These gears are available at: http://www.dandltransmission.com/muncie.html
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.
If you are into formulas, you can calculate the correct gear
- 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
Aug. 2, 2004
Apr. 21, 2005
New info on THM400 Drive Gears added
thanks to Charlie Brecheisen at Fletchs.com
July 17, 2006