I changed to 3.54 gears and was wondering how I get the speedometer to read correctly? Do I just replace the plastic gear? If so what one would I use? I had the 19 tooth natural color with the original gears I think thy were 2.7something.
There are 2 gears relating to accurate Speedometer readings. One outside of the Gearbox and one inside the Gearbox. I had a TH350 from a Pick Up Truck and the internal Gear was such that I could not get an external Gear that could tie up to the Vette Speedometer. The Speedo was 20 mph to fast.
The only solution was to take the Gearbox out and open it up, then I could replace the internal gear to match up with an external gear and the Vette Speedometer, these all have to match up. Naturally I only did this when I had the Gearbox rebuilt.
In your case you would probably get away with just changing the External Gear, but this may not be the case. It is hard to tell what the correct external Gear should be without knowing the internal Gear. It may be a case of trial and error.
Do a search here on the speedo gear. This topic was brought up several times a year or so ago. As I recall, adding or subtracting one tooth made a change of x number of mph. I can't recall which made it show faster or slower or the speed that was suggested.
The gear calculator at bowtieoverdrive says I need a 39 tooth driven gear and a 15 tooth drive gear. Now I just have to find out what the drive gear is in the tranny. Do I have to take the tranny apart if I have to replace the drive gear?I just had the trans rebuilt and I could of had it done then but I didn't think of it.
The more teeth on the gear, the slower it turns...makes speedo slower. The fewer teeth on the gear, the faster it turns....makes the speedo faster.
You can do your own math...example: You have a 20 tooth gear. You replace it with a 22 tooth gear....that's a 10% increase....will decrease your speedo 10%.
You might have to fool with it, but you'll eventually get it close. Chuck
After looking threw the catalogs the gears only go up to 25 teeth for a standard and as far as I can tell I only have to replace the plastic gear.I thought someone might have done this gear swap already and new the color I needed because its $9.00 per gear.I'll try doing the math see how it works out.
Your orginal speedo drive gear of 19 teeth. 1981 vette 4 sp had a 2.72 rear end. Speedometer Driven gear must have been a 38. With this new rear end didn't your RPM/MPH go up by 30%?
Key equations needed to find the correct number of teeth.
Number of reveoutions of the tire in one mile....
Tire rev per mile = 20168/diameter of tire.
If you have 225 70 R15 tires the diameter is 27.4 inches So Tire rev per mile are 736.
Next equation will give you the number of teeth.
Num of drive teeth = Num of driven teeth X1001/axle ratio X number of tire rev per mile
In your case number of teeth = (38 X 1001) / (3.54 X 736)
Or 14.6. Not a good size. Needs to be close to whole number. If you use a 15 then you get a 2.7% error (speedometer slower than actual Shows 60 MPH real speed 62) If you use a 14 then 4.3% error speedometer faster than actual). Thats why the calculator wants a 39 tooth driven gear. Changing that gear is not a 10 minute deal.
If you change the tire size from 225 x 70 xR15, you could adjust this. Get the new diamater of the tire calc the revs per mile and see what you get. A tire with 28.15 inch diameter works with a 15 gear. Something like 235X70 or a 225X75. Not even sure these sizes exsist.
Or live with the 3-4% error. I guess my last choice would be change the driven gear to a 39.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
