I replaced the speedometer with a new OER speedometer for my 67 Camaro 383 cid 12 bolt w 3:73 rear end ratio. The car when driving is reading 20-25mph faster that what I'm actually driving at - I may have to change out teh driven gear .. how do you determine proper tooth count for the plastic gear that goes into the Speedo bullet where the cable connects?


3:73 rear gear end 12 bolt posi
front tires 205 70R15 tire diameter 26.3 inches
rear tires 215 70R15 tire diameter 26.9 inches

 

Might need a different speedo gear on the trans?

 

I don't have the answer to that but a speedometer shop probably would.

 

I have always done this based on ratios. But to do ratios you need two data points. When I was working on the car I would know the current setup and then I would install what it came with or a spare I had and that got me the second value.

(Example before GPS)

12 teeth@2500RPM = 55 MPH

16 teeth@2500RPM = 45MPH (clocked by a friend)

Use these number to create a ratio.

(Option 2)

Drive with your Speedometer at 55 MPH. With the Car in the 1:1 Drive. That is you highest gear on non overdrive transmission. Light on the throttle. Note your RPMs. Then drive with your GPS at 55 MPH Note the difference in RPM. Lets say your at 2500 RPMs with Speedo and 3000 RPMs with GPS That would be 3000/2500=1.2

12 tooth times 1.2. 14 tooth would be the closest gear. The number will never come out exact. Even the wear on the tires effect actual GPS speds

This seems way more complicated than it is. I suck at explaining things. Once you have done it you will see. This also assume the converter is working about the same in both tests. If you have a manual trans you are golden.

Gears used to be cheap compared to shop time so for the example I would by 13 and 15 tooth gears to have.

 

OP

This calculator shows how many teeth a driven gear needs to be based on rear gears and tire size...but you need to know how many teeth your drive gear is. Based on your 411 assuming you have the large 8 tooth drive gear, you need a 22 tooth driven gear

Speedometer Driven Gear Calculator (stl-vettes.com)

generally 1 tooth - 5mph change but that is a variable depending on how fast you are going (read 5 mph off at 65mph is 3 mph off at 35mph)

 

Just a FYI, because this is a ratio saying its 5 miles per hour off does not help us much. Being 5 miles per hour off at 100 Miles per hour is a 5% error. 5 miles per hour off at 10 miles per hour is a 50% error. In the first case you would want to increase the just one tooth and with the second example you would want to double the teeth on the gear.
If you know that you are 5 Miles per hour off at 50 Miles per hour you do not have to guess at how many teeth you need. 5/50=.10 (22 teeth gear) 22*.10=2.2 Or you need a 24 tooth gear.

 

try the 23. without knowing how many teeth your drive gear is, we are just guessing

 

thank you yes i think i need the 23 tooth

 

speedbox is a good GPS MPH app

 

Well if you have a 23 tooth and 24 tooth is the correct gear your speedometer will be off by around 4%. That is about the same error you would have if your tire was 1" smaller in diameter then you think. It is not uncommon for a tires to wear 3/4" on diameter over its life. Also keep in mind the tires do match each other pretty well but the exact diameter vary depending on model and run variation. The point I am trying to make is these systems are not that accurate to start with. Back in the day most precision drivers just used standard transmission and tachometers when they needed accurate/repeatable speeds because the speedometers themselves tended to not repeat exactly.