Transmission speedo gear selection

[ncrobb]

How much difference does a couple teeth make on a Speedo drive gear? My tire diameter is 26.5” (245/60R15) and my rear ratio is 3.55. The Brewer chart shows a 36 tooth for 26” diameter tire and a 34 tooth for 27” diameter. I have a 33 in the trans now. Going through a rebuild and figured to make it right if I need to.

Robb

 

[Nate S]

It’s all proportional. To start with, a 36 is usually pretty good for 3.55’s. So take it for a drive and use a gps app on your phone to get the actual speed when watching the speedo. Let’s say the speedo says 65 and the gps says 60. That means you need 65/60 x 36 teeth on the speedo gear, or 39 teeth. More teeth make the speedo run slower.

Another thing to consider is the odometer. You can do about the same thing. Use a gps app to measure a ride that’s around 10 miles. What does the odo say? Use the same ratio to figure that out. In my car they didn’t match. I made the speedo correct and the odo runs a little slow.

 

[ncrobb]

It’s all proportional. To start with, a 36 is usually pretty good for 3.55’s. So take it for a drive and use a gps app on your phone to get the actual speed when watching the speedo. Let’s say the speedo says 65 and the gps says 60. That means you need 65/60 x 36 teeth on the speedo gear, or 39 teeth. More teeth make the speedo run slower.

Another thing to consider is the odometer. You can do about the same thing. Use a gps app to measure a ride that’s around 10 miles. What does the odo say? Use the same ratio to figure that out. In my car they didn’t match. I made the speedo correct and the odo runs a little slow.

Thanks - that makes sense.

 

[hunt2elk]

If you pick the correct gear the first time, you are way luckier than me. It's usually a trial and error thing. A 1 tooth gear change is a 1-2 mph speedometer change @ 60 mph.

 

[oldkimmer]

I was gonna say 2 teeth are about a 5mph change, but that also depends on ur axle ratio and tire size. Kim

 

[Nate S]

It’s pretty basic math. If you have a 27 tooth gear one tooth is worth 1 mph at 27, 2 mph at 54. If you have a 39 tooth gear, 1 tooth is worth 1 mph at 39, 2 mph at 78.