I upgraded my original 14" Cutlass wheels to 15" Torq Thrusts and wanted to know if anyone can advise what speedo gear I should install in the transmission to keep the speedometer accurate.


Tires are 245-60-15 (sorry this was originally typed but must have deleted)...


TH350 / open rear (probably 2.56 or 2.73)


thanks!


scott

 

Not from the info you've given us. Unless you plan to drive around on the metal rims, we need to know the TIRE size, not the WHEEL size. We also need to know the rear axle ratio EXACTLY, not "probably".

 

I did a little more reading and I'm going to presume I have the 2.73 ratio. That's the standard ratio which appears to be listed for the '72 model Supremes. I haven't found any markings on or around the differential to further specify.

 

Look on the axle tube for a stamping. It might be upside down.

Something like this:

 

This is from 1970.

misc.jpg

 

Your tires are 26.57" in diameter. You want a 37 or 38 tooth driven gear (you're in between sizes), assuming you have an 18 tooth drive gear on the output shaft.

 

Thanks guys for the info - I lucked into an amazing find with my ragtop and I want to make a great driver out of it.


This is a great board -


scott

 

There is also a way to figure out what speedo gear you need without knowing anything about tire size and rear gear.

You need a stopwatch and precisely marked miles (many interstates have these markers). It's easier with two people. Don't start or end with markers that are at exits because these may not be in an accurate location.

You keep the speedo reading exactly at 60 mph for a few miles. Your copilot will use the stopwatch.

Let's say your average time is 56 seconds per mile.

Now look at the color of your driven gear (let's say for example it is Brown, 39 teeth). You would need to put in a gear that has 39 times 56/60 = 36.4 teeth.

You would replace your Brown gear with a White gear (36 teeth).

 

so what count did u end up replacing it with and color I'm in the same boat

 

Smart phone speedo aps negate the need for a correct speedometer and it goes with your from vehicle to vehicle.

 

Installed an 18-42 gear combo based on the 245-60-R15 tires I'm running. Upgraded the rear to a 3.08 posi from its original 2.73 open.


speedo seems to be right on the money for me with this mix.


also installed new housing for good measure