Hi friends,

I have an 89 RS, 700R4. 2.73 open diff.

I have read all of the posts and used all the calculators I could find to try and get to the answer, and I think I have. i was hoping someone could check my math as I have a nonstandard tire setup from the previous owner.

If anyone happens to catch it in the photo, I am aware that my drive gear on the trans is missing the clip. I am midway through working that out on another post, as its Its the cause of my no speedo! Which leads me to this question so I can fix this all once and for all.

I have 255/35/R20 rear tires. The measurement from the ground to the middle of the wheel is 13.5 inches. The tire is 27" top to bottom.

My transmission has a RED 17 tooth drive gear

I should require a 34.63 tooth, which rounded up for safety( right?) puts me at a 35 tooth gear which is orange.

Is this combination correct?

 

Some photos for reference if needed

 

I used this doc to calculate it

 

So this combination shows about 15mph higher than a GPS speedo says. any ideas?

i wasnt able to do a turn test, i know i have open gears based on how they spin in the air. but i am only assuming 2.73 by what the RPO says.

when i tried to spin the tires by the driveshaft i got like 2.2, but it definitely wasnt accurate because for some reason the driveshaft didnt always turn when you spin the wheel and vice versa

 

A mathematical calculation won't necessarily get you an accurate speedo reading in these cars that use a magnetic speedometer. Magnets deteriorate in strength over time and the speedo becomes inaccurate, making any mathematical calculation largely irrelevant. A speedo math chart might be useful to get you in the ballpark, but if you really want to dial it in you need to just buy a combo pack of gears and experiment. I have found each gear tends to make about 2 mph difference.
If you are off by 15mph right now you arent even in the ballpark. Need to actually figure out your rear gear ratio, not guess at it.
Pop the cover off rear differential and check that way. Good excuse to change the oil anyway.

 

FWIW, what's your RPM at 60 mph (actual speed via GPS)? With 27-inch tire and .7 overdrive gear, RPM at 60 mph would be about 1,400 with 2.73 gear. 3.23 would be about 1,700 and 3.42 would be about 1,800.