I am not looking for a specific answer just a ballpark figure.
My car has 3.73 gears and my speedo is off by a mile. I have a 92 RS. What approximate speed am I driving if my speedo says 85mph. I think its ten mph to high but I was hoping someone has a clue.

 

How are we supposed to know that? We can't see how fast you're actually going.

Drive down a measured stretch of highway, such as an expressway. Drive exactly 10 miles. Look at how far youro odometer says you went. That's how far your gears are off. That always works for the gears, because the odometer is basically just a "turns counter" that counts how many times the transducer turns.

If you drive that 10-mile stretch with the speedo at exactly 60 the whole way, and time it, you'll also know how far your speedo is off. Obviously at 60 you go a mile a minute, so if your speedo was right you'd take exactly 10 minutes to drive 10 miles. If it takes 11 minutes (10% more time), then your speedo is saying 60 when you're actually doing 10/11 of 60; or about 54.5. The formula for your actual speed would be (600 / measured time).

 

Ballpark numbers: Check your tach at 85MPH and it should read
About 2900 RPM's with the info you supplied. Or at 60 MPH/2050 RPM's. Also +/- 50 RPM's for the inaccurate tach.

 

Maybe it isn't that complicated. Assuming it had the factory 2.73 gears and was accurate before the 3.73's were put in, 2.73/3.73=0.7319. Multiplied by 85 = 62.2 MPH.

If the gears were something else before, you now know how to do the math.

 

I wouldn't bother with the tach method, if it's a factory tach; they're nearly always wrong (read high) and often GROSSLY inaccurate.

 

That's what a highway, a friend with a stock car, and 2 cell phones are good for

Aren't there certain highways that say "begin speed test" at the 0 mile marker?

 

thats what i was gonna say have a friend with a known accurate speedo run next to you on a 4 lane hiway and use two way radios or cell phones to let each other know when the other is at certain speeds then you know exactly how fast your goin just pace one another..