RPM vs MPH
Thread Starter
Heel & Toe
Joined: Oct 2006
Posts: 16
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From: Granbury Texas
RPM vs MPH
I recently bought a 92 convertible with 71k miles. Upon driving home I found that everyone was passing me when I was doing 60-70 on freeway. I had a speedo check with another car via cell phone. I determined that my speedo was about 10 miles slow. I checked the tires, they are stock size. The code in the glove box indicates that it should have a 2.56 rear end ratio. I feel that the 2.56 rear end would be about 1400 RPM at 70 MPH? I just wonder if someone has 2.56 rear end in car that could give me the RPM at 60 or 70 MPH? I am doing about 2000 RPM at 70 MPH, so I reckon at this time that someone must have changed the rear box. (assuming that my tach is reasonably accurate) Does anyone know if the ratios are identified on the outside of the differential? (if they havent been changed). Thanks my phone is 817 233 7229 or thudplt@alltel.net
Advanced
Joined: Aug 2002
Posts: 99
Likes: 0
From: Lake Jackson TX
MPH= [RPM * LWR (half loaded tire height in inches)] / [ 168 * Gear Ratio]
gear ratio is (tranny gear X rear end ratio)
In your case (2.59 X .7 OD)
This holds true when your converter is locked.
2.59 gear
MPH RPM
50 1208
55 1329
60 1450
65 1571
70 1692
75 1813
80 1933
gear ratio is (tranny gear X rear end ratio)
In your case (2.59 X .7 OD)
This holds true when your converter is locked.
2.59 gear
MPH RPM
50 1208
55 1329
60 1450
65 1571
70 1692
75 1813
80 1933
Last edited by FastVette94; Nov 5, 2006 at 10:52 AM.
Burning Brakes
Joined: Dec 2003
Posts: 1,183
Likes: 1
From: MidOhio
For '89s at least, the original differential info is electro-etched on the bottom of the diff case, including the ratio. I suspect other years are also. It's not very permanent but if you clean the area carefully with a non-abrasive cleaner and shine a flashlight obliquly from different angles you can usually see the data. Otherwise, you may have to do the turns count method to determine what the ratio actually is. 
Le Mans Master
Joined: Jan 2006
Posts: 6,564
Likes: 6
From: Woodstock Georgia
Originally Posted by tonymax2
For '89s at least, the original differential info is electro-etched on the bottom of the diff case, including the ratio. I suspect other years are also. It's not very permanent but if you clean the area carefully with a non-abrasive cleaner and shine a flashlight obliquly from different angles you can usually see the data. Otherwise, you may have to do the turns count method to determine what the ratio actually is.
Elite Torch Red Member
Joined: Sep 1999
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From: Exit 89 GSP,Lakewood,NJ The Land Of Mojo
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Originally Posted by aminnich
Good point, sounds like previous owner might have swapped in a 3.07 performance rear and not had the ECM reprogrammed. You might want to contact the previous owner to see if that happened.
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Race Director
Joined: Sep 2000
Posts: 17,298
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From: The Top of Utah
The 2000 rpms at 70 mph is consistent with 3.07 gears. The diffs are stamped with the info, from the factory, but if the gears have been changed in your diff, the stampings will be meaningless. The best and most positive way to determine your gear ratio, is to jack the car up and count the driveshaft revolutions while you turn both rear wheels, exactly one turn. Once I know the ratio for sure, I can help you with the part numbers for the proper gears to correct your speedometer.
RACE ON!!!
RACE ON!!!
