Speedo/odometer registers fast
Thread Starter
Pro
Joined: Oct 2005
Posts: 642
Likes: 12
From: West Chester Ohio
Speedo/odometer registers fast
Just bought an LT-1 on friday. I knew buying the car the speedometer registers about 20-25% faster, as well as the odometer. Thus if you are going 40 mph the speedo registers 50. I'm inclined to think the speedo needs replaced or rebuilt or its in the gears, a mechanic friend had mentioned that thier is a possibility that the tire size could be the problem. I have 245/60/15 on the rears and 235/60/15 on the fronts. I have a hard time believing the tires could be causing that much difference, any ideas?
scott
scott
Safety Car
Joined: Apr 2001
Posts: 3,581
Likes: 28
From: Spring, Texas
Smaller diameter tires can make your wheels turn more revolutions than taller diameter tires. The 70 - 72 cars (including LT-1) came with G70 x 15 tires, but I can't recall the millimeter width equivalent (e.g. 215, 225). Once you get that information, use the websites below (or Google "tire size calculator" - tons of hits). I've read that you should be no more than 3% off from the factory tire diameter spec.
http://www.1010tires.com/TireSizeCalculator.asp
http://pw1.netcom.com/~sgalaba/tiresize.htm
http://www.1010tires.com/TireSizeCalculator.asp
http://pw1.netcom.com/~sgalaba/tiresize.htm
Last edited by SharkAttack; Jul 3, 2007 at 08:49 AM.
Thread Starter
Pro
Joined: Oct 2005
Posts: 642
Likes: 12
From: West Chester Ohio
Thanks. I'm inclined to beleive its the gears, i know the car was raced before years ago. So i have to pull the gears and count the teeth, my guess is they arent stock and causing more revolutions.
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,355
From: Santa Ana CA
It may be that the rear end ratio was changed. You can spin the rear tires and count the number of times the drive shaft spins to determine the rear end ratio. Then, you can pull the speedo gear out of the trans and look at its color (or count the teeth) to see if it is right for that rear end ratio.
Ah, but the trick is to know which gear goes with which rear end. The Chevy dealer should be able to look it up, and they still stock these gears.
If it comes down to tire size, you can also compensate for that with the right speedo gear (to some extent).
Melting Slicks
Joined: Dec 2003
Posts: 2,487
Likes: 1
From: Sydney
It may be that the rear end ratio was changed. You can spin the rear tires and count the number of times the drive shaft spins to determine the rear end ratio. Then, you can pull the speedo gear out of the trans and look at its color (or count the teeth) to see if it is right for that rear end ratio.
Ah, but the trick is to know which gear goes with which rear end. The Chevy dealer should be able to look it up, and they still stock these gears.
If it comes down to tire size, you can also compensate for that with the right speedo gear (to some extent).
Ah, but the trick is to know which gear goes with which rear end. The Chevy dealer should be able to look it up, and they still stock these gears.
If it comes down to tire size, you can also compensate for that with the right speedo gear (to some extent).
You don't need to pull the rear gears (or even open up the diff center) to check your ratio just spin the tires while it is jacked up and then buy the appropriate transmission gears to correct the speedo.
Instructor
Joined: May 2006
Posts: 221
Likes: 1
From: Albuquerque New Mexico
Pull the speedometer driven gear out of the transmission count the teeth and buy a gear with two more teeth.
If your speedometer reads too high, a driven gear with one more tooth will slow it down approximately 5 MPH. This works for four speed transmissions
If your speedometer reads too high, a driven gear with one more tooth will slow it down approximately 5 MPH. This works for four speed transmissions
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