Correct Tire size?

[Crosshairs]

Hey guys... this may be a dumb question but here goes...

I purchased an 81 vette a little over a year ago and it has Dunlop radial GT Qualifier tires sized -- P255 R60 15 1025.

My speedometer has always been off (it reads faster than I'm actually going eg. if it says 50 mph I'm actually only going 40 mph) and I figured it was the incorrect tire size driving that incorrect reading.

The tires are worn out now so my question is... what sized tires should I get that will make my spedo read accurately? Or do I actually have a different problem?

Thanks!

 

[Paul T]

Hey guys... this may be a dumb question but here goes...

I purchased an 81 vette a little over a year ago and it has Dunlop radial GT Qualifier tires sized -- P255 R60 15 1025.

My speedometer has always been off (it reads faster than I'm actually going eg. if it says 50 mph I'm actually only going 40 mph) and I figured it was the incorrect tire size driving that incorrect reading.

The tires are worn out now so my question is... what sized tires should I get that will make my spedo read accurately? Or do I actually have a different problem?

Thanks!

Nope, you have the correct size tires. Sounds like you may have the wrong speedo gear in the transmission. A 20% error is more than just tires. A second choice is the speedometer itself. Good luck on the hunt.
Paul

 

[akfox1]

I agree. Those tires are 27" tall, exactly what they should be. Someone may have swapped the rear end ratio, or replaced the transmission.

The speedo gear is relatively easy to replace. Do you know what rear end ratio you actually have in the car? Jack it up, put it in neutral, and manually turn the rear tire. Count how many times the driveshaft rotates for each tire revolution.

 

[Bob Chadwick]

According to the CAC Tech Center the 1981 Specifications & Performance Review has three different tire sizes.

Standard: P225/70R15
Optional: P225/60R15 & P255/60R15

So depending on what yours came with you could have the wrong tires. I don't have the calculator handy to determine the difference, if any, in height.