I have never been good at figuring this kind of stuff out, so I thought I'd just punt it over to the experts.

I changed the rear tire size on my 94 from the standard 285/40/17 to 315/35/17. What is the effect of doing this on the accuracy of my spedometer?

 

i think they will come out the same. i did the same exact tire size swap a month ago... and didn't notice any difference. that middle # indicates the height of the tire(from the edge of the rim to the opposite side of the tire, right everyone?) i wouldn't expect speedo probs unless you went to a 275-40-18 in a different sized rim.

 

That middle number is the height of the sidewall as a percentage of the width of the tire. 40% of 285 is 114, 35% of 315 is 110.25. That's in millimeters,
so they're very close, but your 315 will be a slightly shorter tire, and therefore smaller diameter, tire overall. Not enough to make much difference in this case.

 

Since tire diameters can vary, go to http://www.tirerack.com/ and come back with the two tire diameters. Then we can just take the actual ratios to come with the speedo change, if any.

 

Possibly, none. The information you need is not in the tire size. You need to know the circumference or revolutions per mile of the tires involved. The tire store MAY be able to give you this information. Even if they can, I, personally don't care to rely on the books. I like to measure the "roll out" of each of the tires. To get the "loaded" circumference or roll out, mark the tire at the ground. Mark the ground at the same spot. Move the car "exactly" one tire revolution and mark the ground at the new location. Measure between the two marks on the ground. For greater accuracy, move the car more than one (10?) tire revolution, and divide the distance on the ground by the number of revolutions. The speedometer error will be the ratio of the difference of the two circumferences to the origional tire circumference. Simple example: Old tire = 100"; New tire = 80". The difference of 20" is 20% of the old (100") tire. The speedometer will read 20% faster with the new tire, at the same car speed as the old tire did. Clear as mud?

When I started writing this, there were no other responces to your question. At the time of this edit, there are two ahead of me. These guys aren't wrong. I just prefer to use measurements for greater accuracy. Two tires of the same size don't necessarlly have the same roll out. In fact, more often than not, they don't.


[Modified by CFI-EFI, 7:02 PM 11/29/2001]

 

Actually...yes. That makes sense to me.

 

That's a good method, CFI-EIF.

Mathematically, the diameter of the 285/40/17 is 25.97" and the circumference then would be 81.6 inches. The diameter of the 315/35/17 is 25.68" and the circumference would be 80.7.

3 tenths of an inch difference in the overall diameter wouldn't be much more than the amount of tread wear over the life of the tire (talking about two sides).

Hope that helps.