Pit5Bull

What's the consequences of having the wrong size of tire/wheel on your car, as far as performance, gas mileage, etc. is concerned?

I was curious because after visiting the miata tire calculator site, it mentions the effect that specific wheel sizes have on the speedometer (whether it reads faster or slower) accuracy, which I understand, but what other effects will this have on the car's performance? If the sensor is reading the wheel rotation to be faster than it actually is, what can result of it, and vice-versa?

In my particular example, my stock tires are either 195/65/15 or 205/65/15. My MKW's are 215/40/18, (which I plan to switch to 235/40/18) which would mean that the speed sensor is reading 0.8% faster than I am actually travelling. So if my speedometer reads 65mph, I'm only going 64.5mph or something to that effect.

Thanks in advance!

Ammi

iTrader: (26)

Safety and driving enjoyment. Right size tires will help your vehicle respond quickly and safely. Just dont buy undersized tires.

Pit5Bull

Maybe I should rephrase the question. Wrong size tires in relationship to the diameter being measured against the stock tire size. I realize that the wrong "size" of tire is hazardous: what I'm trying to get at is whether or not the overall diameter of the wheel & tire (either too large or too small) affects the gas mileage/performance of the car as far as the ECU/computer system goes. Someone mentioned in an earlier thread that I might be getting crappy gas mileage because the tires on my new wheels were too small for my car (215/40-ZR 18 on an 18x8 wheel). I assumed it was because the wheels appear to be considerably heavier than the stock wheels (which are not much more than full-size spare tires), but his answer got me to wondering if that was necessarily the case.

Sorry for the confusion...

vansterdam

As your tires are very close to stock height, you probably won't notice a difference. Of course, a taller or shorter tire would affect gas mileage. It's not a simple calculation though. This tire calculator will show you the speedometer difference. A different height tire also affects gearing. A shorter tire would accelerate faster, but a taller tire would give better performance in higher gears. The shorter tire should use more gas as your axles would have to spin more to travel the same distance. Different sized tires throw out your speedometer and any internal computers that measure MPG. So it would be difficult to measure, unless you have a GPS.

CanadianMoFo

http://www.miata.net/garage/tirecalc.html

Try this one as well. I've been using it for years and years.

CM

00MaxSE

iTrader: (25)

Um I think that is the one he said that he used in his original post.