Mr Hyde

Using something like the Tire Size Calculator
http://www.miata.net/garage/tirecalc.html, or using the listed RPM (revs per mile)for the specific tire in the exact size you want?

I would think that all tires in the same size would have the same RPM, but they dont. I am not sure if that is due to tread depth differences, differences in deformation, or maybe both?

For examply according to Tire rack, the stock tires have a 819 listed RPM, but the tire size calculator lists it at 792.

The reason I am asking all this is I was just wondering if maybe matching stock tire RPM, would give me more accuracy on the Speedo, and more importantly the odometer than just using a tire size calculator; even if I went to a different size. Looking around on Tire Rack, some 225/45 tires were a closer match to Stock RPM's than some 215/50's, and I dont really want to tack on alot of extra mileage that was never really driven.

Thanx in advance to anyone with any input. .

lou_RENAMED

revs is the most accurate comparison because even a .01 will give you a big difference in revs.

personally I wouldnt worry too much about the revs cause as long as your close it shouldnt be that different

Slimey

The tire calculators are helpful, but you have to look at the specs of the actual tire to accurately size it up.

It's like putting on a pair of pants or jacket. You can pick up 10 same sized pants off the rack and each one fits different.

Compare the new tire's difference in roll-out to the OEM 819/mile. Keep in mind that the roll-out listed is for the tire mounted on the measuring rim (usually the middle size in the size range). If your rim has a different width then the measuring rim, you'll have slightly different roll-out then what the spec lists.

resedaruss

iTrader: (1)

I too have puzzled over the discrepancies in Revs per mile among different manufacturer's same-size tires.

I suspect in some cases, tire manufacturers calculate Revs-per-mile from the "size" specifications with less than perfect accuracy (using truncation instead of rounding, and/or perhaps use of different bias-factors to account for dynamic operation of the tire, etc.).

In other cases, one manufacturer (for example Michelin) puts 10/32nd's of an inch of tread on their tires while another manufacturer (Pirelli) puts on 11/32nd's of an inch. So the tires have slightly different circumfirences.

Let's assume also that tires that have extra-stiff sidewalls to increase the load handling of the tire (the OEM michelin is load-rated at 93 while the Conti Extreme Contact is rated at 90) therefore each might maintain their circumfirence differently at speed.

There could be lots of reasons.

However, keep in mind that there is also a change in revs per mile as a tire's tread wears from an original 10/32nd's of an inch (for example), to 2/32nd's of an inch where the tire is declared worn-out.

So, if revs per mile are relatively close as they would be among different brands of the same size tire, one shouldn't notice glaring differences in their gearing, speedometer or odometer performance.

Mr Hyde

Thanks for the info guys.