Tire size vs mileage calculation

[Joe c]

Not sure anyone's addressed this before. Stock tire sizes for the mini SE are 205/45/17 and 195/55/16. But these are different diameters. The 17 inch are 823 revolutions per mile and the 16 inch are 825 revolutions per mile. There's no button or setting on the mini to tell the mini what you're size is being used. So the mini is going to give an inaccurate measure of miles driven. Does anyone know what the default tire size the mini is using to calculate miles driven? This would affect our efficiency calculations for sure especially when we are comparing efficiency between different stock configurations as shown in MichaelC's great posts.

 

[FrankinCarp]

So over 10,000 miles, the delta between 16" and 17" wheels amounts to 24.24 miles. This is theoretical in a perfect world. I expect there is sufficient tolerance in the "tyre specification" that there would be significant differences between manufacturers and production runs let alone climate.
I would wager tyre inflation pressure varying by 2 psi between cold and heat up to running temperature and tire wear over that distance would exceed that variance of wheel/tyre circumference.
That rolling circumference difference between wheel sizes, I am convinced, becomes insignificant for efficiency calculations.
Would a difference of 24 miles on the odometer of a car with 10,000 miles displayed knowing that it might be off by 24 miles dissuade you from buying it as a used car?

 

[SameGuy]

I was going to say that a two revolutions per mile difference — or roughly 0.24% — is well within any statistical margin. Remember, unless every single trip is driven in a perfectly straight line, all four tires are actually turning a different number of times per mile.

 

[DJCoopster]

BML - below measurable limit in the real world.

 

[yoomini]

I'm getting a slightly different revolutions per mile number. Also, the overall diameter of the 16" wheel setup is larger than the 17" wheel setup. Is that correct? Check my work:

Diameter:
17: 205/45/17 = 24.26" overall diameter
16: 195/55/16 = 24.44" overall diameter

Circumference:
17: pi*24.26" = 76.22"
16: pi*24.44" = 76.78"

Revolutions per mile (63,360 inches per mile from google):
17: 63,360" / 76.22" = 831.3 revolutions per mile
16: 63,360" / 76.78" = 825.2 revolutions per mile

Maybe the difference in tire pressure between the two setups negates any difference in clocked miles.

 

[DJCoopster]

While I'm a math guy, I also like well built calculators like tiresize.com's

 

[Brawndo]

Fun topic. Note, as the tire rolls, it deforms, and the tread squirms. So the stated revs/mile don't quite match up to the straight math circumference calculation.

Also the revs/mile are different by tire. The OE Star Mini rated 17" Dunlop vs 17" Pirelli - in the same OE size - differ by 5 rev/mile.

There's 8/32" of tread radius to burn away which also varies the rev/mile.

tl;dr there's a good reason the term "your mileage may vary" exists.

 

[Joe c]

My bad. I meant 832 vs 825, error in the original post!

 

[Joe c]

So the main reason I wrote this post was that I wonder why we can't put in the lightweight 15 inch wheel (heli spoke) which I believe only weigh 11 lbs each that mini offers and use 185/65/15 tires. They would be 824 revs/mile which would be very close to the standard 16 inch wheel.
I would imagine the heli spoke wheels with very efficient tires could get some really good efficiency numbers out of the mini SE.

 

[Carsten Haase]

So the main reason I wrote this post was that I wonder why we can't put in the lightweight 15 inch wheel

Probably not enough brake caliper clearance with the 15s

 

[Puppethead]

Probably not enough brake caliper clearance with the 15s

Exactly this. The SE has the "S" brakes, which are bigger than the non-S brakes, which can use 15" wheels. ICE "S" models are the same.

 

[Joe c]

I see. Well I guess we're stuck with stock wheels.

 

[SameGuy]

The published info suggests the 16” Revolites are quite light. You’d have to find out the weights of the different 195/55-16s to get the lightest combo, but rolling resistance is also a factor in economy results.

 

[Puppethead]

The published info suggests the 16” Revolites are quite light.

My wheel weights:

33.2 lb. 16" Spectre Grey Revolites with Hakkapeliitta R3 (and TPMS)
39.4 lb. 27" Corona Spokes with Goodyear Eagle F1 Asymmetric 3 (and TPMS)​

Both the Eagles and Hakkapeliittas are 18 lbs., so that would put the Revolites at 15.2 lbs. max and the Corona Spokes at 21.4 lbs. max.