I am about to change from worn 205/55/15 tyres to new 205/45/16 Bridgestone tyres. On paper, the new tyres will be 2.6% undersized, but wonder what's the difference in rolling diameter between a new and an old tyre due to the thread's depth. Need the info to recalibrate my fuel consumption and speedo reading.

Thanks & regards.

 

Why are you fitting 205/45/16 ? The standard sized subaru 16" tyre is 205/50/16 .
I don't think the thread depth will make any significant difference to the rolling diameter as regards the speedo reading or fuel consumption .

 

Assuming that the tread depth on a new tyre is around 8mm and that you'll change tyres when they're down to around 2mm, then the diameter of the tyre will reduce by approx 12mm over its life.

A new 205/45x16 will have a rolling circumference of 1855mm, a worn 205/55x15's rolling circumference will be around 1866mm so you wouldn't notice the difference.

Over the life of the tyre, the 205/45x16 will wear to the extent that it's rolling circumference would be, as you said, around 2.6% less than the equivalently-worn 205/55x15. In mpg terms, the difference is only around 50+ yards/mile so assuming Scooby-like consumption of around 20mpg , it would make less than 1mpg difference so isn't worth bothering about.

As for calibrating your speedo, it depends how accurate you want to be. Police traffic cars have theirs calibrated daily to minimise the discrepancy caused by tyre wear. In normal driving, a few mph one way or the other shouldn't matter. Police (and Gatsos, etc) will always (IMHO) allow for that margin of error. If you want to be precise, though, most roads will provide distance markers in some form or other - on Mways, they're a mile apart; on most A roads, look for the painted squares/circles which will generally be 1/4 or 1/2 mile apart (depends on police area).

Given that there's a general acceptance that speedos can be up to 10% out (over-reading), I wouldn't worry about it. But that's me, of course!

Brian

 

I know the standard 16 inch rubber for the car is 205/50, but I bought a set of used Speedline and they came with two new Bridgestone SO2 205/45/16, so rather than waste them, I might as well make use of them first.

I have been keeping a running log of my fuel consumption and the figures hardly varied more than 1 mpg (around 24-25 mpg) over almost two years of ownership, hence the need to make allowance for difference in tyre diameter even of such a small magnitude. The other reason is that I usually incorporate the 'margin of error' in my top speed, hence the need to avoid compounding of errors.

Thanks for the info.