AnswerID: 264863 Submitted: Wednesday, Oct 03, 2007 at 13:41
Steve L
replied:
Definitely using a
GPS is the way to go.
The problem with ALL the tyre size calculators out there is this - not all tyre manufacturers make
tyres that match the specifications printed on the sidewall.
For example, a 285/75R16 'should' have a diameter of 833.9mm if you calculate it using the tyre section, sidewall and rim diameter figures. Unfortunately, you will find that most manufacturers do not stick to this, and most 285/75R16s (with the exceptions of Coopers and Mickey T - and maybe a couple of others) have their 285
tyres usually with a diameter of 840+mm, with BFG having their AT at 852mm!!! (I.e. not even close....)
That results in a sigfnificantly different speed reading compared to what the tyre calculator suggests. This is the case for most tyre sizes, and when the diameter is out, the circumference, etc. are all out as well (as are the comparisons between tyre sizes).
Use a
GPS or get the manufaturers specifications for the specific tyre you're using (or wanting to buy) and do your own calculations from there if you want to be sure you've got it right.
Reply 3 of 3
FollowupID: 526702 Submitted:
Wednesday, Oct 03, 2007 at 16:35
Steve L posted:
It's not really something to do with tread depths.
For example, a Coopers ST or STT 285/75R16 is 833mm in diameter, while the BFG AT in the same size is 852mm and many others are around the 841mm diameter. The BFGs total tread depth is not 19mm, let alone its tread being 19mm deeper than the Coopers.
I think it's simply the manufacturers building in tolerances or something. There is a 3% variance in the ADRs or similar that they are allowed to be within in terms of their sizing, so perhaps they are taking advantage of that?
FollowUp 2 of 2
