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[Shackeng]

This seems to be a useful calculator: http://www.boosttown.com/wheels_tyres/tyre_size_calculator.php

[MV6Matt]

That looks like a good one!

I use this one as it has a great visual guide too

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

Matthew

[Martian]

Also available on the market, and I expect there is also something similar for those taking it up the backside from crapple.

[2woody]

This seems to be a useful calculator: http://www.boosttown.com/wheels_tyres/tyre_size_calculator.php

just tried that one out - it's over an inch out in tyre diameter

[2woody]

from last week's thread.... (adjusted to suit)

the maths on all of these websites and in the hand calcs is worng, sorry.

the tyre business and the auto business doesn't use them because they're wildly inaccurate. The problem stems from the static figures vs. what actually happens on the road. It's for that reason that every profile is tested and the measured figures are what's used. There are two important figures, the wheel revs per mile ( which gives you the gearing ) and the hub height, around which the suspension geometry is designed. Using measured figures takes out the installed sag and the effects of slip.

Omega B is designed around 817 wheel revs per mile. Anything between 807 and 827 will be acceptable ( i.e. 5 either way )

some comparisons :-

205/55 x 16     811 wrpm     594mm diameter on the car     654mm diameter calculated
245/50 x 16     811 wrpm     594mm diameter on the car     651mm diameter calculated
235/45 x 17     820 wrpm     595mm diameter on the car     643mm diameter calculated

tell you what - I'll write a sticky on the subject......

[Kevin Wood]

It would be great if someone could write an accurate calculator, actually.

Oh, 5h1t! that's as good as volunteering.

[aaronjb]

It would be great if someone could write an accurate calculator, actually.

Oh, 5h1t! that's as good as volunteering.

Yeah.. Hop to, Kevin!

[2woody]

that's just the point - every size, profile and tread combination has a different figure, so they are measured, rather than calculated.

[Kevin Wood]

that's just the point - every size, profile and tread combination has a different figure, so they are measured, rather than calculated.

I was thinking of something that works on a table of known values, perhaps?

[Shackeng]

This seems to be a useful calculator: http://www.boosttown.com/wheels_tyres/tyre_size_calculator.php

just tried that one out - it's over an inch out in tyre diameter

Is that for every alternative Stewart? I was looking at going from 225/55 R16 to 205/60 R16 for winter tyres, which it claims is only 2mm difference.

[2woody]

unfortunately, it's not a "blanket" error - it's a different error for every combination.

why 205/60 x 16 ? why not the size from the owner's manual ?

[2woody]

I was thinking of something that works on a table of known values, perhaps?

I have the known values.......

[Shackeng]

This seems to be a useful calculator: http://www.boosttown.com/wheels_tyres/tyre_size_calculator.php

just tried that one out - it's over an inch out in tyre diameter

Is that for every alternative Stewart? I was looking at going from 225/55 R16 to 205/60 R16 for winter tyres, which it claims is only 2mm difference.

As recommended to use narrower tyres for winter, this looked closest on that table. How far out is it?

[2woody]

that's a very good question, as my figures only go down to 55 profile - I need to do a search and get back to you.

[Shackeng]