Unless the aspect ratio is marked on the tire, it will be around 80%. So
a n 185-15 would be 185/80. Different brands always have slightly
different circumferences (take it from a former rallyist, we measured
mileage to the 1000/th of a mile!) Rule of thumb for wider/lower tires,
to keep speedometer and ride height accurate, is to increase WIDTH (1st
number) by 20 for each 10% less ratio (second number). So the stock
155-13s (155/80 actually) can be replaced by 175/70s on my Spitfire.
Bear in mind that as you go to wider and lower profile tires, that wheel
width is more critical. So on a Spitfire, you would want wider than stock
wheels if you wanted 195/60 tires (assuming no rubbing on the suspension
due to the wide rubber.)
Some of the tire manufacturers or retailers have more in-depth
explanations, especially regarding Plus 1 or Plus 2 sizing.
(www.tirerack.com comes to mind)
Atwell Haines
'79 Spitfire FM96062
'88 Lotus Esprit
'88 Mustang 5.0
CarBuff@scooter.net
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> Tony:
>
> It is my understanding that the proper OEM tire size for TR6 is not
> 185/70R15, but rather 185/15... no aspect ratio for this size. Otherwise,
> I could calculate the rolling circumference of the tires based on my
> understanding of the sizing nomenclature. My car is fitted with 185/70R15
> Bridgestone SF371s and they are too short. Someone told me once that the
> OEM size was closer to a 75% aspect ratio. Go find, for instance, the
> aspect ratio on a set of Michelin Redlines. I don't think they have one
> designated. They just say 185/R15. Anyone know more about this?
>
> Pete Chadwell
>
> 1973 TR6
>
> >Check out the site at www.powerdog.com/tiresize.cgi/. If you know the
> >tire size (i.e.- 185/70R15), this site will give you every dimension of
> >the tire you need. You can even list different size on the form and it
> >will do a comparison of circumferences indicating the difference in
> >percentages. This is useful if you want to go to another tire size
> >(wider?) with minimal effect on your speedo. Check it out!
> >
> >Tony Childs
> >"72 Spitfire - Resurrected!
>
>
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