The "aspect ratio"; that is the height of the sidewall expressed as a percentage of the tread width.

Wow, I didn't know that, I thought it was an absolute measurement, ie 45mm, or something like that :rotz:- thanks!

So based on how wide it is, depends on how big the wall is at 40/45 on that profile.

So anyone handy with maths fancy working out how much smaller/bigger the tyre wall is from a 205/45 to a 215/40??

A guess::

205 * 0.45 = 92.25

215 * 0.40 = 86

205/45 is 7.2% bigger ?

This bookmark proves useful time and time again

Car Bibles : The Wheel and Tyre Bible Page 1 of 2

Steve

So 7.2 per cent in favour of throwing out the speedo EVEN more than it is already out? or putting itback in line? Interested as I run 215's and I know my speedo is out a tad, but wondered how much it was out at 160mph

Check this:

http://www.tyresite.com/tyrecalc.asp

Going from 205/45 R 16 to 215/40 R 16 is a -2.12% speedo reading - speedo reads 70mph, actual speed 68.52mph - difference in tyre DIAMETER overall is -12.5mm and CIRCUMFERENCE overall is -39.27mm

Just as a word of warning, don't some insurance companies count a tyre size that is different to stock as a declarable modification?

Check this:

http://www.tyresite.com/tyrecalc.asp

Going from 205/45 R 16 to 215/40 R 16 is a -2.12% speedo reading - speedo reads 70mph, actual speed 68.52mph - difference in tyre DIAMETER overall is -12.5mm and CIRCUMFERENCE overall is -39.27mm

So does that mean with 215's the speedo is more out than with 205?? im confused

running the OE tyre size my speedo is scarily accurate, all the cars ive had before over read by 5mph or so, but not this one, those speed signs at the side of the road always match my speedo by at most 1mph difference

When I 1st got mine I went out with the SatNav to calibrate how out it was.

Rule of thumb in mine is if your doing 30, for example, 30 is actually right on the far edge of the 0, rather than between the 3 and 0, so I guess that's 1 -2 mph out?

I did wonder if it was like this on purpose, as sub consciously it feels much easier to keep the needle on and around the 30 and you actively have to go past it to start speeding. if it was exactly right I'd find my self constantly trying to get the needle between the 3 and the 0.