75 mph =3600?

Wonder why my 2.5L M50TU. 16x7 tyres size is 205\50 3.15

120 kmh\75 mph =3600 rpm

how about your?

Thanks.

That is the same with my car, and I'm running on 16s with 205/55, I think.  Runs at about 3500rpm at 70/80ish... so putting lower profiles on would reduce that? Would that improve fuel consumption??

Sheep

Sheep,

if you are running on 16's then 205/55 or 225/50 are the correct tyre sizes. The profile is a percentage of the tyre width, so for example the profile of a 205/55 is 55% of 205mm = 112.75. Similarly the profile of a 225/50 tyre is = 112.5 ie the same as a 205/50. So you need to keep this profile the same in order to maintain the same rolling radius. Unless of course if you fit bigger or smaller wheels. For smaller wheels you need a bigger profile and for larger wheels a smaller profile.

Putting lower profile tyres on the same size wheels will reduce the rolling radius and hence reduce the gearing of the car. This in the long run will probably increase fuel consumption and it will put the speedo out.

Also as your car is a four cylinder it may well be slightly lower geared than the 6 cylinder cars anyway. 

325ism50. I assume that your car will have the same gearing as mine. Had another look the other day at the revs and mine is 3000 rpm = 80mph. Like I said if you are running 16 inch wheels then 205/50s are the wrong tyres, you need 205/55 or 225/50.