Thread: Quick question re tyre size/RPM - real world experience

Originally Posted by George130

I did it for a while. Bought a set of tyres second hand for $100. It has a big impact on your speedo and RPM. extra cost in fuel makes it a wast of time.

Diference in diamiter is -7.69% or -62mm

revelutions per km 394.9743778199558 (235/85 R16)
revelutions per km 427.8933338958226 (225/75 R16)

Hope this helps

And a useful formula is

kph/1000rpm = 60000/(gear x trans x diff x tyre rev per km)


gear = gear ratio ie 1 for say 4th gear
trans = tranfer ratio
Diff = diff ratio (3.54 std defender)
tyre rev per km 448 for 205R16
409 for 750R16 etc


So for a RRC running 205R16 4th gear high


kph/1000rpm = 60000/(1 x 1.113 x 3.54 x 448)

= 33.99kph/1000rpm




Martyn

Originally Posted by isuzu110

Pete

Ok, I googled your Vic Vehicle Standards available here

Seems they may be technically more restrictive than Qld on tyre modifications as they mention only +/- 15mm change in diameter versus the "largest diameter tyre specificed by the manufacturer" in that link. By all means, check it yourself, and don't take my word for it.

No I don't claim to be an expert on Vic regs and yes, I too saw plenty of large tyred 4WDs in my time living in Vic that didn't seem to attract police attention.

Not trying to be alarmist or a kill joy, just do your own homework and know what risks you are taking.

Enjoy your rubber!

Its not always the "boys in uniform" thats the worry about mods we do ...its the Insurance Companys who will look for any reason not to pay up on a claim.