Quote:
Originally Posted by no golf clubs
Why does a 235/50/18 show 59MPH at 60 and a 245/50/18 show 60MPH at 60MPH?
I thought that the 235 bit was the tire width only, why does that affect the circumference, ie rolling distance? I would have thought that the profile (50) and the wheel size (18) would have been the only contributors to the equation C=PI*D,
Reason I ask is I have a chance of some decent 235s at a good price and I need an answer....
Should I buy these, I dont have to worry about a staggered set up and xdrive as I have the 2.0d all round.
Any advice would be welcome.
|
The rolling circumference of 245/50 R18 is 2143mm (all the wheel/tyre options for the F25 are between 2141 & 2146mm).
The RC of a 235/50 R18 is 2113mm. ie 1.4% smaller.
So at the same wheel revolutions that (with BMW spec'd tyres) gives 60 Miles, with 235/50 R18 the car will physically only travel 60/1.014=59.17 miles.
The complexity is that speedometers actually measure wheel revolutions (not distance travelled) and convert that to distance using an average wheel/tyre set and then deliberately over read the MPH by 7 or 10% depending on manufacturer and model.