Speedo and Wheels
 

Does anyone know much about the effects of changing wheel size on the speedo reading ?

I've currently got 18's on and want to change up to 20's. But before doing that I'd like to understand the effect this will have on the speedo. I'm thinking that after the change the speedo reading will be understated.

Is there some mathematical formula I wonder ? Like every one inch increase means x MPH effect on the speedo ? That'd be handy .....

It all depends on the tires that are mounted on the new diameter wheels - and how the new combination changes the circumference of the tire. May make your speedometer faster, slower, or about the same if you use significantly lower profile tires.

Here is a handy calculator for it:

WebCars! Tire/Wheel Compensation Calculator

Assuming you have standard spec tyres (255/55/18) and you want to move to standard spec 20s (275/40/20) there´s a difference of 1.31%,meaning that with the 20s tyres your speedo will read fast by the same 1.31%(eg,speedo showing 70 mph ,your true speed will be 69.08.

Formula example(Use it for any tyres :))
Old Tyres
255-55-18
MM
255 x 55 /50 =280.50
25.4 x 18 =457.20
Total dia. =737.70

New tyres
275-40-20
MM
275 X 40 / 50 = 220
25.4 X 20 = 508
Total dia. = 728

Difference =9.7 mm (-1.31)

Speedo interference: -1.31% .Will read fast!At 70 mph ,true speed will 69.08 mph

I wouldn't really mind if I thought I was doing 70 but was actually doing 69. Only 1 MPH after all and at least it's in the right direction, as in I wouldn't be going over the speed limit. Not worth worrying about I'm thinking .....

from my experience with 19 inch rims and factory sized tyres at an indicated 100 kmph im really only doing 95kmph

actually across the whole speed range im really 5 k slower than indicated

Its only when you fit 21 or 22" wheels that the speedo difference is greater,due to the lack of more compatible tyres sizes not being commercially available.
There are some members running 295/40/20 all round ,so they can swap front to rear.:)

E53s come have everything between 17s and 20s as standard, or optional. With progressively bigger wheels, the tyre sidewall is lower so that your overall rolling circumference remains effectively the same. It's designed to work that way.

You must be talking about comparing your Speedo reading and actual GPS reading,surely?Otherwise how could the reading flactuate :dunno:

19" standard tyres in relation to the OP 18" will actually make the speedo read almost exactly the same,just -0.01% difference!

Regardless of whether you're on 17s, 18s, 19s or 20s with the factory tyre sizes your speedo will overread. All cars do it by somewhere between 2-3% and 10%.