When I changed my standard 16" wheels for 17" Oz Superleggera's my speedo is reading differently. My car is a MY99 WRX STI Type-R. Oddly when I compared back to back my new and old wheels, my new wheels were actually smaller in height than my old wheels, therefore the rolling diameter will be different. I noticed that my mileage I do on a full tank shot up and a journey that I have been doing for years now also reads diffently. On my two last cars the journey read 245 miles, on my Scooby with new wheels the same journey reads 258 miles!!!
I assume that the differing diameters of the wheels will give different speedo readings. This will mean that I will put more mileage on my car than I actually am.
Can my speedo be recalibrated to dial this offset out?

Cheers,

Craig.:D

Can anyone help?

Anyone going from original 16" to 18" or 19" must have had this problem.

Craig.:D

I thought that the different aspects of the tyres was meant to take this into account - at least largely??
Got me worried now too :confused:

They are all slightly different....in the sense that 205 55 15 tyres (standard on 93WRX) are slightly smaller than 205 50 16s which I have on my car now. Difference is almost completely compensated for by the change in profile, so if you get larger alloys but get lower profiles, then you get the circumference of the tyres to match up, but obviously none of them match completely perfectly.

Mine remained within a few miles over a 145 mile trip though.

Bigger circumference gives higher indicated mph than actual dunnit?

Not sure about calibration but you could try to match circumferences as best as possible with the profile tyres you fit?

Dunno hope this helps !

Cheers
Alex

Thanks for the replies guys,

I am now going to do some phoning around. It seems strange that no-one has mentioned it before. Jumping to 18 or 19" must make a huge difference.

Cheers,

Craig.:D

Having a larger overall rolling radius will actually result in the car travelling faster than the speedo indicates. There's no real need to go into the relationship of how man revs per mph. A quick way is to work out the linear speed of the rotating wheel. This is the product of the radius multiplied by the angular speed. However, for a given rpm the difference in linear speed is simply the ratio of the two wheel radii. Most of the time when one goes up an inch on the rim the prfile comes down a step so the difference is only marginal. throw into the mix that different tyre manufacturers produce tyres with differnt actual dimensions even for a given size - eg. 205 50r16 - then the indicated speed can fluctuate even just using a different set of tyres.

Take 3 diffrent standard combos for Scoobies - 205 55 15, 205 50 16, 215 45 17.

The 15" wheels have a theoretical rolling radius of 606.5 mm
The 16" wheels have a theoretical rolling radius of 611.4 mm
The 17" wheels have a theoretical rolling radius of 625.3 mm

The 17" combo is only going to produce a theoretical inaccuracy of +3%. So for every 100mph you'd be going an extra 3mph faster. With the different tyre manufacturer specs this could be + or -. In other words, you will either not notice much difference or you might - it's not an exact science.

After all that #, I too am interested in getting my speedo tested because I often feel that I'm travelling much slower than indicated.