Larger wheels - Will it affect my fuel computer readings?

[Evo]

Can someone help with a query I have.

My C Class has 18” AMG wheels fitted on 45 profile tyres. The over circumference is larger than the original 16” wheels with 65 profile tyres, probably larger by some 1 ½ inches. Will this throw out the readings I get from the fuel computer relating to the current and average mpg??

If the mpg average reads 25 for example is the real mpg higher or lower in reality?

Thanks in advance

[gog]

Lower

[steve-V8s]

Speedo will be wrong as well

[MGwob]

I have always been led to believe that if you go up 1 inch on the wheel diameter that you come down 10 on the profile of the tyre:

So if you had 16" wheels with a 65 profile and you fitted 17" wheels, you would need a 55 profile.

[Magog]

Try this;

http://www.etyres.co.uk/tyre-size-calculator.htm

[mrmr96]

gog said:

Lower

No, it will be higher.

Travelling one mile on the small wheel will turn them, say, 1,000 times.
Travelling one mile on the new larger wheels will turn them, say, 900 times.

So if the trip computer says you've used up one unit of fuel and according to the computer travelled one mile, the wheels have turned 1,000 times. But clearly you will have gone slightly furhter, as 1,000 is more than 900, therefore more than a mile.

Therefore if it says x mpg, you're actually done slightly better than the computer would suggest.

[mrmr96]

steve-V8s said:

Speedo will be wrong as well

Speedo will read too low. So indicated 70mph will actually be, say, 75mph. Measure the circumference of the new wheels and compare to the old. If the circumference is, say, 105% of the old, then add 5% to what the speedo says to work out your real speed.

[Evo]

Well done chaps, you can always rely on PH'ers to sort it out