Tyre size change knackered my MPG!!

**Last Turkey** · 17th December 2009, 16:19

I changed my front tyres from factory fitted goodyears (sri 17 inch alloys) last weekend. Now my mpg is around 5-10% worse! Its not the cold weather, i know what i should be getting. Someone on here, said that VX can change the tyre cal in the ECU via tech2. How much would they charge for this?

**Diet1985** · 17th December 2009, 16:20

what did u change them too?

**Dan_BlackSRi** · 17th December 2009, 16:20

Tyre pressures right..? That can have a noticeable effect on mpg....

**Ruperts Trooper** · 17th December 2009, 16:25

If your new tyres are larger rolling radius it is possible to get an improvement in mpg but still show a decrease - it's all to do with the arithmetic and gearing.

Most (maybe all?) OE tyres are ECO low resistance fuel-conserving tyres - it's very likely that any change will increase consumption.

Don't forget that weather, traffic conditions and driving style all affect fuel consumption so it's normal to get a variation in results, even over longer mileages.

**Last Turkey** · 17th December 2009, 16:31

Sorry re the missing info. Changed them from 215/50/17 to 225/45/17 (Sri 17 inch alloys).

I understand about driving style etc, but I know what mpg i should be getting. It definately went with the tyre change.

**Big Sig** · 17th December 2009, 16:32

Are they cheap tyres or a decent brand?

**Last Turkey** · 17th December 2009, 16:34

Tyre pressure should be OK, as they have just been fitted.

The brand: they are Avon z33 (or zz3). The old ones were goodyear nct's.

**gregc** · 17th December 2009, 16:39

check the pressure when garage fitted mine only put 30psi in and should have been 38psi, don't trust the garage to put correct psi

**Diet1985** · 17th December 2009, 16:43

Originally Posted by gregc

check the pressure when garage fitted mine only put 30psi in and should have been 38psi, don't trust the garage to put correct psi

tis true...garage refused to put mine up to 43psi as they said it was way to high... even though i explained i was going on a long journey full loaded up and it said it in the book!

**Handyandy-UK** · 17th December 2009, 16:45

Tech2 or Op-Com can as you suspect, tell the car it has a different wheel size fitted. From looking at mine though all it does is change between 17, 18 and 19 inch to accomodate the change for wheelspeed sensors/speedo, etc.
As has been said, I'd be looking at the cheap options before worrying out paying out more dosh.
Pressures first - air is free