Does size matter ?

[JulianG]

Hi,

I recently bought myself a set of 19" alloys for my Leon TDi Cupra, however since fitting them I seem to be getting on average 10mpg less on my journey to work.
I used to get around 50 - 55mpg on average, its now 40 - 45mpg !

Does anyone know if changing my standard 225/45/17's to 235/35/19's should be making this much difference to the mpg ?

Maybe I haven't got the tyres inflated to the right pressure ? I've currently got them at 30psi all round....

Any advise or assistance would be great.

Cheers

Boolandy

 

[skullmunki]

isnt it just a case of physics?

wider tyres = more friction = lower mpg

well, it makes sense to me!

 

[daveyonthemove]

I'd tend to agree with the above comment. The more rubber in contact with the road will add more traction when you need it, but more friction when you don't.
Try checking the tyre pressures too, that could make a difference. I would also expect a bigger wheel to be a bit heavier, so all 3 of those points considered, it think you could expect a drop in economy.

 

[aceman]

Also don't forget that with larger diameter rims there is alot more weight on the outer circumference of the wheel hance it takes more effort to rotate the wheels hence less mpg.

The law of physics strikes again.

 

[truCido]

Yeah you will only see advantages when travelling say on the motorway....but travelling around town, stopping and starting it will definately drop as like aceman said it takes more effort to rotate

 

[andycupra]

you should get more 'roll', so if you adapt your driving you could gain back some l ost getting the car going...

 

[aceman]

There is another negative effect to large rims also in the braking department it will take more effort to haul the car to a stop but depending on how good your brakes are you might not feel this effect.

 

[JulianG]

Thanks for your comments guys

I always hated physics at school !

I see that more rubber on the road will mean more friction and hense less mpg though... but I thought 10mpg on average was extreme

Anyway I'll find out the correct tyre pressures and see if that changes anything.

As for braking though, I haven't noticed any difference in stopping ability.
But then I did have new pads and disc all round not that long ago so should all be in good condition.

Cheers again for your advice

Boolandy

 

[muddyboots]

Going from a 225 width to 235 width will only increase the amount of rubber contacting the road by a tiny amount. And I doubt whether you'll have increased the overall circumference much either.

Ian (Aceman) is right, the biggest difference is that the bulk of the wheel weight (the outer rim) is now further from the centre of the wheel, making the wheel harder to accelerate and decelerate. This will have more effect if you are slowing down & accelerating a lot, would be less noticeable during constant cruising.

 

[DPJ]

30psi seems rather low!

I'd try it at 35/36 and see what mpg you get................

 

[andycupra]

you may also find that the suspension feels a little harsher due to an increase in un-sprung weight and also due to the shallower tyre side walls.