I have just changed from standard 205/50/R17 tyres to 225/45/R17 Pirelli P Zero Rossos. Although they seem fine my average fuel economy has dropped from around 35mpg to less than 30. I realise that the computer will now be slightly inaccurate as a result of the slightly smaller tyre circumference but surely it can't make that much difference. Has anyone else experienced a similar drop when changing tyres?

There's a bit more rubber in contact with the road isn't there? I see almost a 20% improvement in economy when using my 14" steels with ecocontact tyres, compared to 17s with 205 wide toyo proxes tyres.

It's not the same situation as yours, but I've noticed that when I swap worn tyres for new ones then the trip computer mpg drops by 2 or 3 mpg with the new ones, even when they're an identical make, type and size to the old ones.

I think it's because the new tyres, with more tread, have a diameter that's a few mm bigger than worn ones of the same type.

This means every mile is measured by the car differently and gives errors in mpg and also speed (with worn tyres you're going slower than you think - which may also mean you use less fuel).

It's not the same situation as yours' date=' but I've noticed that when I swap worn tyres for new ones then the trip computer mpg drops by 2 or 3 mpg with the new ones, even when they're an identical make, type and size to the old ones.

I think it's because the new tyres, with more tread, have a diameter that's a few mm bigger than worn ones of the same type.

This means every mile is measured by the car differently and gives errors in mpg and also speed (with worn tyres you're going slower than you think - which may also mean you use less fuel).[/quote']

7mm difference I assume, on each side, so 14mm bigger diameter overall = ahem, my brain hurts! Well assume a normal 17" alloy + tyre is 530mm in diameter with a worn tyre. ahem (17" + 2*50mm profile), then adding a new tyre takes a diameter to 544mm, which is a 2.64% increase...

These sums might be totally to ****, but what the hell. 2 mpg out of a 35mpg average though is a little bigger than 2.5% eh?

7mm difference I assume' date=' on each side, so 14mm bigger diameter overall = ahem, my brain hurts! Well assume a normal 17" alloy + tyre is 530mm in diameter with a worn tyre. ahem (17" + 2*50mm profile), then adding a new tyre takes a diameter to 544mm, which is a 2.64% increase...

These sums might be totally to ****, but what the hell. 2 mpg out of a 35mpg average though is a little bigger than 2.5% eh?[/quote']Oooh my brain hurts too - and I'm sure I'm getting this horribly wrong but...

I reckon your 2.64% counts twice - once when you measure the mpg, but again when you measure the speed.

So if you drive at an indicated 60mph, then with worn tyres you're actually going 2.64% slower and only really doing 58 and a half mph. (Obviously this is in the non-existent traffic conditions when you drive based on the speedo rather than other traffic slowing you down...)

Friction from air resistance rises with the square of speed and 58.5/60 squared is about 0.95. So in terms of real fuel usage, you may get 5% better economy just because you're going slower.

Then you factor in your original 2.64 % because the mileage and hence mpg figure is measured wrongly. So could it be up to 7.64%? I dunno - my head really hurts now! But 7.6% is a couple of mpg when your base figure is 35mpg.

For my Furby, which averages nearer 55 mpg, it could explain where those 2-3 mpg go when I put new tyres on.

:confused: :confused: :confused:

Ouch! I'm gonna rest my brain now! I lost it about half way through that. :(

Aha - vRS Furby is different! 16" + 2 x 45mm = 496mm worn, and say 510 with tread, which is a marginally higher percentage 2.82%.... I give up. lol - but I'd say 3% of 55mpg isn't far off the 2 or 3 mpg mentioned.

To further compound the difficult equation, factor in approx 10% additional rolling resistance for the tyres being 225 wide instead of 205..... presumably engine must work harder to maintain 60mph with wider tyres, than it would with slimmer ones.

Diameter of a 17" wheel with 205/50 tyres (un-worn) is 637mm.

With 225/45 tyres, diameter is 634mm or 0.47%. So not a lot.

Also, Pirelli P Zeros have a softer compound and higher rolling resistance than most other tyres of the same dimensions.

.

Also' date=' Pirelli P Zeros have a softer compound and higher rolling resistance than most other tyres of the same dimensions.[/quote']

This will be the reason...Autocar mentioned roughly a 10% decrease in economy when testing these tyres.

From experience though....a little reduction in mpg is well worth the extra grip and feel.

10% reduction sounds about right on mie too

I have just changed from standard 205/50/R17 tyres to 225/45/R17 Pirelli P Zero Rossos. Although they seem fine my average fuel economy has dropped from around 35mpg to less than 30. I realise that the computer will now be slightly inaccurate as a result of the slightly smaller tyre circumference but surely it can't make that much difference. Has anyone else experienced a similar drop when changing tyres?

Paramol

if you are getting more than a 10% drop in economy try up-ing the tyre pressure a bit. the rolling radius will be almost the same at those sizes but the frontal area is increased so you will get more resistance to the air flow. the contact patch may not actually be any bigger as that is more to do with the weight of the vehicle vs the tyre pressure than the width of the tyre.

what if...........................no, never mind.