I recently put some Goodyear Eagle F1 Asymmetrics on the vRS (225s) and straight away have noticed a difference in mpg. I was expecting a bit, but not 5mpg average. I had Conti Sport Contacts (205s) on before and yes, I now they were a diffrerent compound and size, but blimey.:eek:

On the plus side, the cornering capabilities of the car are now much improved, as is my ability to get the power down when setting off. Previous places on the commute to work that would have had the fronts spinning away are now much better. The wet grip is also amazing in comparison.

On the plus side, the cornering capabilities of the car are now much improved, as is my ability to get the power down when setting off. Previous places on the commute to work that would have had the fronts spinning away are now much better. The wet grip is also amazing in comparison.

You sure your not just driving it harder now?

i remember from my physics lessons at school, that less tyre on the road lead to less resistance, meant that if cars had razor blade thin tyres (in width) they we get better mpg to an identical vehicle running on wide tyres. obviously now we can argue the grip etc. but even the enviromentalist claim you get better mpg on smaller width tyres to wider width.

Yeah but 5mpg is what, somewhere between 1/6 and 1/7 of the total? That's more than you typically use overcoming all mechanical resistance at legal speeds!

This is the reason i went with the 205 profile.

Don't forget that by changing the profile of the tyre, you also change the overall rolling diameter presuming you have kept to the same aspect ratio.

You speedo will be reading differently with 205/55's on than it will with 225/55's on for example. This will obviously affect the MPG if you use the odometer reading

Oh, hello, first post and all that

That won't explain it either; that I know is about a 1% difference, and the same again but the other way for the new treads.

That won't explain it either; that I know is about a 1% difference, and the same again but the other way for the new treads.

I know it won't cover 5% but add it all up and it is more plausible.

225 instead of 205 gives a larger rolling radius, as you say, new treads give a bit more of an increase to rolling radius, wider tyres increase rolling resistance, new compound probably does the same, and top it off with a change in driving style 'ability to get the power down' then it all seems a bit more reasonable.

I know it won't cover 5% but add it all up and it is more plausible.

225 instead of 205 gives a larger rolling radius, as you say, new treads give a bit more of an increase to rolling radius, wider tyres increase rolling resistance, new compound probably does the same, and top it off with a change in driving style 'ability to get the power down' then it all seems a bit more reasonable.

I can't remember the actual aspect ratios used, but doing this change you go from something like 205/55 to 225/45, which reduces the rolling radius maybe 1%.

I can't remember the actual aspect ratios used, but doing this change you go from something like 205/55 to 225/45, which reduces the rolling radius maybe 1%.

Hence in my first post I said 'presuming you kept the same aspect ratio'

It is probably more to do with poking it about because there is more mechanical grip but just trying to help.

205/55/17 gives a rolling circumference of 2005mm

225/45/17 gives 1935mm, which equates to roughly 2.2 MPH slower at 60MPH which should have the effect of increasing percieved economy

If they are 225/55/17 then they will have a rolling circumference of 2072mm which is 2 MPH at 60 faster.

I've a notion it's actually 45 section down to 40, which reduces the rolling radius by 2mm.

I've a notion it's actually 45 section down to 40, which reduces the rolling radius by 2mm.

Which is less than 1/2 MPH and the wrong way at that.

Hmm, no idea then

its 205 50 17 to 225 45 17

FFS.

why do people pontificate about stuff if they dont know the facts. Rolling resistance has more to do with compound than width.. a low resistance 225 will have better mpg than a sticky 205..... the width is more aero related @ higher speeds.

Should the comparison not be 205/50/17 and 225/45/17? Where did 205/55/17 appear from? :confused:

its 205 50 17 to 225 45 17

FFS.

What is the difference in delta radius between 205/50 to 225/45 and 205/45 to 225/40? Precisely nothing, like what you're getting upset about! The delta R, which changes your overall gearing, is still 2mm!

why do people pontificate about stuff if they dont know the facts.

Rolling resistance has more to do with compound than width.. a low resistance 225 will have better mpg than a sticky 205..... the width is more aero related @ higher speeds.

I have a Mr Kettle for you on line 2! Rolling resistance, as distinct from slip resistance, is primarily a function of tyre construction, particularly sidewall stiffness, not compounding!

Tyres eh........ who'd have 'em

Rolling resistance - Wikipedia, the free encyclopedia

Not quite sure who your 'pontificating' comment was aimed at redbaron but your wikipedia link tends to agree with KenONiell's comment about sidewall stiffness being a greater factor than compound makeup.

If you read what I wrote you will see I was simply having a discussion using figures and examples rather than trying to lecture anybody. Especially as we were discussing rolling radius and you have a bee in your bonnet about rolling resistance, which is different.

E=mc2

Should the comparison not be 205/50/17 and 225/45/17? Where did 205/55/17 appear from? :confused:

Yep, it is 225 45 17". Overall I'm more than happy with the new tyre, but an increase in mpg is just one downside.

I figured that enough people had made the leap to the new size for it not to be too much of an issue speedometer wise. I also didn't mean to cause people to argue..