Do Large Alloys = Lower Mpg?

[Gordon in Angus]

HI all, just curious but looking round the forum there is a common consensus backed up by the info in the Toyota brochures that a larger alloy wheel with a lower profile tyre would result in reduced MPG. Just wondering if anyone can provide a straightforward explanation as to why. Is it a weight thing or a tyre flex thing or something else?

Sorry of its been gone over before but I'm curious as to why... It never seemed to be as prevalent in non hybrid discussions but I suppose hybrid owners are more mpg conscious than normal car owners

thanks

Gordon

[MEP's Yaris GS]

I believe its to do with the ride height / aero dynamics of the car which changes and affects the MPG, but I could be wrong.

[Sooty]

Try this link for some info:

www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested.

It mentions that it has something to do with weight amongst other things.

[Gordon in Angus]

Thanks Sooty, that was usefull info there.

Not really sure as to Toyota's logic selling the Yaris Hybrid in Excel trim with the bigger wheels if it drops the MPG. Might sell the car to people more style conscious but I imagine the comfort and noise drops a little with the bigger wheels too. I'd personally prefer an MPG increase over the marginally better handling of the 16" wheel over the 15" but the Excel trim came with the 16" wheels as standard. Never mind, I suppose if I get round to buying a set of winter tyres and wheels it would be better getting the 15" wheels so you claw back a wee bit of the MPG lost when using the winter tyres then.

Cheers.

Gordon

[FROSTYBALLS]

As regards changing wheel size for winter tyres, check with your insurer first, as some would view the change in wheel size to be a change to specification - see post 27 of the following: http://www.toyotaownersclub.com/forums/topic/145281-toyota-motor-insurance/page-3

[Gordon in Angus]

Fair point Frosty, probably something I'd have done anyway but worth reminding us all of it..

[Cyker]

It's pretty much all down to the weight - Bigger rims are heavier, and the heaviest bit (The actual rim surface) is further away and bigger the larger the rims get.

It wouldn't be so bad if the weight was concentrated at the hub, but as it's at the rim, there's that much more more momentum to overcome when accelerating and decelerating. Wheel rims act like giant flywheels in a way, and it's well known the performance and responsiveness benefits of having a lighter flywheel!

AFAIK the only real benefit of bigger rims, aside from the looks, is you can fit bigger brake discs, but this is a bit pointless on HSDs since they have excellent regen braking and their discs are tiny!

[kithmo]

As well as the extra weight, the wider profile also has an effect on drag and tyre rolling resistance.

[Devon Aygo]

Thanks Sooty, that was usefull info there.

Not really sure as to Toyota's logic selling the Yaris Hybrid in Excel trim with the bigger wheels if it drops the MPG. Might sell the car to people more style conscious but I imagine the comfort and noise drops a little with the bigger wheels too. I'd personally prefer an MPG increase over the marginally better handling of the 16" wheel over the 15" but the Excel trim came with the 16" wheels as standard. Never mind, I suppose if I get round to buying a set of winter tyres and wheels it would be better getting the 15" wheels so you claw back a wee bit of the MPG lost when using the winter tyres then.

Cheers.

Gordon

Yaris Excel Hyrbrid can be factory ordered with 15" alloys and then officially benefits from the lower CO2 & and higher fuel economy, it also allows the 15" Excel to qualify for London congestion charge exemption denied to 16" models

[ToyotaAuris]

Hi I did a project on this. Basically it is because of the surface area of the wheel. As the surface of the alloy wheel is decreased due to its size, it will make the engine put more power into turning the wheel. Depending on the type of alloy it will allow more air to flow through it, increasing the resistance, causing engine to put in more power. It doesn't seem as if it would use alot of power but if 4 wheels are all using extra power then it does make a difference.

[PeteB]

As well as the differences on CO2 and mpg quoted for the T3 and the T4/Spirit Prius, the drag coefficient goes up from 0.25 to 0.27.

Whilst it's possible extra equipment may weigh slightly more, the only external difference is the wheel/tyres, which I think are wider with the 17" ones, hence increased frontal area.

On the Gen 3 Prius, heaven knows how many millions were spent getting the CD (drag factor) down from 0.26 on the Gen 2 to 0.25 on the T3 (15" wheels). Toyota claimed at the launch that the Gen 3 had the highest amount of wind tunnel testing of any Toyota in history.

Then they fitted 17" wheels to the T4 and T-Spirit, which lifted the CD back to 0.27 - WORSE than the Gen 2 !!! - with the consequent hit on mpg, CO2, comfort and spend on tyres for owners!

[unclepoo]

Considering the frontal area of any car, or a slippery prius, the wheel size or tyre width will make very little difference to mpg from wind resistance...but the rolling resistance of the wider tyres on t-spirit models certainly will. Try doing back to back mpg tests with all 4 tyres at 40psi, then at 30psi.....and you`ll see a difference.....and all thats changed is rolling resistance.

[kithmo]

Was there some reason Toyota couldn't fit 17" wheels with a narrower profile ?

BMW did this with the i3.

[MEP's Yaris GS]

Was there some reason Toyota couldn't fit 17" wheels with a narrower profile ?

BMW did this with the i3.

Possibly a size / weight thing? The i3 is a bit smaller from memory.

[PeteB]

Was there some reason Toyota couldn't fit 17" wheels with a narrower profile ?

BMW did this with the i3.

Personally when I had a test drive on the i3, I was horrified when I saw 19" wheels on a car the size of a Yaris - the salesman seemed to think they were wonderful, but I thought it looked like a mock-up someone had made for a joke. To my mind they also seemed rather wide for such a small car.

This is the same size that Lexus use on their 2½ ton LS limo, and the RX 4x4!

At least they didn't spoil the ride, which was a surprise, and the turning circle was still excellent.

I couldn't help thinking that 14" wheels (like the original Prius) and smaller arches would have helped give a bit more range per charge, and give some chance of at least an optional space saver spare wheel in the front storage compartment (I used to refuse to buy a car that didn't have a full sized spare wheel, now I have to be happy that they even offer a space saver!).

Perhaps BMW think the average EV buyer is still a petrol head impressed by big wheels, and doesn't mind running out of electricity a bit sooner.

But it meant carrying a spare wheel was virtually ruled out, unless you don't mind losing just over half the boot! Not to mention the £1100 or so for an extra wheel & tyre.

[kithmo]

Wow, I didn't realise they were as big as 19" on the i3, but I was thinking, on the Prius, Toyota could have maybe had the same 17" diameter wheels, but a narrower profile.

It's just my opinion and as they say, "beauty is in the eye of the beholder", but I think the 15" wheel just look a little to small and make the car look a bit slab sided. Maybe a compromise would be 16" as on the old Gen2.

(Maybe white wall tyres would make it look better ?)

[PeteB]

I sometime park next to a T-Spirit at the beach car park, and the overall size at the edge of the tyre looks identical to me, just mine has a wider black bit and narrower silver bit.

Personally, I don't give a hoot what it looks like, I just want the best balance between safety, longevity, replacements costs, comfort and noise levels.

[kithmo]

The overall size (rolling radius) is the same.