Grip v Economy v Bling

[CortinaGT]

The latest Which? On-Line has some amazing 'real world' information on how wheel sizes affect economy. Toyota and Lexus are the first company to give true information. The following is quoted:

 

“However, official figures could be even further from the truth – depending on the wheel-size of the car you buy. Opt for big alloys (as many style-conscious buyers do) and your fuel economy could drop significantly.

 

Different wheels, different mpg

At the moment, Toyota and its sister-brand Lexus are the only companies to quote separate economy and CO2 emissions figures depending on wheel size.

For example, buy a Lexus IS 300h in entry-level SE spec and it comes with 16-inch wheels. Quoted economy is 65.7mpg with CO2 emissions of 99g/km (grams of carbon dioxide per kilometre driven). However, stretch your budget to the Luxury model and you’ll also need to stretch your fuel allowance; its 17-inch alloys reduce efficiency to 64.2mpg (103g/km CO2).

At the top of the Lexus IS range is the F Sport version. This has exactly the same engine and electric motor, but with 18-inch rims its claimed economy is 60.1mpg (109g/km CO2). That’s an increase of nearly 10% over the SE model – almost entirely because of those larger wheels and tyres.

To put that in perspective, a driver covering 12,000 miles a year would spend £107 more on fuel for the F Sport than the IS 300h SE. Plus, annual car tax (VED) costs £20 – rather than being free.

 

 

Same car, different wheels

Now let’s look at one of the IS’s rivals. The BMW 3 series is the UK’s bestselling large car, yet its maker quotes identical fuel economy and CO2 emissions for all models, whether with 17, 18 or 19-inch alloys.

We believe Toyota and Lexus deserve credit for being upfront about fuel economy figures. It’s time other car companies followed suit.”

 

I found this very interesting! I am also aware that another manufacturer adds a further small spoiler on its top of the range model purely for stying - it does look good, but in testing it actually reduces mpg and does nothing for the handling either! :think:

[bluecar1]

the actual rubber has an effect as well

 

many "performance" tyres are very grippy but poor rolling resistance rating which means lower fuel economy

 

difference between "A" and "G" rating is apparently about 6% fuel economy which could account for the differences in the Toyota figure

[Gambo]

Also bigger wheels with smaller tyre depth ruins ride and comfort......but hey it looks good......

[fabandy]

Exactly - that's why show cars always have huge wheels. But it's a fair point - the tyres and the wheels make a difference. It's also the weight of the wheels ('un-sprung weight').

[bluecar1]

low profile tyres on bigger wheels have 2 effects

 

1:-  lower profile means less sidewall height, this has the effect of reducing comfort of ride as part of the suspension travel is in the flexibility of the tyres sidewall

 

2:- road holding, due to higher sidewall when you corner hard the sidewall flexes and the tread tries to stay put but the lateral forces cause the car to push the tread under and try to use the sidewall leading to lower grip, on low profile due to thinner stiffer sidewall it resists the lateral forces more which means more of the main tread is on the road and more grip

[Jane_i10]

Well its logical isnt it? Surly no one ever thought that bigger wheels and tyres increased MPG? The more contact with the road the more force required to push it along.

 

Tbh most wheel and tyre sizes are far too big nowadays, theres no way in hell my vRS needed the 18's but to keep up with modern look and the appearance of the car they opted for them, it handled adequately fine on its 16 steel wheels. Only when grip is number 1 target do big feet make more sense.

Like mentioned the tyre and tred make the difference i think, im sure at some point you will of come across an off roader with the big chunky tyres that make a hell of a racket and with it not being a smooth contact they have a much more rolling resistant 

[Yeti_Man]

Well its logical isnt it? Surly no one ever thought that bigger wheels and tyres increased MPG? The more contact with the road the more force required to push it along.

Tbh most wheel and tyre sizes are far too big nowadays, theres no way in hell my vRS needed the 18's but to keep up with modern look and the appearance of the car they opted for them, it handled adequately fine on its 16 steel wheels. Only when grip is number 1 target do big feet make more sense.

Like mentioned the tyre and tred make the difference i think, im sure at some point you will of come across an off roader with the big chunky tyres that make a hell of a racket and with it not being a smooth contact they have a much more rolling resistant

smart a$$. I won't bother posting my 'isn't it obvious though?' post now as someone has beat me to it! In Germany they don't buy into this big wheel, low profile tyre thing but the German cars on sale here are usually worse off for their sports suspension and low profile sports tyres and big alloys. :( I'm also totally unconvinced that the car will be any quicker on real world roads than something that's got a little more suspension and sidewall 'give'.

[CortinaGT]

The point I was trying to bring to make is that no other manufacturer is bringing to prospective purchasers attention the fact that their new car choice might look good and probably handle better with the larger wheels, but that it will do less to the gallon!

Note Which? said BMW for example have not mentioned it! I had an Octy L&K on 17", next was an Elegance where I deliberately went to 16" and now proudly own a Citigo on 14".

[Defenderben]

But TBH does it matter as you will probably never achieve the ultimate manufactures "official" fuel figure!

Not unless you empty the car of everything including running with just enough fuel for your journey and having your tyres over inflated.

All manufactures fuel figures are a fair way off the mark of real life and should be taken as an indication and not necessarily an achievable figure.

If a wheel/tyre size combo is going to make a few mpg difference is it going to be that much of a deal breaker on your final choice of a certain model....?

Not for me.

Do Toyota/Lexus give the tyre manufactures name on each model tested with these fuel figures?

We all know the same size tyre in various brands will also give varying mpg's and then there is tyre pressures/temperatures.....where do you stop? It all starts getting too silly.

Edited 15 September, 2013 by Defenderben

[CortinaGT]

I originally posted this in the Citigo forum where there has been some discussion regarding whether to buy a Sport or Elegance version. We know the suspensions are identical except for the Sport having larger and wider 15" wheels and non Eco tyres.

My intention was simply to introduce this as another thing to be considered in the choice when buying a car where low running costs are an important issue.

If you are buying a larger vehicle you have already introduced the higher running cost into the equation! ;-)

Incidentally when I used to rally my mk1 Cortina GT back in the 60's it was upgraded to 5 1/2 x 13"J wheels (from the standard 4 1/2) and running on 175 tyres! Oh, how things have changed!

(The moderators must have moved the topic!)

[silver1011]

But TBH does it matter as you will probably never achieve the ultimate manufactures "official" fuel figure!

Not unless you empty the car of everything including running with just enough fuel for your journey and having your tyres over inflated.

 

No, but the effects of larger wheels and tyres will still have an adverse effect on your actual MPG, whether that be close to the manufacturers claims or not.

[Dr Zoidberg]

Honda show worse economy for the top spec CRV with bigger wheels, so they aren't the only ones.

[Aspman]

the actual rubber has an effect as well

 

many "performance" tyres are very grippy but poor rolling resistance rating which means lower fuel economy

 

difference between "A" and "G" rating is apparently about 6% fuel economy which could account for the differences in the Toyota figure

 

 

I've just switched from G to A rates tyres and my economy has gone up  ~9% on my daily commute.

Edited 16 September, 2013 by Aspman

[ColinD]

Learnt a couple of things the other week. The 'eco' tyres are not all equal, the better ones apparently; compound aside, will have a stiffer sidewall, the aim is to reduce the amount of pushing the car has to do to initiate and maintain rolling. A softer sidewall will 'squish' more. Thus an eco tyre might not be compatible with comfort, so you should remember the tyres are a component of suspension. Coupled with reducing unsprung weight, so lighter wheels are

 

The second which left most of us a little gobsmacked. The new 'eco' rating, you know the wet grip, noise, and rolling resistance stickers. Well they are currently self certified by the manufacturer, no independent quality gatekeeper etc... Thus I could go to china, get some tyres imported and put a AA rating on them... i.e. they are almost meaningless. The only benefit, as was loosely put is on the middle/top end brands who can't afford quality and perception to be questioned, thus they are possibly more accurate. But also bear in mind a 165 and 295 in the same tyre might have the same rating; oh rly?... Just to bear in mind when comparing tyres on ratings alone.

 

We also tested in some x drive 3series bm's back to back on two 'premium' brands and there was a noticeable difference in wet grip. The test was 30mph a quick sharp turn, one set required opposite lock the others didn't.

[Aspman]

Strangely I find the ECO tyres (Goodyear) have a squishier feel than myprevious tyres, Maxxis. The Maxxis had quite a lot of 'bite'.

 

I actually bought because of the noise rating of the Goodyears not the 'eco' ness. The mpg was and added bonus.

 

On the wider topic I get the feeling (anecdotally) that larger more powerful engines come across more accurately in the current tests that the 'eco' engines. The more 'eco' they are the further from reality they seem to be.

 

Even before my green tyres I could regularly beat the combined figure (35.7) for the VRS.

[aToOoOoMboi]

You might want to take a look here..: http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

[LadyVienna]

Interesting thread

[Aspman]

You might want to take a look here..: http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested

 

That was interesting. More complex results than I'd have expected.

[clamberer]

I haven't read all of the articles, but I imagine that the wheel size is only part of the equation.  As the comparisons are for different trim levels, there will be a certain weight of extras between the entry level and high trim models - e.g. heated, electronically adjustable leather seats will weigh more than unheated textile ones.

 

Also, how much of the difference from the wheels is a result of differing weight, and how much is from rolling characteristics? How much is to do with wheel width (and therefore area in contact with the road), and how much is from wheel diameter/overall diameter/tyre profile?

[KenONeill]

Assign the mass of the 15s the value 1 at this point. Regarding the effective extra mass, using R&T's figures, the 19s weigh 1.4x what the 15s do.

 

Power absorbed by rotating mass at constant rotating speed and distance fron hub varies as the 4th power of mass, which gives us that the 19s absorb 3.8x more power than the 14s.

 

Working this back on the 40lb per corner for the 15s means that the 19s effectively add 608lb to the mass of the moving car.

[Lady Elanore]

Assign the mass of the 15s the value 1 at this point. Regarding the effective extra mass, using R&T's figures, the 19s weigh 1.4x what the 15s do.

 

Power absorbed by rotating mass at constant rotating speed and distance fron hub varies as the 4th power of mass, which gives us that the 19s absorb 3.8x more power than the 14s.

 

Working this back on the 40lb per corner for the 15s means that the 19s effectively add 608lb to the mass of the moving car.

...............which according to Prof Eric Laithwaite means the car can fly.....or something like that?