Tyres And Mpg

[monya]

Got myself a new Auris Excel last week. Seems ok but not best pleased to find 225/45 /R17 tyres which are low profile? The last one had 225/55. According to the brochure these seem to lower the mpg which seems to be contrary to the concept of a hybrid. What's the justification?

[MEP's Yaris GS]

What's the justification?

Looks, and potential handling differences, but thats about it.

[FROSTYBALLS]

Did you not check the spec before you ordered the car?

[monya]

Yes but I guess tyres didn't figure in my observations. I did notice about a 10 mpg difference between 15 and 17 " wheel size though but didn't see the variation in tyre wall depth. But I can't quite figure the purpose of varying wheel size other than poser cred.

[FROSTYBALLS]

From Toyota's OE tyre size chart, the Auris has never been fitted with 225/55 tyres as Original Equipment. OE tyre sizes have been 195/65/R15, 205/55/R16, 215/45/R17 or 225/45/R17.

[CPN]

From Toyota's OE tyre size chart, the Auris has never been fitted with 225/55 tyres as Original Equipment. OE tyre sizes have been 195/65/R15, 205/55/R16, 215/45/R17 or 225/45/R17.

Right and the difference between the first and second size in terms of rolling circumference is 8.1mm, the difference between the first and third is 28.9mm and finally the first to the fourth size is almost identical at 0.6mm difference. None of which would result in anything close to a 10 mpg difference in fuel consumption on pure mechanics alone. Out of interest, the difference between the (base) 195/65/R15 and 225/55/(R17?) would be an enormous 140.8mm and that would result in the speedo reading a full 4 mph low at 60 mph... (and probably explain a 10 mpg drop in real terms)

[YarisHybrid2016]

The difference in MPG between wheel sizes is *significant*. I noticed this when I was spec'ing my Yaris. In fact, there is an option to have the 15" wheels on the Excel spec, as it makes *that* much of a difference.

The brochures also reference wheel size when discussing fuel economy. The CO2 figure for a basic Yaris with 15" wheels is 78 (or 79?) g/km, whilst the same model with 16" jumps to 80 g/km. Of course, equipment levels change things too due to additional weight/power requirements.

One thing to consider with all this, is due to the low CO2/high MPG figures, it takes relatively little to throw them out, compared to a vehicle with higher CO2/lower MPG. These highly efficient cars are more sensitive to subtle changes, where you probably wouldn't notice it on anything else.

[cuius]

My Mk 1 Auris ran on 15" wheels and over 3 years averaged 54mpg - current Mk 2 with 17" wheels is averaging 52mpg after 7 months and 5.5k mls. Same type of motoring throughout

[monya]

My bad. My old Auris 2010 had 215/45/R17 tyres and the new one has 225/45/R17 - they just look lower to me and so far the fuel consumption is in the low 50s which is less than I had before with the T spirit. Early days I guess.

[CPN]

Ok, I'm going to put my head on the block because I see and read so much rubbish about wheels/tyres and their effect on mpg and would like to "put this to bed" once and for all.

In automotive engineering terms, wheels/tyres come under the heading of "unsprung weight" and unless there is a radical difference in rolling circumference and/or aerodynamic profile (in relation to vehicle shape) then the variation of this weight is the only other thing (apart from sizing) which will affect fuel consumption since the drivetrain has to overcome it in the process of propelling the car forward.

Now, not for nothing do car companies like Toyota give choices when it comes to specs for wheels and tyres but one thing they are very careful to do (for a given car) is to make sure that the choices they recommend are just about equivalent in rolling circumference terms; +/- a few millimetres.

Taking the Yaris' choices as an example, they are currently (per the manual) 175/65R15, 185/60R15 & 195/50R16. Notice that as the tyre width goes up, the profile (2nd figure) goes down and this is to keep the rolling circumference within tolerances. Otherwise, they'd have to change the speedo ratio each time they changed the wheel/tyre size and they don't want to have to do that. Using the 175/65R15 as the base, the rolling circumference difference between it and options 2 and 3 is 0.91% and 1.18% in terms of speedo error (over in both cases) respectively. This results in over reads at 60 mph of 0.55 mph and 0.71 mph in their turn. As you can see, really close. Close enough not to produce a wide variation in fuel consumption between the different installations.

So, we come down now to weight. Usually the lower spec option (175/65R15) is the perceived "tall" option and is usually a steel wheel and in the case of the Yaris, even the second option will probably be a steel wheel also (someone correct me if I'm wrong) because it's still a fairly "tall" tyre profile but the final option of a 50% profile tyre would be considered the "sporty" or good looking option and would probably come shod on an alloy rim. Now some will probably say "ah well, the steel wheel is bound to be heavier than the alloy option so there's the answer" but ironically (and I've actually weighed a selection of steel and alloy wheels to compare) unless you go for really expensive magnesium alloy wheels (which Toyota won't because of cost) the weights comparing the steel rims of 15 & 16 inch and an OEM alloy wheel rim can be very similar and not very far apart in the overall scheme of things.

For example, a 15" set of Enzo alloys I have w/Continental WinterContact TS850 185/60R15 84T as my winter tyre set, weigh 17.7Kilos each whereas the wheels my car came with (195/50R16 - also alloys) weigh 17.5 kilos each. Just 200 grammes per corner difference. A similarly shod (to the 1st set) 15" set of steel wheels I had shod with Avon Ice Touring for my previous Hyundai weighed in at 17.6 kilos. As you can see, pretty close...

The only thing left is aerodynamic drag and the effect of change in wheel/tyre profile on it... If you check out page 428 of the manual, we are talking 25mm from one extreme to the other in tread (i.e. 1" overall). Personally, I can't see that making a huge difference to fuel consumption either. Now, driving style and terrain? Of course...

[monya]

Thanks for that BUT how else, apart from wheels/tyres can one account for the following fuel consumtion figures from the new Auris brochure:

15'' / 16'' / 17'' wheels

80.7 / 78.5 / 72.4 combined fuel consumption.

[Cyker]

Yoy mis-read him - he IS saying that the wheels DO affect the MPG; However it has nothing to do with the size if the overall radius (i.e. wheel + tyre) is the same.

What he's saying is that it's purely down to the weight - A 17" alloy is going to weigh a significant amount more than a 15" alloy, and that is all unsprung rotating mass. I read somewhere that 1kg of unsprung mass is equivalent to 4kg of sprung mass in terms of its effects on MPG, and assuming that's true I figure the effect on mpg between 15" and 17" is near that of carrying a concrete slab or two in the back of the car

Just a small amendment; You don't have to go for Mag alloys for light weight - Aluminium is good enough.

However, almost all OE manufacturers use cast alloys, which are only slightly lighter than equiv steel wheels because the alloy is weaker than steel and more material has to be used.

Forged aluminium wheels, however, tend to be massively lighter; When I bought PaulT's old 14" Toyota alloys off him, he was showing off the Rays Engineering forged ally wheels he'd bought to replace them and they were so light you could lift them with one finger!
However the cost was an order of magnitude more than what I paid him for the Toyota alloys, so you pays yer money as they say ;)

[PeteB]

As well as the differences on CO2 and mpg quoted for the Gen 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. Some early Gen 3 owners with 17" wheels said they didn't fit nicely under the boot floor after they'd had to use the space saver - 15" ones do!

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 (and later the plugin - both 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 replacement tyre cost for owners!

[DarrenM]

When I was specing my car up, it was the Icon+ with 16" alloys and 85g/km CO2 or Excel for more money with 17" alloys, 88g/km, worse ride and worse MPG. As the 16" alloys looked fine it was a no brainer not to go for the smaller wheels.

[CPN]

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

That's one test figure I'd really like to see although it's probably impossible now I guess... I'd like to see the difference in mpg with the Prius when the only factor that changed was that drag coefficient from 0.27 down to 0.25 (without changing the wheel size/shape) by just changing body shape. Or in other words, what, in mpg terms, does a drop of 0.02 directly equate to? Is there such a figure anywhere?

[offashead]

When I was specing my car up, it was the Icon+ with 16" alloys and 85g/km CO2 or Excel for more money with 17" alloys, 88g/km, worse ride and worse MPG. As the 16" alloys looked fine it was a no brainer not to go for the smaller wheels.

Me too. Very important decisions were made when I sourced my Auris, the wheels/tyres being one if not the main one. I also believe driving style/technique plays a major part in fuel consumption, not just wheels and tyres

[Ten Ninety]

Having run 17" OEM wheels and tyres for a year, followed by aftermarket 15" wheels and tyres on the same car doing the same journeys for a further 16 months, I would add the following observations:

Whatever difference there may be in terms of weight and contact patch can be completely offset by the choice of tyre. My 15" Goodyear EfficientGrip Performance tyres (rated B for economy) have proved to be utterly pathetic on every measure compared to the OEM 17" Michelins (C-rated), including economy.

I stopped logging stats on Fuelly shortly after switching wheels (when they decided to do weekly server maintenance at the precise time I fill up each week). However, my own records show an overall marginal decline in fuel economy across a like-for-like 12-month period. I still managed to keep above 70mpg, but only just. That's ~.5mpg worse on 15s than on 17s, which can be explained away by all the other factors which affect economy but which certainly isn't an improvement. Summer performance was marginally better whilst winter performance (despite a milder winter) was notably worse. This is almost certainly related to the Goodyears delivering terrible economy in the wet, with a ~10mpg drop on the Goodyears in wet conditions, compared to a ~5mpg drop on the Michelins.

Any benefit in ride comfort is negated by the need to inflate the Goodyears to well above 40psi to deliver acceptable handling. As you may imagine from those average stats, I am not a rapid driver. But even at my sedate pace, I couldn't cope with driving a blancmange. The 15s were slightly quieter when first fitted but now they have worn, interior road noise is back to the usual (unacceptably high) Prius levels. To add insult to injury, they are wearing fast as well - at a guess at least twice as fast as the Michelins. Consequently the cost saving in replacing 15s compared to 17s is negated. And, of course, my car looks like a shopping trolley with its stupid little wheels as well. Good job I didn't buy it for its looks.

tl;dr Forget wheel size and make sure you select a good tyre instead. It is perfectly possible to look good and deliver superb economy, with the right 17" wheel/tyre combo.

[YarisHybrid2016]

I've filled up twice now, driving approximately 300 miles each time (different types of journey, with the second including 200 miles of motorway at 70 MPH) and the pump-based MPG is 55 and 53 MPG respectively. This is with the 16" stock alloys of the Excel spec.

[CPN]

I did have some bad early tankfuls but those figures match the kind of figures I'm now getting in mine on long runs...

[PeteB]

...Whatever difference there may be in terms of weight and contact patch can be completely offset by the choice of tyre. My 15" Goodyear EfficientGrip Performance tyres (rated B for economy) have proved to be utterly pathetic on every measure compared to the OEM 17" Michelins (C-rated), including economy...

Any benefit in ride comfort is negated by the need to inflate the Goodyears to well above 40psi to deliver acceptable handling...

tl;dr Forget wheel size and make sure you select a good tyre instead. It is perfectly possible to look good and deliver superb economy, with the right 17" wheel/tyre combo.

Each to their own - you're quite right that apart from wheel sizes, lots of other things can have an effect. However, all other things being equal, Toyota's own data * has said the (UK) Prius models with 17" rims have a drag factor (CD) of 0.27, Vs 0.25 for the 15" ones.

That's a massive difference, and whilst I struggle to see how wider tyre profiles can account for all of it, I can't see any other external differences between the model to explain it.

My T3 (15" rims) ran on original Bridgestone B220s when I got it and is now on Dunlop Sport BluResponse (Eco B Wet A 68 dB) in the summer, Nokian WR D3 (Eco C Wet C 72 dB) in the winter.

The Dunlops remain quieter than the Bridgestones (66-68 dB at 60 mph in the front of the cabin according to my meter, except on very rough surfaces) which I find pretty good. Personally, I'm more than happy with the handing at the recommended pressures of 36/35 psi F/R, but we all have different tastes.

Being old enough to remember when no (road) cars had low profile tyres, the 'old fashioned' ones look better to me, but I would have no hesitation is switching if there was some benefit that appealed to me. But as I say, we're all different.

* when I looked on the latest version of Toyota's web site yesterday it said all models of Prius had a CD of 0.25, but I doubt very much they've made changes ot the car to make that correct.

[CPN]

That's a massive difference, and whilst I struggle to see how wider tyre profiles can account for all of it, I can't see any other external differences between the model to explain it.

I'm struggling to understand that massive change from just a change in wheel width/size also... Would love to see the actual test figures because frankly, until I do, I won't believe it! (...that a delta of +0.02 is all down to the wheels)

The only way I could see it making that much difference is if the underflow of the car is so close to the edge that raising it a few millimetres is causing a massive increase in downforce?

[valmiki]

perhaps it's Toyota's own 'defeat' technology to rig the mpg/emissions tests? here's looking at you VW and your superior diesel technology

[PeteB]

That's a massive difference, and whilst I struggle to see how wider tyre profiles can account for all of it, I can't see any other external differences between the model to explain it.

...The only way I could see it making that much difference is if the underflow of the car is so close to the edge that raising it a few millimetres is causing a massive increase in downforce?

Apparently the circumference (and therefore ride hight) are almost identical, just more alloy and less rubber between the hub and the road. It only seems to be the width of the wheel/tyres that's greater on the 17" rims.

I have heard of owners with 17" wheels saying they won't fit under the boot floor when the space saver has been put on the car, while a 15" will.