Larger Wheels More Consumption!!

[hindesbeans]

Hi this topic was touched on in a previous thread that i started, but i have a question for Skymastor who posted this thread. (or anyone else who has the answer)

"When I upgraded to from 16'' to 18'' I noticed that i went from 330 miles to a full tank to (at worst) 280 miles! I wasn't too chuffed..

After a little tinkering with the air pressures and being a little more conservative with the accelerator, I managed to get it back to 330 miles, but it was hard graft!!.. Strange!!"



My question is what did you do to tyre pressures to gain some of your mpg back? increase/reduce and by how much, and is whatever you have done wearing your tyres quicker.

I have just put 18's on and already notice on a motorway run of 25 miles to peterbourough i am down to 27mpg from 34mpg, Thats minus 20%. (thirsty Kompressor, knew it wasnt good mpg when i bought the car) This is driving with a very light foot with cruise control set to 70mph.

Now i dont mind having a nice looking thirsty car, but when the trouble and strife notices i need some answers.

Thanks all

Danny

 

[kjay]

Increasing tyre pressure will help a little. But I wouldn't go much above the manufacturers max guide. If you do, you can end up wearing the center of the tyres out prematurely.

Car looks great, so a small price to pay! But then it's not my money.

.

 

[amwebby]

Did you recalibrate your speedometer? I can't see how bigger wheels would increase your fuel consumption, if anything it should decrease it as you're using less revolutions of the wheels to cover the same distance.

However, if your speedometer is still reading as if it had 16" wheels on, it will be reading 60 when you're actually doing 64.9 mph, this will affect the consumption calculation negatively as well as it will only show you've covered 60 miles when you've actually covered 64.9

This calculator will show the difference: http://www.miata.net/garage/tirecalc.html

 

[230K]

Hi

My car went from 235/45/17 to a staggered set up of 265/35/18 and 235/40/18. It uses more fuel thats it. Good tyre calculator site. Thanks.


I started a thread at one stage to try and find out what the recommended pressures are for the really low profile tyres as fitted standard. I wanted various pics of different fuel filer caps and a description of what standard tyres they were fitted with but not many contributed. Cant find it now though!

See here http://www.mbclub.co.uk/forums/showthread.php?t=23385&highlight=tyre

Autodata reckon 39psi front and 50 psi rear for 18" staggered set up. MAD!
230K

 

[Dieselman]

Did you recalibrate your speedometer? I can't see how bigger wheels would increase your fuel consumption, if anything it should decrease it as you're using less revolutions of the wheels to cover the same distance.

The wheel/tyre combo is the same diameter so no calibration is required.
The reason bigger wheels affect mpg is two-fold, one they offer less flex than higher profile tyres so use more energy to flex and two they are usually wider so offer higher wind resistance and greater rolling friction.

Just check the new car Co2 tables for clarification.

 

[grober]

Assuming that the rolling radius of your new wheel and tyre set up is the same ( It is unlikely they will be exactly the same which will effect your gearing and speedo readings slightly-but lets assume that it is for now ) the other 2 factors are aerodynamic drag and rolling resistance (drag) associated with the tire. The first is easy since a wider tyre is going to present a wider cross sectional area to the air flowing past. The second is more complex but if you assume that the tyre deforms slightly where it touches the road to yield the important "tyre contact area" ---wider wheel= greater sideways grip = greater tyre contact area--- and this deformation requires energy to happen--- derived from wheel rotation-- then a wider low profile tyre "soaks "up more energy "deforming" than a narrow higher aspect tyre causing greater drag or rolling resistance. This deformation can be reduced by increasing tyre pressure which in turn decreases rolling resistance due to a smaller contact area at the expense of grip. If you look at the emission figures for various Mercedes models you will notice the gms/Km CO2 figures are slightly higher for the wider wheeled versions of the same models. In a similar fashion "eco" models from various manufacturers usually come with slightly narrower wheel/tyre setup as standard for that very reason.

edit I see Dieselman has just said the same thing in a lot fewer words.

 

[rf065]

I've noticed the biggest difference in mpg from wider tyres are when the roads are wet, as the tyres are having to move more water from underneath which equals even greater resistance than when dry.

Russ

 

[Dieselman]

One thing neither of us have mentioned grober, is that due to lack of sidewall flex the suspension has to soak up more vertical movement when the wheel passes over bumps. That also takes energy.

 

[hindesbeans]

Thanks all, i will try increasing tyre pressures slightly and see what happens,

I have also just added some redex to my petrol to give the fuel system a good clean, i have read on forums that some people have found doing this has had a surprising increase in fuel efficiency, but obviously as many people disagreed.

Danny

 

[jaymanek]

i cant help but think all these little adjustements like tyre pressures can hardly make a material effect?

I mean just taking off slightly quicker at the lights than normal will affect your MPG to a point that would outweigh the effects of tyre pressures and slightly wider tyres.

No?

 

[Bobby Dazzler]

Larger alloy wheels usually weigh more than smaller alloy wheels, especially at the cheaper end of the market. It's not strictly true with very expensive wheels.

This extra weight adds to the unsprung mass, which can have an effect on the handling, and also requires more energy to over come inertia and get the wheel rotating.

In isolation it would probably have a very small effect, but coupled with other impacts mentioned above start to become more significant.

Whether it's enough to become noticeable without averaging over huge distances I'm not sure.

 

[Uberwagon]

IMO wide tyres are for looks alone on 90% of cars which have them.

With regards to tyre pressures, just use the reccomended factory settings, that small increase in mileage you gain from higher pressures will be ruined once you wear a set of tyres out quicker.

Also, my friends M5 with 275 rear tyres on 18" alloys and what I believe is a 40 profile tyre, runs the same pressure as I do with a 16" tyres, 55 profile and 225 width. I've read on 500E.com about people fitting 18" tyres and using 45psi in the rears and to me this seems utterly mad.

Dave!

 

[c240yaz]

I don't understand how you guys are getting all these reductions in MPG.

I changed from 16" to 18" 6 weeks ago, do the same jouney every week and nothing in my fuel consumption has changed.

Guage and trip computor average has not changed at all. 600 miles per week.

 

[pammy]

I went from 16" to 17" about 3 months ago now and also have not noticed any difference

 

[crockers]

I went from 16" to 17" about 3 months ago now and also have not noticed any difference

So what you're saying is size doesn't matter ...

 

[pammy]

So what you're saying is size doesn't matter ...

In certain aspects of life maybe - but often one needs to look at what one is doing with what one has to ensure you're getting the the desired outcomes

 

[niva441]

I don't understand how you guys are getting all these reductions in MPG.

I changed from 16" to 18" 6 weeks ago, do the same jouney every week and nothing in my fuel consumption has changed.

Guage and trip computor average has not changed at all. 600 miles per week.

Based on the official fuel consumption and emissions data the tyre size does have some effect on most (if not all) vehicles.

 

[Alfie]

One of the reasons that high speed cyclists use very narrow tyres is beacuse of the drag they cause. This can easily be felt by simply changing your mountain bike knobblies for road tyres and see the difference in energy requird to travel at pseed. Its the same for cars. Its basic physics and applies to all vehcles where there are no areodynamics to deflect the air flow around the wheel.

Wider tyres = higher fuel consumption.

 

[c240yaz]

Wider tyres = higher fuel consumption. = Minimum impact and not worth worrying about.

I would be more concerned driving with the widow open causing drag, 3 passengers or unnecessarry heavy items in the boot.

 

[wheels-inmotion]

Personally i would change the Toe position. The suggested positions no longer apply so with a wider tyre there is added lateral drag.

For example a 15" wheel miss-aligned by 2mm drags the tyre 14ft sideways per mile.