General Larger wheels decrease fuel economy:Discuss

rallycinq · May 28, 2008

Borrowed the dealer's 1.3 Lounge demo, with 16 inch wheels. Its done 4200 miles. When I got the car the average mileage was showing 50 mpg. I thought this a bit low, so I reset the trip and did a 40 mile run of mixed roads and got it up to 58.7 mpg.

Back at the dealers tonight to collect my 1.3 Sport on 15 inch wheels, done 3400 miles. Reset the trip and did exactly the same route at the same time of day in the same traffic conditions. 64.4mpg.

Thats more than 10% of a difference. There was a bit of motorway, so the superior aerodynamics of the Sport might explain a bit, but not 10%.

Any other explanations, other than the different wheels?

Cheers

Gormless Cretin

Ulpian · May 28, 2008

Wouldn't the tyre diameter be the same? I mean, smaller wheels, wider tyres - rolling diameter identical.

Or am I wrong?

Could be wind strength, different road conditions, slightly different engine characteristics....all sorts?

Nuova500 · May 28, 2008

The overall diameter of the wheel/tyre combinations are virually identical on 15s and 16s. Different cars can do different economys, it happens that way. LOL, how is the sport aerodynamically superior? That little rear spolier will almost certainly create drag, no decrease it

rallycinq · May 28, 2008

nuovapanda said:
The overall diameter of the wheel/tyre combinations are virually identical on 15s and 16s. Different cars can do different economys, it happens that way. LOL, how is the sport aerodynamically superior? That little rear spolier will almost certainly create drag, no decrease it

Check the facts, the Cd is lower on the Sport than the lounge. Are the 16s wider? Are they a softer compound?

Cheers

Gormless Cretin

Nuova500 · May 28, 2008

The tyres on the 16s are wider, maybe it being a demo the tyres were underinflated? That can knock a good 5% off.

rallycinq · May 28, 2008

Certainly there was no washer fluid in it and the oil was a bit low, but I topped them all up for my nce dealer. I also reset the clock to summer time.

Cheers

Gormless Cretin

John R Smith · May 29, 2008

The other thing to check would have been the make of the tyres. Some of the cars are fitted with Michelin Energy, which are reputed to have a lower rolling resistance, hence more mpg. Aerodynamics will really only have much of an effect at speeds over 60 - 70 mph.

John

Mark Elvin · May 29, 2008

I'm sure the 16" wheels must be heavier, & the centre of mass is certaim;y further out, so the enrgy required to accelerate the wheels is greater (think of it as a big flywheel)

Dawg Dee-Lux · May 29, 2008

rallycinq said:
Check the facts, the Cd is lower on the Sport than the lounge. Are the 16s wider? Are they a softer compound?

Cheers

Gormless Cretin

You are making the case for sportssuspension on my 1.3 Pop

white 500 · May 29, 2008

Mark Elvin said:
I'm sure the 16" wheels must be heavier, & the centre of mass is certaim;y further out, so the enrgy required to accelerate the wheels is greater (think of it as a big flywheel)

unsprung wieght is the term

certainly 16" tyres are heavyer than 15's also the wheels have to be ( theres more metal ) so could be down to weight

Mark Elvin · May 29, 2008

white 500 said:
unsprung wieght is the term

Nope, that relates to the mass of componants that are not supported by the weight of the road springs. e.g. Disc' caliper, hub, wheel bearing etc, something we aim to keep as low as possible on the F1 car.

The unsprung weight will have little noticable effect on fuel economy.

The term is actually rotatinal inertia.

gumball · May 29, 2008

I would have thought rolling resistance would be the largest factor, either tread design, tread width and tyre pressure.

white 500 · May 29, 2008

Mark Elvin said:
Nope, that relates to the mass of componants that are not supported by the weight of the road springs. e.g. Disc' caliper, hub, wheel bearing etc, something we aim to keep as low as possible on the F1 car.

The unsprung weight will have little noticable effect on fuel economy.

The term is actually rotatinal inertia.

the wheel is bolted directly to the disc ,hub,wheel bearing etc so the wheel like all these other components is unsprung and you can't say otherwise because it contradicts the disc ,hub etc being unsprung.wheels don't have rotational inertia as they do not keep a steady speed once an outside force has been removed

Venters · May 29, 2008

white 500 said:
wheels don't have rotational inertia as they do not keep a steady speed once an outside force has been removed

Yes they do as they would continue at a constant speed if it weren't for outside forces acting against the wheel.

I thikn.

white 500 · May 29, 2008

back on topic

could it be just the difference between two cars ?

rallycinq · May 29, 2008

white 500 said:
back on topic
could it be just the difference between two cars ?

Could be, but why more than 10 percent on what are supposed to be the same mechanicals.

Cheers

Gormless Cretin

JimMagill · May 30, 2008

Not that we're interested in the i10 but they do make an interesting point about wheels and co2/emissions

http://www.autoexpress.co.uk/carreviews/firstdrives/222474/hyundai_i10.html

Jim

306maxi · May 31, 2008

white 500 said:
the wheel is bolted directly to the disc ,hub,wheel bearing etc so the wheel like all these other components is unsprung and you can't say otherwise because it contradicts the disc ,hub etc being unsprung.wheels don't have rotational inertia as they do not keep a steady speed once an outside force has been removed

What Mark is describing is NOT unsprung weight.

Think of it like this. You've got a bike wheel in your hand and you spin it while you're standing still. You've got to overcome the interia right? Then you start running with the wheel. You now weight extra so you also need to overcome even more inertia. So you have to accellerate the wheel up to speed twice in effect.

Unsprung weight isn't just the hub, wheel and brakes either. Anything that's bolted to your car is considered as being half unsprung also. So a suspension strut is considered half unspring but a disc is totally unsprung.

The Sultan · Jun 1, 2008

306maxi said:
What Mark is describing is NOT unsprung weight.

Think of it like this. You've got a bike wheel in your hand and you spin it while you're standing still. You've got to overcome the interia right? Then you start running with the wheel. You now weight extra so you also need to overcome even more inertia. So you have to accellerate the wheel up to speed twice in effect.

Unsprung weight isn't just the hub, wheel and brakes either. Anything that's bolted to your car is considered as being half unsprung also. So a suspension strut is considered half unspring but a disc is totally unsprung.

forget all that

1 two different cars
2 different fuel from different fuel stations
3 loan car or demo what ever it was has more than likely had it's head kicked in buy prospective buyers
4 the particulate filter may have been blocked on the demo short journeys no time to regenorate rite
5 it may have just had it's head gasket replaced you know the recall
6 the ecu may have a different map on it it's when cars are new that fiat are constantly updating it to improve things and iron out faults
7 tyre pressures may have been different
8 tyres may have been different
9 brakes might have been seised sitting around at the dealer ship and having it's wheels cleaned with that horrible acid stuff
10 air temperature at the time
11 humidity at the time

there are just so many variables it's easy to account for the 10 percent fuel difference

Ulpian · Jun 1, 2008

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