(G) Larger wheels decrease fuel economy:Discuss

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

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?

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

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

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

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

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)

You are making the case for sportssuspension on my 1.3 Pop

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

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.

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

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

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

I thikn.

back on topic
could it be just the difference between two cars ?