Possibly dumb question about wheel sizes!

devorguila

Evening all, apologies if this has been asked before,but here goes... If I take the existing standard wheels off my '95 diesel Fiesta (70% SVO for 4 months and no problems...although I carry a spare fuel pump in the boot just in case...) and replace them with ones that are an inch or two bigger and have original standard tyres (as opposed to lower-profile ones) so that the total diameter of the wheel and tyre is also a few inches bigger, will I get a better MPG because each rotation will move me a bit further forward than normal; or will the extra weight and engine effort make it worse? Been a bit of a heated debate in the office about this, so any thoughts are appreciated!

steveo3002

there wont be room for a few extra inches , maybe room for 10% on the profile

remember it will throw the speedo out and while the added size will act like a overdrive in top gear the engine might have to work harder to accelerate to cruising speed

imo...leave it alone, the manf take alot of effort figuring what size tyres to use

anewman

Wrong size tyres also equal wrong speedo reading.

tomstickland

If you fitted larger diameter tyres to a vehicle you will not get a pro-rata increase in mpg.

This is because even though you will travel further for one revoltion of a wheel, the various forces acting on the car will now require a greater torque at the wheels to be overcome with. So for a given road speed the engine will be turning more slowly but with a higher torque output required.
Since power is torque multiplied by engine speed, the engine will still be required to produce the same power as before. So you would expect the mpg to be about the same. In reality it might be very slightly better or very slightly worse, depending on the exact details of the speed-load-consumption map. The effect would be so small that you would not detect it, since other variables already have a much larger effect on mpg.

redux

Apart from the above remarks, oversize wheels and tyres may rub on the inside of the wheelarches or suspension parts at full steering deflections. Within the same model range, the cars with wider tyres tend to have a larger turning circle in order to avoid this.

As well as that problem, the performance of suspension parts like springs and dampers will be adversely affected by having heavier wheels.

Actually, you might get better aerodynamics and rolling resistance by having narrower tyres, but I'd still really avoid changing things. Anything that isn't designed as standard fitment for that particular engine size in the range will mess things up - the heavier engine options have bigger brakes wheels and tyres.

And the insurance would be adversely affected too. Forget it.

devorguila

Many thanks to all for replies - should've known it wouldn't be as simple as it seemed! Back to the drawing board, I guess...

steveo3002

if its time for new tyres anyway...certain brands market fuel saver tyres, not sure if they offer much savings?

you can also play around running a little higher pressure...some sites suggest 80% of the max load pressure , but youd need to keep an eye out for uneven wear