bigger tyres and fuel economy

You're not calling Bendigo 'city driving' are you Jim?

Hi,

Is it possible that the improved economy is now because your odometer recording is out because of the larger tyres?

I am not sure of this but thought I would raise it as a possible reason for the ? improved economy.

Have a good one.

Diamond, you got 22lt per 100km previously? Thats heaps. Am I missing something...? I'm just suprised. I got 14lt or so in my old Jack with stock tyre size. Mine was petrol, maybe there's a difference if yours is diesel, not sure. But anyway, guess it doesn't matter what was before, but its great you got better economy now.

As consumption = dist divided by litres your bigger tyres will show less distance therefore worse economy .
Maybe the improvement is due to less engine revs per km.

Am I reading this wrong.

22 litres /100k is only 12 MPG . that is worse than a Petrol Landcruiser Auto.

You did say you had a Jackaroo. Wow they are gas guzzlers.

I'd recommend checking the figures again. It sounds like 22L/100km is too high.

Think about it: with larger diameter tyres, your speedo shows you going slower than you actually are, as well as your odometer counting up the Km's slower than you actually do them. End result is you think you have travelled 100km, but its more like 110km.

Add to this the increase in rolling resistance, the extra weight you are making rotate, the extra petrol being burnt to get the low down torque you now rely on more....

Do you still reckon you are getting that 32% increase in economy??

Strange buddy, yes the autos do have a thirst, i would have expected 17-18 tops around town.

I went from 245/70 to 265/75 and it was 106% increase.

My figures went from 12/100 on 245/70 at pattern to 14-15/100 265/75 mtr's. and you know i'm not shy of giving her some stick.

Stop/start driving is not a great way to measure economy....too many variables.
IMHO you need to travel a given route (eg: Bendigo to Melbourne) and start with a full tank each time and see how much fuel it takes to re-fill at the end of the trip. Do this with the smaller tyres, then the bigger ones. Still not real scientific cos temperatures have an effect, wind etc.
Cheers
Roachie

When I went from 265/70 to 265/75 the economy got worse by about 1 litre per 100k. According to the GPS the speedo is now almost perfectly accurate so it must have been out before (didn't have the GPS then). There should be no way that your economy will get better with bigger tyres as the engine is working harder to get the car moving. I also went from HT to AT and that affects the economy as there is more rolling resistance.

Seems like a big difference with no reason so something is strange.

Rolling resistance is affected by low tyre pressures, but with tubeless radial ply tyres running at the correct pressures, raising pressure further makes only marginal difference.

Fitting larger diameter tyres may reduce consumption very slightly in some circumstances (the cooling fan rotates marginally less fast and thus absorbs slightly less energy, and there are marginally less friction losses ) but any apparantly observable difference in fuel consumption is far more likely to be due to slight changes in driving habits, and/or experimental error.
Collyn Rivers