Thread: Fuel Economy vs Tyre Pressure

I run placarded pressures and get 10-11l/100km highway and 12-13l/100km around town. My D2 is fairly standard externally - no lift, and stock bumper, 235/70R16 Michelin Latitude Cross. I do have a second battery in the engine bay, plus tirfor, air compressor, tools, etc in the back. It's a stealth tourer rather than an inyerface 4bee rig.

"Correct" tyre pressures primarily related to axle loading and the maximum load rating of the tyre, rather than tyre size.

Size and aspect ratio have a relatively small influence. The difference in tyre pressure between 235/70R16 and 245/75R16 if all other characteristics were the same is less than 1 kpa (0.14psi).

Once you start adding weight to the front end in the form of second batteries and bull bars you need to increase the tyre pressure over standard to maintain the same load to load rating ratio. Assuming the battery and bull-bar add 50kg over the standard rig, you'd need to increase the front pressures by 2 psi to get back to the same inflation relative to load as the placarded pressures.

The placarded pressures on the D2 assume kerb weight plus 5 passengers. If you are travelling two up, I'd guess you could carry a load of 150kg in the rear before you really needed to inflate above the placard pressures.

I'd hazard a guess that those who claim big improvements in economy are running heavily loaded trucks and as a result have under inflated tyres at placarded pressures.


cheers
Paul

36 all round but the roofrack will be killing your economy.

I run 32 at the front and 36 at the rear and get 11-11.5L/100km around the city on Cooper ST MAXs, 245/75/R16s. Carry about 150kg over factory weight. I've run up to 4psi higher than that front and rear however it makes very little difference in economy but is a much harsher ride.

Originally Posted by OffTrack

I run placarded pressures and get 10-11l/100km highway and 12-13l/100km around town. My D2 is fairly standard externally - no lift, and stock bumper, 235/70R16 Michelin Latitude Cross. I do have a second battery in the engine bay, plus tirfor, air compressor, tools, etc in the back. It's a stealth tourer rather than an inyerface 4bee rig.

"Correct" tyre pressures primarily related to axle loading and the maximum load rating of the tyre, rather than tyre size.

Size and aspect ratio have a relatively small influence. The difference in tyre pressure between 235/70R16 and 245/75R16 if all other characteristics were the same is less than 1 kpa (0.14psi).

Once you start adding weight to the front end in the form of second batteries and bull bars you need to increase the tyre pressure over standard to maintain the same load to load rating ratio. Assuming the battery and bull-bar add 50kg over the standard rig, you'd need to increase the front pressures by 2 psi to get back to the same inflation relative to load as the placarded pressures.

The placarded pressures on the D2 assume kerb weight plus 5 passengers. If you are travelling two up, I'd guess you could carry a load of 150kg in the rear before you really needed to inflate above the placard pressures.

I'd hazard a guess that those who claim big improvements in economy are running heavily loaded trucks and as a result have under inflated tyres at placarded pressures.


cheers
Paul

Manual or auto?

I suspect that some people assume that just because there are published figures (eg here Gas Mileage Tips - Keeping Your Vehicle in Shape ) that show that there can be a 0.3% increase in consumption for every 1 psi of underinflation, that there will be the same improvement for each psi increase in pressure.

it doesn't necessarily work that way. Look at Figure 14 on page 72 of this document.
http://www.goodyear.com/truck/pdf/ra...tread_S9_V.pdf
The information is about truck economy and there are quite significant differences between truck tyres and car tyres. However, the fact that the graph is not a straight line applies to both vehicles.

The extra drag cause by 5 psi underinflation will not necessarily be matched by a similar reduction in drag for 5 psi overinflation.

That graph also suggests that the benefits might not be worth getting too excited about. It takes about 10% overinflation to get a 1% improvement in economy. You can get much bigger improvements by small changes in driving technique.

I would have thought the biggest expense of over/under inflation was not getting the full life out of the tyres.

There are plenty of sites on the web that give pros and cons, one thing I was told many years ago was that, recommended tyre pressures are mainly about ride comfort.

I always remember back to my many years pushing pedals to school and back, about a 40k round trip.

If the tyres were down it was harder to push. If you went onto soft ground and grass from off the road it was harder to push. So in both cases you are using more energy to move forward.

The trick is finding where you start effecting the tyres life, what is to much that will wear out the centre of the tryes.

This is my take on it anyway, happy to be proved wrong.

Originally Posted by Franz

Manual or auto?

Auto.