Tyre pressures and fuel consumption.

[FRVPrado Lurker Find all posts View Profile Close FRVPrado]

I’m running a set of AT3W on a new GXL with mods including a rhino platform and bush company 270XT awning. A recent trip to Little Desert had me dropping pressures to 18psi and I got me thinking about my road pressures. I was running at the factory recommended 29psi which was also the pressure set by Bob Jane when new tyres were purchased. Recent posts had me upping these pressures to 38psi similar to other previously owned vehicles with the reasons being better wear vs comfort. What I can report to those interested is that to my surprise after 3 months of road (highway and city) use fuel consumption is exactly the same 11.0/100km. (Ambient temp these 3 months have been quite low 2-15deg C.) Anyone have similar or differing experience???

[RPP Senior Member Find all posts View Profile Close RPP]

Your tyre pressure on the bitumen are too low.
I run with 45psi. My 1st Dealer set them at 38psi, but they pump em up to 40+ after every service.
Economy is heavily dependent on tyre type (eg Hwy vs AT vs MT), tyre size, air speed (not ground speed) and frontal cross section.

My economy stats for the first 130,000Kms:

[404pug Avid PP Poster! Find all posts View Profile Close 404pug]

45 psi and the ride is absolute crap.. Around town I find 32 works well. 38 good on highway and about 40 if loaded. All cold pressures. 29 is very soft and way too low at highway speeds.

[amts Ninja Poster. Find all posts View Profile Close amts]

Tyre pressures needs to be adjusted according to load too. If too low in pressure and high load, then tyre temps will increase from sidewall flex.

[Arg Lurker Find all posts View Profile Close Arg]

May I ask, why does the plate on the door of the car say 29 psi, if it is a terrible thing to do?

[krypto Avid PP Poster! Find all posts View Profile Close krypto]

May I ask, why does the plate on the door of the car say 29 psi, if it is a terrible thing to do?

The manufacturer tyre pressure is all about comfort. At 29 psi the tyre will squirm and handle very poorly in emergency situations. Great thig to try out if you ever get on a skid pan. Biggest surprise for me was how much better the tyre handles in the wet with higher pressure, which makes sense if you think about it.

Tyres deform as they rotate, the less tyre pressure the greater the deformation which also generates heat into the tyre. Hence change in tyre temp is a good indicator of inflation. I never run less than 40 and up to 55 in the rear when doing highway towing.

Obviously on gravel and offroad you want deformation to absorb some of the shock so pressures are very different. And on sand you want deformation to increase the contact patch and reduce contact pressure, but that's a whole different story.

AT/MT tyres are a lot heavier than road tyres and a lot less efficient, I've seen tests where they add about 10% to fuel consumption. Unfortunately I run AT for the 15% of gravel/offroad work that I do.

[Mike-B Junior Member Find all posts View Profile Close Mike-B]

You will be better served by speaking to the tyre manufacturer, some LT rated tyres do not recommend going below 35psi cold road pressures. Too low a pressure can cause excessive heat build up, the 3 psi rule is a good starting point but better to get factual advise. From speaking to falken ( i run 285/70r17 LT rated ) they suggest going 35psi, the door tyre placard recommends 32psi but that is below minimum recommended pressure for that specific tyre according to falken