How high can we safely run the tire pressure?

[toddjb122]

I drove down to OBX this weekend with my new Yakima Skybox 18 and was curious to see MPG difference. Well, I didn't notice much. I clocked 19.8 on the way down and got 18.6 on the way home. (I drive around 70mph, in Sport mode w/ cruise control.)

A couple things to note, I have always gotten a pretty reliable 20 mpg on highway trips with the LR3. Lately, it had been dropping down to 17 or so and I didn't know why. I had it in for scheduled 52.5K service recently and don't expect an oil change to make a difference, but for some reason my tires came back pretty juiced up!

I checked the tires when I got down to OBX (the next morning) and found that both front and rears were running at 47psi! It could have been a careless top off by someone, or it could have been that the last time they were filled it was 30 degrees out instead of 65, but in any case...I got great gas mileage! I dropped them back down to the factory recommended 42/33 (or something like that) and had my 18.6 MPG on the way home. While I'm on the topic, I have found that nobody, including the dealer, can be trusted to get the front vs. rear pressure correct when they rotate your tires. Check it yourself because it is often overlooked.

And if none of this has *anything* to do with it, I did have a tailwind on the way down and a headwind on the way back. FWIW.

In any event, I was curious if it was safe to run the tires a little hard to get better mileage? At least for long trips, anyway.


yes, i use the trip computer for this and for my use i have found it to be fairly accurate. i reset it before a long trip when i want a clean reading of the entire trip.

 

[Houm_WA]

That is a good questions...how high can we go?

I drive barefoot (in the summer) and get better mileage !
....and why Sport mode? I would think that this would make your mileage worse.

 

[tommithy]

I agree with Houm on "why sport mode"? I use sport around town but I found that on the highway it will sometimes fail to switch into 6th gear thus affecting mileage.

Oh, and fwiw, I accomplished 22.5 mpg on a Land Rover sponsored mileage challenge. The challenge was a nice trip down HWY 84 and 14 along the Oregon-Washington border from Portland to The Dalles and back. The winner got 23.3 mpg (he was driving a v6 SE and I think that made a difference.) One of the LR employees actually got 25 mpg. His claim to fame was that he puts an uncooked, unbroken egg between his foot and the throttle and drives like that.

 

[duckdive]

You mean when you try manually to upshift into 6th gear it won't let you ?

 

[richpike]

I have found that nobody, including the dealer, can be trusted to get the front vs. rear pressure correct when they rotate your tires. Check it yourself because it is often overlooked.

Hahahaha - classic. So true

Back on topic - when you checked the air pressure, were the tires cold (you said the next morning, so I'm assuming yes, but thought I'd check)?

I'd also say that you should be able to run your tires as high as the tires themselves state (for their maximum). But you might get some weird wear. It seems like (at least for our LR3 with General Grabber AT2s) the best setting is 38F/42R.

-Rich

 

[toddjb122]

....and why Sport mode? I would think that this would make your mileage worse.

Oh, I don't drive Sport mode for MPGs. I just like the way the truck drives much better. It seems to have power the instant I press the gas, while normal mode has that delay followed by a downshift. So, for onramps, pulling out, passing, etc...I like it a lot better. Actually, when someone else drives my truck I tell them to put it in Sport mode because I think it drives like a heavy vehicle in Normal. Maybe that is because I never use it so the computer has never adapted to me. ?


Yes, tire pressure was measured mostly cold the next morning. (short drive in town at 25-45 mph...so it wasn't highway driving, but wasn't still).

Good tip on that front air pressure. Maybe I'll leave the backs at the factory setting but boost up the fronts a bit.

What is the physics reason for having the fronts at a lower pressure than the backs? Is it something to do with steering? I would have thought high pressure in front would be good to offset the weight of the engine.


There's always a good discussion on this forum no matter what my silly question is. Thanks!

 

[tommithy]

You mean when you try manually to upshift into 6th gear it won't let you ?

That would be Command Shift mode and it works fine in that mode. I'm talking just Sport mode where the shifter is moved to the left but no gears are manually selected. I've found that sometimes at highway speeds (60+ mph) it will stay in 5th gear. If I flip it to normal it'll shift to 6th.

 

[Houm_WA]

BTW Tommithy, I don't think that this is a "failure" to shift up into 6th...I think that is intentional and it's what puts the "sport" in sport mode. I think if you cranked the RPMs up enough on the freeway it would shift into 6th, but in Sport mode it holds each gear a lot longer.

 

[richpike]

What is the physics reason for having the fronts at a lower pressure than the backs? Is it something to do with steering? I would have thought high pressure in front would be good to offset the weight of the engine.

My guess (based on discussions I've seen elsewhere) is that most car manufacturers have the front tires at a lower pressure to create understeer if the car is pushed beyond its limits. Understeer (at least for a novice driver) is considerably safer than oversteer - most people don't know how to correct an oversteer event and end up spinning out. With understeer, the car basically won't turn - this sucks too, but at least people can see where they are going and are less likely to overcorrect and flip their vehicle.

-Rich

 

[toddjb122]

My guess (based on discussions I've seen elsewhere) is that most car manufacturers have the front tires at a lower pressure to create understeer if the car is pushed beyond its limits. Understeer (at least for a novice driver) is considerably safer than oversteer - most people don't know how to correct an oversteer event and end up spinning out. With understeer, the car basically won't turn - this sucks too, but at least people can see where they are going and are less likely to overcorrect and flip their vehicle.

-Rich

Good info.

So, since you brought it up...how do you correct from an oversteer?

(I had a bad overcorrection, on my part, when towing a trailer a several years ago...a normal overcorrection for the car called the trailer to oscillate for a bit at highway speeds. It was scary, but luckily self corrected.)