tire pressure vs fuel economy

pianotuna wrote:
Believe me, I was hoping I could do something about the teeth jarring ride on my E-450.

Don ... I assume that you have read in these forums about using Koni FSD shocks to take some of the sting out of the E450's rear springs?

First try two of them in the rear. It sure helped in our case ... and we even have only a ~11,800 lb. 24 footer on the E450 ... and I run 80 lbs. pressure in the rear duallies, too. I'll probably install them in the front to complete the set before our Alaska trip next year.

For fuel economy, you want the tires at their max pressure for least rolling resistance.
You can demonstrate this to yourself by trying to push a car with low tire pressure, then pushing it with its tires inflated all the way. You'll feel a very large difference.

You may have other reasons you want to deflate your tires, but fuel economy isn't one of them.

I gotta put in a plug for rig weighing, not just on a CAT-type scale, but by wheel position. Had it done this summer and it was a real eye-opener. Even though I was near my GVWR, turns out I was well-UNDER for each axle. By knowing the actual weight at each wheel position, I was able to LOWER the pressure in my tires from sidewall max of 80psi ( which I incorrectly thought I needed based on overall vehicle weight) to 60 in the fronts and 65 in the rear duallys. End result will be better ride comfort and increased tire life. Actual tire psi's are based on each axle weight in addition to the tire manufacturers weight chart. ST

I run Max for the tires because I'm near the GCCW most of the time.

Something is very wrong with those calculations. I would think that it is impossible that a difference of 10 psi of air in the tires could make a change of mileage of 20%. You state 140 K of gravel road, the rolling resistance in the gravel would have much more effect than the 10 psi. I don't have a 20% difference in mileage between not towing or towing my 4500lb. Jeep that has very aggressive offroad tires on it.

OP,
I suggest you do your test again !
http://www.popularmechanics.com/cars/a940/4199963/

I went to Firestone Air Ride on the drive and tag axles. The tag has a massive 55 lb sway bar. Ride comfort is a Freedman high back polyester with eleven adjustments. The #3 support setting trades butt comfort for maybe a little too much variance between seat height and wheel. The wheel does telescope. I'm mentioning this because I think it is a disgrace that a fantastically high priced rig forces the driver to put up with a buckboard ride. I knew what I was in for on Mexican roads so no effort was spared on cabinetry and bracing. Those 1100x22.5 tires are unforgiving. I was lucky. I raided a truck wrecking yard in Reno and another in Four Courners and it's annex in Adelanto. I snared this stuff for about ten cents on the dollar.

But a fragile spine snapping seventy thousand dollar rig is hard for me to swallow.

I do agree that 20% seems incredible. I would have to take multiple trips to prove that it was simply an anomaly--but I do know that handling was degraded on gravel and that mileage was poorer.

Believe me, I was hoping I could do something about the teeth jarring ride on my E-450.

Tight, repeated dozens-of-trips with the toad revealed running door sticker tire pressures front & rear taxes me thirty point four US cents per adjusted gallon of gasoline (liters and pesos down here) running 35# front and rear presents flat tire wear across the tread. I adjusted tire pressure on Quicksilver to yield exactly flat wear across the 11x22.5" tires. It would take a lot of convincing to sway me from the idea that flat tire wear on a correctly loaded chassis is not the optimum target.

Hi Earl,

Not even hills, let alone mountains.

There was 140 k of gravel.

There were no significant head winds.

I normally drive 79 kph (49 mph).

Earl E wrote:
I just don't believe that tire pressure could result in anything like a 20% change. Something else had to be going on like wind, speed, mountainous terrain.