Amazed at tire pressures affect on MPG
 

So here recently (2 tanks of fuel ago) I had my van in to have the tires rotated and balanced. For the 2 tanks of fuel I had since then my mileage was down 1-1.5 mpg. I got 15 and 15.5mpg where I was getting 16-17mpg prior to the rotate and balance on the same daily commute. I had been running 80psi cold tire pressure on all 4 corners prior to the service. When I checked them tonight they were all at 65psi. The door sticker says 65psi front and 80psi rear. I inflated them all back up to 80 psi, so we will see if my theory is correct.

I vaguely remember this happening before when I had tires put on. They set them to the factory specs and my mileage dropped.....then instantly recovered when I increased the pressure back up to 80psi.

Anyone else seen this large of a difference with a change in tire pressure?

Just the change in seasons I can tell when the tires are getting low. It won't even move when idling in drive and take my foot off the brake............

Trucks with manual gearbox (Armstrong steering) have noticed that just a few pounds shy makes a big difference in the ease of turning. And every kid has ridden a bicycle that needs air in the tires and noticed right away what a tremendous difference this makes in the pedal. I can believe it.

One thing to keep in mind though, pumping tires way up and exceeding recommended pressures beats the hell out of wheel bearings.

I really don't think the tires made that much of a difference. You're talking about burning 10% more energy, all due to tire flex with "properly" inflated tires. I don't think that's possible.

If you were dumping that much energy into your tires' sidewalls they would overheat and fail in short order. I'd expect that if you were down to 25 PSI, but certainly not 65.

Well, a gallon of gas has about 114,000 BTU. 10% of that is 11,400 BTU, divided amongst 4 tires, which is 2850 BTU per tire over the course of 15 miles--15 minutes at 60 MPH. And the tires are getting air-cooled. I'd say it's plausible.

Jason

I filled up the tires last night shortly after I filled up with fuel, so we will see. I track all my mileage. I was consistently getting 16.3-16.8mpg prior to the rotation and tire pressure adjustment. Since I have been getting 15-15.6mpg. I will be interested to see if my theory is correct.....maybe the station I use got a bad batch of diesel.

The rated maximum tire pressure is for maximum load, if you pump them up that high with an empty vehicle you will just wear out the centerline of the tread more quickly. If you notice a decrease in steering force with increased pressure this just confirms that the tires are being overinflated and the contact patch is decreasing, I guess that can increase fuel milage slightly but will it be enough to offset the decreased life of your tires.

65 psi should be enough that you don't have any kind of major tread sag and scrub happening unless you are running a load in the van.

With my E150, the recommended pressures are 41 lbs and the van gets great gas mileage. But my GVW is 7200 lbs and yours is gonna be way more, 9500 lbs or so(?)

One way you can tell if your tires are squirming and dragging is to feel the tread with your hand after a drive of some distance (like a freeway rest area maybe). If you are wasting a lot of energy dragging the tread rubber around, they will be really hot. I have a set of Eco tires on one of our cars, and even after a long freeway drive, the tire treads feel no warmer than the ambient temperature. Makes perfect sense to me. In the old days, bias ply tires would have all the cloth plies and the tread rubber squirming around bigtime--modern radials are designed to roll the tread down the road like a smooth tank tread with no squirming of the tread pattern.

I once put some AT tires on my van and lost 2 mpg or more (my 17-18 mpg on the road went down to 15). They were Firestones and had a 30 day money back warranty which I took advantage of and got a refund (except for mount/balance cost) and went back to Michelin LTX M/S's. I remember feeling the treads of the Firestones after a drive and they were HOT. I was using extra fuel to flex and squirm rubber across the pavement...

This is really unscientific, but it's an easy thing to do... And look at your tire tracks after driving thru a puddle onto dry pavement. If the patch is narrow and center-only, you're probably over inflated.

George

IIRC my GVWR is somewhere between 8600-9000lbs. I may take my IR temp gun along just to do an experiment. Next time I get tires I am going to go for some LRR tires to see if it makes a difference. Fuel prices have dropped some (can't believe I think 2.30 a gallon is good) so increasing my mileage 1mpg doesn't save as much as it would of last year, but it still would have saved me almost $400. That would be enough for another 1 or 2 weekends at the track fr the year.

I know that is the theory. The door sticker says 65psi front and 80psi rear for normal/unloaded use and 80psi front/rear if loaded. The thing is that I get better front tire wear with 80psi. At 65psi I wear the inside and outside edges of the tire much more than the middle. At 80ps the edge wear reduces and the middle wear is not any more noticeable. I have put about 30k miles on these tires and I am still in the middle of the "Green" tread depth section at Discount tire.

It makes me wonder if all the e250 had the same inflation specs regardless of power-train configuration. I know that the 7.3 IDI in mine weighs substantially more that some other motors that these came with. I am thinking that that may contribute to the front tire wear characteristics that I have noticed.

I could be wrong. Not sure about the science of it, but I know from watching my EGT gauge that it stays a bit lower when I am cruising with my tire pressure at 80psi. We will have to see how the experiment turns out.

An IR temp gun would be a great way to get some real life readings of your tires. I just looked on Amazon and there are some really inexpensive ones (I thought they were $100). Makes me want to buy one--I can think of many uses for one of these.

The Michelin LTX M/S tires that were OEM on my van (and on many of the 250 and 350 vans) were never expressly sold as LRR tires, but they certainly get better gas mileage than alternate brands. Likewise, I have bought numerous sets of MXV auto tires over the years (starting with the OEM MXV's on my '91 BMW, then MXV4's, then Energy MXV4's, then Primacy MXV4's). Those have always been really long lasting tires with a great ride and excellent gas mileage.

In general, LRR tires will also be longer lasting because they won't chew the rubber off the treads as much--that takes energy to do.

If your front tires wear well at 80psi, by all means use that pressure if they don't skitter across bumpy pavement in turns or rattle the fillings out of your teeth :)

Good luck,
George

Try 90 psi.

Maybe. At 80psi the ride is fine on most the roads around here. Not too many pot holes and patched roads. The ride is a little more jarring on rough roads at 80psi, but luckily even the country roads are not rough around here. Just pretty bouncy.

I feel pretty comfortable at 80psi. If I have a long highway trip I may try 90psi to see if there is a difference. Who knows, more may be better. my track car loves higher than recommended tire pressure. Although the track usually doesn't have any pot holes.

I run my fronts at 80 all the time, right now they are a yokohama all season. rears are also yokohama but a directional tread pattern, maybe a rain tire?


they work ok but I do seem to use more fuel to do the same driving.


previously I had a Bridgestone rib tread on the front and the truck did roll easier and no uneven wear at all. they were very comparable to the XPS rib.


I have never gone over max rated psi on my tires but if I am on any longer trailering trips I put my rears up to 80psi from the 70psi they are usually at.


65psi to 80psi will definitely make a difference in your fuel economy.


If I was using my van for commercial use I would go back to a Michelin XPS rib and traction. ride not the best but no issues at all and wear like iron.


when my tires get down to 20-25% tread they go onto my trailer.:D
took my bridgestones right to bald on the trailer before any belts shifted.:-X0A6


post those findings when you get into it.:-drink

Didn't realize he had a diesel engine
 

His engine probably weighs many hundred pounds more than us gassers...
Maybe he's onto something.

If I usually ran 65psi in my rear tires with empty van (which seems reasonable), and I had a 300-400lb payload to add, I would definitely crank them up to 75 or 80psi..
It just so happens that this guy (OP) has added weight on the front (compared to us v8 and v10 guys)

There is no reason to guess, this IS an exact science.

For the factory LT225/75R16 that comes on an E-350 the tires are rated at 1,940 lbs each @ 50 PSI. At 65 PSI they are rated at 2,335 lbs each, and 2,680 at the max of 80 PSI.

I've never weighed an empty van, but the max front GAWR is 4,850 lbs for an E350. So a loaded diesel van should never be over that, so nothing over 70 PSI should ever be needed.

The extra 15 PSI adds 690 lbs of payload capacity. Do you think you're getting close to 5,360 lbs on that axle?