Inflation Testing Question

[SuperTrooper]

I know that under-inflation of tires lowers MPG. Does anybody out there have definitive A-B-A testing showing increases in MPG with over-inflation?

We talk about it ad nauseum and throw numbers around, but who has the data to back up the theory?

[MetroMPG]

Have you seen this thread?

Experiment: coast down distances @ various tire pressures

[SuperTrooper]

Originally Posted by MetroMPG

Have you seen this thread?

Experiment: coast down distances @ various tire pressures

Yes, and while the data is promising it doesn't give us the direct relation ship of pressure:MPG.

In one of the threads about LRR tires there is an article about upcoming govt standards for LRR tires. They quote a spokesman from the TireRack as saying that a 25% reduction in RR would only result in a 1% increase in MPG.

Using your figures, your biggest avg gain (at 60psi) was 19'. That's a 0.03% increase. WHOOPIE!!!

I think we all took the well documented MPG penalties of under-inflation and extrapolated them equally to over-inflation. Modern radial tire construction probably limits the amount a tire can grow without going to dangerous extremes in pressure.

Unless someone can present reliable data showing a definite increase in MPG related solely to tire over-inflation we might have to declare this eco myth BUSTED!

[Daox]

Its not as simple as pressure vs mpg. There are so many other variables that effect mpg you'd never be able to get solid info unless you put your car on a dyno or something incredibly controlled.

[SuperTrooper]

OOPS! My wife, the teacher, just looked at my math and found an error. It's not a 0.03% increase; it's a 3% increase. That sounds much better.

I'm still looking for reliable data supporting an MPG increase.

[tasdrouille]

Google is your friend. There are plenty of studies that have proven this long ago. Higher pressure = lower rolling resistance which can't translate to anything else than better mpg.

If you care to read a bit look near the end of this list of research papers.

[MetroMPG]

The coastdown vs. PSI test essentially proves that you'll improve MPG by inflating tires to where the curves level out.

As Daox suggested, the actual MPG difference is going to vary from car to car and tire brand to tire brand. You'll never find any info that says "you will increase MPG by 4% if you inflate your tires 15% over placard rating".

And even if you did, it would be useless to you unless you have the same car & tires!

[SuperTrooper]

My, that was a fascinating morning of reading. Thank goodness I'm at work!

I learned a lot, but nothing that would indicate that over-inflation offers SIGNIFICANT increases in MPG. The main fact behind my conclusion: at least 3 studies agree that tire rolling resistance (RR) is the source of 25% of energy consumption for a vehicle moving at 100kph/62mph. You have to make LARGE gains in RR to reduce total energy consumption (i.e. MPG). All things being equal, you have to reduce RR 4% to make a 1% improvement in MPG. Since MetroMPG's Metro gets significantly better milage than most it should see the larger gain: about 0.7 MPG. Most wouldn't see half that.

Then you have to factor in the other important things related to tires: accelerating, braking, turning. (I'll just call it handling.) The Dutch study proved the negative impact of over-inflation on handling, enough that they deemed it significant enough to overshadow any benefit to over-inflation. Surprisingly, they found wet road performance even further degraded by over-inflation, which is contrary to what other posters here have been known to say.

Other tidbits I learned:

RR increases singificantly at speeds above 60 MPH. Another reason why slowing down is important.

Warmer tires have a lower RR. No kidding. The point being that in winter conditions some tires never get warm enough. Surely a factor in lower MPG often seen in winter.

Lots of talk about critical pressure increases. Air expands at a given ratio to temperature. Over-inflated tires reach critical temps sooner, but since expansion is limited more heat is retained. If you over-inflate you MUST slow down. If I were going to over-inflate I would change over to nitrogen since the expansion ratios are lower.

According to the Finnish study one way to reduce RR is to go to a higher circumference (taller), lower aspect ratio (short sidewall) tire/wheel combo.

The "spring rate" of air in the tires is a critical factor involved when the factory chooses a tire. It's part of the whole load/pressure/heat formula. This is why when you go +1 or more on wheel size you MUST widen the tire so that the volume of air in the tire remains essentially the same. Otherwise, the load increases, which increases temp, yada yada.

I've also been googling trying to find DATA to support the claims. All I find are anecdotal claims on sites similar to this one. Nothing scientific.

I'm still skeptical.

[tasdrouille]

First, congrats on your reading. If only more people would do what you did...

Let's just look at rolling resistance alone for a second. Why does a railroad steel wheel on steel rail has a RRC of roughly 0.0005 (according to wikipedia)? Because the hysteresis of both the wheel and the rolling surface are very low. Cover that steel wheel with a layer of rubber and RR will increase becauseof the energy lost in the hysteresis process.

Now lets switch back to tires. Although there is a significant amount of hysteresis happening in the tread (that's why as tires get used the RR goes down, less tread material, less hysteresis), the vast majority of the hysteresis happens in the sidewalls. If you increase the pressure of the tire you effectively reduce the property of the sidewall to flex as the tire rolls, hence reducing hysteresis losses.

As for everything else I'll have to go back to check the studies as I don't remember getting to the same conclusions as you.

[gascort]

Originally Posted by MetroMPG

Have you seen this thread?

Experiment: coast down distances @ various tire pressures

If you're hypermiling your vehicle, this data is enough to conclude higher FE. Otherwise, there's just good suggestion. I'm trying to establish the inflation effect on my car with my next experiment; I'm at factory pressure now.

I know the supermiling teams with the homemade cars running with lawn mower engines use bike tires inflated to 100+ psi to make them rock hard.
Does anyone know how the Michelin Energy Tires are different from others?

BTW, someone mentioned bringing an air tank to these tests. All you need is a bike pump. My bicycle tires are inflated to 65 psi. The only difference with the car tires are they're larger so they take more pumps.