phil7782 Loc: Rancho Cucamonga, CA

From a Popular Mechanics article.

I guess I'll soften up the tires and get a more comfortable ride..

Phil

Debunking a Mileage Myth: Can You Really "Pump Up" Your Fuel Economy?

http://www.popularmechanics.com/cars/news/4199963
October 1, 2009 12:00 AM

Here at PM’s automotive department, we’ve debunked most of the devices on the market today that claim to increase your fuel economy. They simply don’t work. And in some cases we’ve found they can actually make your vehicle run worse. But what about a free mileage booster?

We’ve heard folks claim mileage gains by running their tire pressure above the manufacturer’s recommendations. In theory, that makes sense: With more inflation pressure, the tire bulges in the center of the tread and creates a smaller, skinnier contact patch with the road. This should mean less rolling resistance and increased mileage.

What’s more, we know under-inflated tires have a negative effect on fuel economy. The U.S. Department of Energy says that for every 1-psi drop in pressure, you can expect your gas mileage to lower by 0.4 percent.

If under-inflated tires drop your fuel economy, then over-inflated tires must give that mileage a boost—right?

We decided to find out. So, after saying goodbye to half our test car fleet, we took our long-term Honda Fit on an 800-mile road trip from Los Angeles to Phoenix and back again—setting our cruise control to 70 mph except for dips down to 65 mph in central Phoenix and downtown L.A.

The door placard on the Honda recommends 32 psi in all four tires. The sidewall says the maximum pressure for the tires is 50 psi. So for the trip to Arizona, we inflated the tires to 45 psi. We figured 5 psi under that maximum inflation pressure would reduce drag enough to boost our mileage by some measurable amount. Before reaching Phoenix’s West Valley, we were able to travel 394.1 miles, then filled up in Glendale with 9.34 gallons and recorded 42.19 mpg. That’s the best mileage we’ve recorded so far in this car.

Two days later, we dropped the pressure back down to the recommended 32 psi and pointed the Fit’s nose home toward L.A. This time, over the exact same route, at the same speeds, the Fit went 411.3 miles. At the Chevron station in Santa Monica, our Fit drank 9.76 gallons, translating into 42.14 mpg—nearly the exact same mileage.

To invoke the MythBusters, we’d now consider the myth of increased fuel economy from over-inflated tires officially busted. So why didn’t the harder tires with less drag result in better mileage? Well, you’ve got to figure that, at 70 mph or so, the aerodynamic drag of the car’s body is probably outweighing the small improvements the tires gave us.

And we wouldn’t recommend anyone try this one at home either. The vehicle’s handling is compromised when the tires are over-inflated. At 45 psi, the ride was brittle. In fact, the tires were so stiff they nearly vibrated our garage-door opener right off the sun visor. Ouch. –Ben Stewart



Read more: Debunking a Mileage Myth: Can You Really "Pump Up" Your Fuel Economy? - Popular Mechanics
Follow us: @PopMech on Twitter | popularmechanics on Facebook
Visit us at PopularMechanics.com

TucsonCoyote Loc: Tucson AZ

Have seen quite a few car tires over 100psi...that is scary!
When asked what that was all about the owners mentioned reading about higher pressure fuel economy and figured the more the better !

phil7782 Loc: Rancho Cucamonga, CA

When I was younger in the early 70's, I used to go and watch the GYMKHANAS where cars of all stripes would race for time around a course laid out in a shopping center parking lot.

That was when tires would rarely be rated over 32 psi.

However, a lot of racers would bump up the tire pressures to 50 psi.

I think it was more to keep the tires on the rims.

Phil

Ka2azman Loc: Tucson, Az

Problem with this test, LA to Phx is a gain in elevation of circa 2500 ft. Your climbing. You lose MPG climbing.
Trip from Phx to LA is downhill of 2500 ft elevation. You gain MPG going downhill.
This test is BUSTED. Not all apples.

TucsonCoyote Loc: Tucson AZ

Let's also remember that the show "Mythbusters" is just a show.....sometimes they don't have a clue what they are doing !
....but it's mostly fun to watch anyway. :)

Black Bart Loc: Indiana

This is the first thing I thought of when reading the article.
You are right this proves nothing other than they wasted gas to prove nothing.
Also have to take into consideration head wind tail wind that would affect mileage more than tire inflation.

These dummies proved nothing.

Bunko.T Loc: Western Australia.

By over inflating tyres you increase the diameter/ circumference thus altering the revs per mile hence difference in distance recorded over same course.
Bigger diam tyre also increases load on engine therefor a higher fuel consumption.
Your figures, although miniscule, are what I would call par for the course.
Wind direction on return journey may have been higher too.