Higher tire PSI lower MPG

Higher tire pressure == stiffer sidewalls
Stiffer sidewalls == less sidewall flex
Less sidewall flex == lower rolling resistance
Lower rolling resistance == higher MPG

 

+1.

 

I didn't follow the meaning of the tire pressure sensor sentence, but it doesn't seem relevant to the rest of the argument anyway.
You have a point, but as priusenvy points out, higher tire pressure mostly makes for stiffer sidewalls. The increase in tire height is quite minimal, since the tread is pretty strong and includes steel belts, so it won't meaningfully expand once already at full size (which probably happens at about 25-30 psi).

 

Your thinking is whacked.

His thinking is good. He points out the important factors.

His thinking is also good. He points out that the tire diameter between a fully inflated tire and a tire at a higher pressure is not very significant which pretty much deflates (pun intended) your initial logic.

If you don't believe it, get a tape measure and a tire gauge and see for yourself.

Also, I would just like to point out that your responses when someone doesn't give you the answer you want to hear are very rude. But I think you already realize that.

 

No. The diameter of a steel-belted radial tire is almost completely insensitive to the tire pressure, until the pressure gets so low that the tire starts to collapse. And, because the sidewalls are stiffer at higher pressure, for a given tire the *actual* (and reported) MPG will increase.

The only way tires will yield a lower *reported* MPG without necessarily affecting actual MPGs is by mounting larger tires.

 

Increasing air in a tire will not increase the diameter so much that it is like putting the next size diameter tire on the vehicle.

 

Let me try to put a little water on the fire that's kindling here. If I understand the issue and question asked, it's just a misunderstanding of the difference between axle height (Ground clearance) and distance traveled per revolution. If I understand the physics right, then:

1) with steel belted tires, the circumference doesn't change significantly with tire pressure
2) But higher pressure does affect ground clearance.
3) How can both statements be true? Because the increased pressure makes for a rounder circumference (less oval).
4) Note that as you force a very oval tire to roll, the side wall gets flexed a lot. That's why underpresurized tires can heat up and rupture.

 

I believe that at highway speeds, even an underinflated tire will grow (due to it spinning) to the same diameter as a properly inflated tire. And a properly inflated tire doesn't change that much in diameter with pressure. So low tire pressure doesn't really reduce the distance traveled in one tire rotation. Maybe at 3mph it might make a difference but not at 60mph.

 

One more bit of information: the tire pressure sensors on the Prius are just that, tire pressure sensors. Some cars use tire RPMs from the antilock break sensors to estimate tire pressure, but not the Prius. Each sensor is inside the wheel on the back of the valve stem. It uses a radio link to transmit pressure information.

Tom

 

There is a very simple test you can perform, a hill rolling test. Find a shallow hill that does not have much traffic and you can start from a dead stop in "N". Release the brakes and measure your speed at the same point on the hill after it flattens out. Be sure and run at least 3 runs so you can average the results.

Now go to the filling station and fill your tires to the maximum sidewall pressure and run three runs. Reduce the pressure by 5 psi and do another three tests. Continue until you are 5 psi below the door jam tire pressure.

Bring the data here and we'll load it into a spreadsheet and plot the results.

Bob Wilson

 

The Prius TPMS doesn't work this way. It uses sensors in the wheels to directly read the pressure and then transmits the reading to the TPMS ECU. See the PDF attachment.

You might have this confused with a system that Hobbit devised for indirectly determining the difference in tire pressure.

 

Just another thought:

The diameter is not going to matter as much as the circumference. More to the point: The entire tread still rotates and comes into contact with the road surface, regardless of the diameter of the tire and/or its "circular" quality.

Yes, the car may actually sit closer to the ground with a lower air pressure, but that doesn't mean there is any less tire tread touching the road surface with each rotation of the wheel. What is does mean is that you still have the same distance travelled with each rotation of the wheel. The difference is that you have more tread touching the ground with a lower inflation than you do with a higher inflation.

It sounds to me as though a lower inflation then leads to higher rolling resistance.

Does this make sense?

 

Do the hill rolling test!:car:

No speculation, no guesses, just direct observation. :laser::ear:

:smash:Bob Wilson:target:

 

Very astute observation - I hadn't even considered that.

 

Thank you! I'm flattered. Until hobbit or one of those danged and intrepid engineer/physicists points out the flaws in the thinking, I'll hang on to this compliment with a tight grip.

 

Higher pressure isn't making a diameter change, it's simply making the tire HARDER. That's why your local choo choo has STEEL wheels ... because a steel wheel is HARD. Hard wheels have lower rolling resistance. Low pressure is SOFT, and SOFT has more resistance. A low pressure tire is akin to a tire made out of a sponge. Soft. Soft grips better, and thus slows you down. That simple. Our 2004 doesn't have sensors, but I'd imagine the sensors simply sense the lower pressure ... which has nothing to do with effecting your odometer reading, because your tire diameter simply isn't effected much, if at all. All the ply in the tires hold them to a certain shape ... it aint like your party baloons that'll grow & grow.

That said, if your mpg's went down after hardening your tires, and your MPG's are getting low, then it's because of something else ... shorter trips, gas quality, terrain, temps, A.C., heavier foot on the pedal, whatever.

 

I see you have a touring 6, same as what I have.

Interestingly I have notice the same problem you described, I have found my optimal MPG when tires are set at 38/36 cold, anything beyond that I start to lose MPG, I am averaging about 50-52 MPG now, I usually travel alone with no luggage in the back.

Wondering if anybody is seeing this as well.