Let's talk tire pressure...

I run the maximum safe air pressure as stipulated by the manufacturer that actually made the tires that are currently on my car...... 44 psi, front and rear. When I do eventually change tires, if the maximum safe pressure is different, then I will adjust to that new pressure.

If I wanted a smooth and quiet ride, I wouldn't have bought a Prius.

 

I use the info at the pump with my miles traveled to get an accurate calculation.
My speedometer is less than 1 MPH off and I fill my tank to the top of the filler neck every-time.
Testing with an old Buick 225 Your outcome will only be as good as your tools.

 

Still not as accurate as measuring (via a separate fuel cell, by weight) the actual amount of fuel consumed in the engine vs. miles traveled, as the Mythbusters do.

 

Did they use a Prius ?

 

It doesn't matter what they used, they are measuring fuel used through the same vehicle with different tire PSI.

Does the Prius change the laws of physics that revolve around tire to ground friction?

Not trying to be argumentative, perhaps we are talking about 2 different things.

 

From the video I saw, They used a small compact car but definitely not a Prius. I agree that the basic principle can be tested in any car, but the exact results could well change with the aerodynamics and tires of different models. In particular, if there is an inflection point where you get diminishing returns for increases in tire pressures, that inflection point could occur at a different tire pressure setting for different model/tire combinations. Hence I would like data from Prius gen III with stock Yokohama tires. I have just reset my pressures to 39/37 and will start collecting data.

 

It makes a big difference. A standard car isn't gonna reflect minimal changes like a Prius can.
Drivetrain drag alone could mask the tire pressure benefit. You're trying to compare apples to grapefruits

I've proven it to myself with 2 cars. An 08 Prius and the 2013 Prius C. Both return better MPG's with a 5 to 7 PSI increase in tire inflation alone. I tested it on the stock tires as well as the replacements.
Same exact results, every time. Try it and see.

 

I'm not doubting that the Prius has nice gains with pressure. I have experienced the same.

I'm only responding to the idea that the Mythbusters' methodology is off or that they weren't getting accurate fuel use measurements. "We" may not like the car they used, or think their findings apply to "our" car, but they were getting extremely accurate results for the test they were running and their method of fuel use measurement is far more accurate then the generally used Odometer indicated miles, gallons pumped at fill up method.

 

• I don't think that the Mythbusters did a very good job. If you plot their data (see attached), there's obviously something wrong with the 40psi data point. It looks like they did short trips, didn't check wheel alignment, didn't allow tires to come back to ambient temperature after every inflation change and run, skipped over certain inflation pressures, used a human driver, etc. Mythbusters are primarily entertainers.
• I also don't think that the Prius is the best test bed for determining rolling resistance. For a short trip, some of the energy used during that trip could have been derived from the burning of gasoline before the trip even started! One would have to do a lot of absolutely identical trips, with constant temperature, wind, etc. and would have to make sure that the state of charge was identical at the beginning and end of each trip --- if this is even possible. But, with enough driving some trend might be noticeable.
• I still believe that the NHTSA tests, done in a laboratory on a large test roller are the best indicator of the effect of inflation on fuel economy. They estimated a 0.3% MPG increase for each psi. This could obviously vary by vehicle and tire -- others have estimated as much as 0.6%. Interestingly, if you throw away the "bad" data point in the Mythbuster's data you also get about 0.3% per psi.
• Finally, I don't think there's anyone who thinks that increasing inflation pressure doesn't increase MPG. It's only an issue of how much and whether you like the change in the ride characteristic.

 

Yep! TV entertainment

Your chart shows a baseline of 35 PSI with the the biggest differential increase @ the 42 PSI mark

 

I know that is a commonly accepted response in regards to why "supposedly" the manufacturers recommendation for tire pressure is arguably or debatably not optimal.

But has anyone ever had that statement confirmed?

Some of the MPG gains reportedly happening with just minimal increases in PSI, I would think would be a huge selling factor in selling a Prius.

And really, today most test drives are so short in duration. Have you ever heard someone say they took a Prius for a test drive BUT then decided to NOT to buy one because the ride was so bumpy? Or have you ever heard the converse, that someone test drove a Prius and was very impressed by the "comfort" of the ride, but then became disgruntled when later they increased the PSI and the ride became bumpier and stiffer?

All I'm throwing out, is that it is possible the manufacturers recommended PSI...is really simply the best PSI to be running. Because most people that don't run it, seem almost to stubbornly conclude it isn't the best based on some somewhat unconfirmed assumptions. Assumptions such as, the manufacturers choice was made because they want the vehicle as comfortable as possible for a test drive.

One thing also, is I test drove the vehicle I actually purchased and drove off the lot. I think the morning after I had purchased it, one of the first things I did was check the PSI....expecting it to be low, or at least at exactly the recommended PSI.

What I can tell you is the vehicle I bought came from the Dealership with the PSI way above the recommended PSI. I was actually surprised. So at least at my dealership no "comfort" respect was being paid to the recommended PSI. I actually had to bleed air out...to get down to PSI still above the recommended sticker.

 

I actually do run the pressures indicated on the Prius placard and am satisfied with both ride and MPG. But what is really interesting to me is that Prius (or perhaps Toyota) doesn't provide recommendations based on vehicle load. All my other vehicles do -- an example attached.

 

We had the opposite experience; test drove the car, purchased it next day, and when a cold snap hit the following weekend, that tire pressure warning light on the MFD came on. So I got out my old airspace and inflated them all to the door jamb specifications. Caution light then went away.
Shifted to 39/37 pressures two days ago and am now collecting comparison data to see if it makes an mpg difference. Ride is somewhat harsher but not obnoxiously so.

 

I have always been sensitive to a tire's road performance, or non-performance as is the case with the original Bridgestone Ecopia EP20's. I understand and appreciate the tire choice, but they trade a lot to bring better mileage via LLR.

When testing tire pressure I'm looking for the best combination of cornering grip, front-to-rear balance (understeer/oversteer control), comfort, and tire wear. Here's what I observed.

Recommended 35/33

Test F-35 R-33 - Poor balance, little steering feel, fairly soft ride
Test F-38 R-35 - Better steering feel, rear tires are noisy over small bumps
Test F-37 R-34 - Front feels slightly mushy, no balance improvement
Test F-40 R-38 - Fair front ride/noise/control balance, improved rear noise but added wheel hop
Test F-39 R-38 - Better front ride/noise/control balance, stiff rear with strong wheel hop
Test F-40 R-34 - Front feel harsh, balance is good, with some mid corner bump wheel hop
Test F-39 R-34 - Best front-to-rear balance with some mid corner bump wheel hop
Test F-39 R-33 - Best combination of balance, control and comfort

Been running F39/R33 for about 5500 miles and the wear appears even.

 

I remember when radials 1st came on the market. Pirelli and Michelin recommended 32 psi front and rear on 13" wheels. Within a short time they were recommending 28 psi. because people were complaining about the rough ride.

The Toyota recommended pressure on my Aussie 2010 is 36/35. I'm running 40/38 and happy with it. when my car comes back from the dealers after a service it is 40/40.

 

I use factory settings on mine as i believe the tire manufactures engineers know a whole lot about tire pressure than
most of us.

 

All sorts of commercial and political decisions go into those recommendations. The only thing you possibly can rely on is that those recommendations are safe.