Tire pressure question

Higher pressure does not always improve fuel economy. Poor road surfaces and high pressure is actually worse than lower pressure on the same surface.

High speed driving doesn't benefit from high pressure as much as low speed driving because rolling resistance plays a larger role in fuel economy while aerodynamic drag plays the largest role at high speeds.

So, if you drive 95% city and the roads are not too bad I would go with as high a pressure as you can stand. Especially if you make a lot of short trips where the tires don't have a chance to warm up.

 

If I can ever remember to do so, I plan on having my tires done to 40/38. They are rated up to 55 and are probably around 35 at the moment.

 

My Prius C came from the dealership with 40/40 psi. I have averaged 65 mpg since I purchased the car on 07/21/12.

 

I was going to ask this very question. I checked my tires, and they were at 32psi, but the rating on the side very clearly said 51psi.
So I pumped up to 50psi. My avg mpg seems to have gone up a little. I say seems, because I bought the vehicle for my service work, and I literally never drive the same place twice.

Today in the pouring down rain, no defroster running, freeway speeds and in town, I ended the day at 53.7mpg. Before, in the pouring down rain I would consistently end up 49-ish.

 

The placard on the door jamb of your car states the manufacturers recommended pressure. The pressure listed on the tire sidewall is the max cold pressure for the tire. That does not mean you should run max pressure. In fact, I do not recommend doing so until you understand the potential drawbacks of running such pressures. A safer route is to run somewhere between 38-45psi. There is not much benefit above 45psi unless you are an extreme hypermiler.

 

+1 F8L's wisdom. If you're not a tire guru, follow the driver side door jamb sticker for recommended tire pressures. Tire sidewalls clearly state "max pressure", which is rarely a good idea. Overinflated [just like underinflated] tires can cause unpredictable handling response and strange tread wear.

 

More than you want to know?

Car Bibles : The Wheel and Tyre Bible Page 3 of 4

Scroll down a bit.

 

Now, see, I may not be an expert, but my understanding is that that sticker is rather generic and a safe minimum so Toyota can cover its butt. I would not go as high as 50, but see nothing wrong with PSI in the 38-44 range.

 

OK, I'll be the guinea pig then at 50psi. Somebody's gotta do it, right? So far, I am seeing a definite improvement. 4-5mpg consistently. Yesterday it was mostly dry out(which is rare), and I had to run approx 180 miles here in Seattle. Combination of hills, city, and freeway. Got home, it said 57mpg. Prior to going 50psi, I was averaging 52-ish after a dry day of work. In the rain, I've gone from 49-ish to 53-ish.

Worth noting - I'm an old fart. I have several vehicles. C5 Corvette, Newer Jeep Wrangler, lifted on 35's, and a Ford F350 Dually, which I use as a real truck. It doesn't get driven unless something very big or very heavy is being hauled. I have E Rated tires on the truck, running at the sidewall max of 85psi. I have a 5800lb pickup-camper I haul with it, an Arctic Fox 1150, their biggest. If I don't run the stated sidewall pressure of 85psi, the rear duals actually bulge at the contact patch and rub each other. They also get much warmer if they are not at the stated sidewall pressure. I've run these tires now for 3 years, no issues, no abnormal tread wear.

The Corvette DEFINITELY needs to be at it's sidewall rated pressure of 35psi, otherwise it corners extremely sloppy and unpredictable.

The Jeep rarely sees the road, mostly trails, and I keep it air'd down to 10-ish PSI. It gets hauled to the trail by the truck on a flatbed trailer. Can't count it in this game.

So I would like to go on record as saying if you DON'T run the pressure listed on the side, you are more apt to cause problems, not vise versa. Remember, these not normal tires, these are LRR tires. The goal is for them to have the absolute minimum rolling resistance possible. It is my opinion that these new LRR tires say 51psi, and need to be there for them to work as designed by Goodyear for the least amount of rolling resistance.

Just like my truck won't haul it's rated load unless I'm aired up, and my Corvette won't corner as it should, my Prius needs to be where the tire Mfg said it should for them to do they're stated purpose.

This is MHO... We'll see.

 

Over nflating the tire may cause them to wear out faster. So any money you save on gas will just go into buying more tires. If the criteria is "over all cost of ownership." the thing to do is measure tread wear across your tires and check that the wear is the same in the center as the edges. Over inflation will cause the center to wear first. Yo will have to replace the tire even if there is still rubber left on the edges. Trouble is that it will take 20,000 miles before you have your answer about the most economical tire pressure. So,.... let's trust that Toyota figured this out for us. You'd need a LOT of test time to find the most economical tire pressure that balanced tire tread life and gas consumption.

 

If Toyota selected the tire pressure that gave best tire wear I would agree with you. Instead I choose to agree with the dozens of PC users reporting even tread wear with pressures in the 40s. The belts in radial tires greatly reduce the tendency of the tread to wear in the center at high pressures

 

Please do more research on this topic. Your opinion is not backed up by current information. There are thousands of posts about tire inflation and many of us have tested different inflation pressures. Our experience as well as those on many other forums has shown your opinion is not true in the case of the Prius and the narrow tires they use.

No disrespect intended.

 

In this instance, I'm also more likely going to trust the folks who actually made the tire rather than the car manufacturer.

 

I guess in a nutshell, that's my point. Goodyear says run 51, I'll run what they say.

My truck tires say 85psi, and sure enough, they work best at 85psi. My Corvette, same thing, says 35psi, I run 35psi. Prius tires say 51psi, so technically I'm UNDER inflating them at 50psi.

 

The car manufacturer and tire manufacturer arrive at their pressures two very different ways.

The car manufacturers recommended pressure is a compromise between comfort, tire life, handling, etc.

The tire manufacturers pressure is the maximum safe pressure for that tire. That is pressure with the tire at ambient temperature, not after the tires have warmed up from driving.

We can safely experiment with pressures between those two pressures. Most people don't care for the comfort at tire manufacturers max pressure, some do. Some go above the manufacturers max pressure and don't sweat it because there is some unknown safety factor built into the tire design, there is little reason to do that unless you are a hard-core hypermiler.

 

I've been running at ~50psi since the day I took delivery. I ran the 2006 at ~45psi as I recall. At max sidewall pressure the 15" Goodyear Integrities still showed edge wear, a sign of underinflation. The 17" tires do not show abnormal tread wear.

 

You guys are talking about some pretty high pressures here. Not that I think the tires won't take it, just that the ride quality will suffer. Running 42/40 in my C makes the ride pretty stiff from my perspective.

 

I had my 51 PSI rated tires inflated to 40 PSI. Ride is ever so slightly stiffer, but not so much that I think anyone else would notice.