Tire Inflation

When we got the Prius from the dealer, the air pressure was 32/32 - not good. I bumped it up to 39/37 to increase the gas mileage a bit. With the roads around here (especially in the District), I'm reluctant to go any higher. I guess I'm looking for a good compromise between economy and comfort.

 

Yeah, it's more for mileage than anything. It's amazing how one low tire can affect it. I keep mine at 42/40 and have had no problems.

 

One thing that I wish I didn't have to mention is to check the tire pressure after you have the car serviced. Our very efficient former Toyota service people took it upon themselves to remove eight pounds of pressure from each of our tires when the car was in for an oil change! And they put 4.0 quarts (roughly 0.5 quart too much, depending on who you talk to...) of oil in the car. And, oh yeah, they wanted to use 10W30. They told us that every Toyota uses it. Our answer, "Well, this isn't every Toyota, is it?"

I would suggest checking the tires once a week when they are cold. I use a pretty inexpensive digital guage and a compressor run off of one of the car's 12 volt outlets.

 

Yeah, I seem to have an ongoing issue with the service shop when it comes to tire pressures. The sticker on the side of the car says 35 psi fronts, 33 psi rears. Whenever I get my oil changed, etc., the car usually returns with 32 psi all around.

I just got back from picking up my Prius after having the SSC 60C work performed. I left a typewritten note with the service people to check the pressures and adjust it to 35/33. I checked the pressures before I even got into the car - 32/31. *sigh* I guess if you want something done right, you gotta do it yourself!

*EDIT* Hey Richard, can you recommend a 12-volt air compressor to me? I'm in the market for one like you describe.

 

I use 36/34. Even at this pressure the car rattles like an old car, at 42/40 the suspension or the dash may fall off.

 

Another point to consider is the tire you are riding on. Complaints of a rough ride or noise may be attributed to the OEM tires. I changed to Goodyear ComforTred tires and 42/40 pressure and like the ride just fine. No excessive tire wear at that pressure, and not harsh riding.

 

Glad to hear you like the Comfortreds, Bill. I just put them on, and am also using 42/40. It is at least as smooth as the bald Integrities at 38/36 (probably a combination of the sushioning layer in the tire, and the full new tread). The Integrities wore out on the shoulders, requiring early replacement. I am hoping the higher 42/40 will avoid this with the Comfortreds.

I did notice a return to the twitchiness. Could be the sticky new treads vs baldness, and also the new tires taking a set on the car.

MPG dropped by a good 5mpg (only one tank burned so far), but the new tread and tire set may be the cause, not the tire design. Remember the Integrities are not LRR, either. In a couple of months I'll see if it creeps back up again.

 

<div class='quotetop'>QUOTE(Goofa @ Jul 6 2006, 10:06 AM) [snapback]282084[/snapback]</div>

First of all, having them higher than 35/33 is NOT NOT NOT overinflating! The tire is rated for up to 44 psi cold. I generally fill mine to 44/42 or 43/41 because I don't check them quite as often as I should. I have never found the ride to be excessively harsh, but then again I really hate the "mushy" ride that some big luxury American cars have (think of my sister's Buick Park Avenue...).

Second, you'll get a lot more premature wear if you keep your tires at 35/33. IMO that figure was arrived at by Toyota for more ride comfort, NOT for tire longevity. Things like braking distance, wear, mileage, or just about anything are improved at higher psi. I think the only things improved at low psi are ride comfort (a subjective thing) and if you drive through gravel.

The Classic Prius had Bridgestone Potenzas, and they were rated at 50 psi. Toyota recommended 35/33, and everyone's tires wore out WAY too quickly. There was eventually a TSB that changed the recommendation to 42/40 (I think). The only reason to stay with 35/33 is if your butt can't stand anything higher....

 

I started with 44/42 on my ComforTreads and kept letting air out in 2psi increments until I was happy with the ride. (It was easier doing it that way than adding air). I ended up with 40/38, lower than many people here, but the roads in Chicago, especially in the city, aren't that great. Else I would run 42/40.

 

Currently 41/39, with sidewall maximum pressure of 50 psi. The "C" has recommended pressures of 35 front/33 rear; I kept them at that level through the winter and allowed the warming weather to increase the cold inflation pressure (with "cold" now being 50 F. or so rather than zero F.). Better mpg with higher pressure, clearly. Firmer ride, for sure; at the levels I am currently running, not so much firmer as to be unpleasant. Tread wear? I have no idea. I'm not seeing anything abnormal, but I haven't been doing this for very long. There are some online who say that tires with higher pressures have longer tread life; others who say that you get center tread wear. Stopping distances/traction? Likewise, I have no idea, beyond some saying that traction on snow/ice is diminished by higher pressures; that is why I was down at 35/33 through the winter. I would be very interested in knowing more about this part of it.

A careful re-reading of the applicable part of the Prius C owner's manual gives multiple notices as to the dangers of underinflation, also not to allow cold inflation pressure to exceed the maximum printed on the sidewall. But they never quite come out and say "don't inflate over what we recommend."

Here is a pretty good page on tire inflation issues from Cooper Tires:

Proper tire inflation | Cooper Tire

"Overinflated tires (over the maximum molded on the tire sidewall) are more likely to be cut, punctured, or broken by sudden impact from hitting an obstacle, such as a pothole." That makes sense. But again, nowhere do they say to not exceed the manufacturer's recommended pressure, and they make a point of defining "overinflated,"

 

LOL. Until I saw Bill Merchant I didn't realize how old this thread is. Where is Bill?

 

Bring out your dead, bring out your dead...

Funny though: front-heavy cars are nothing new, but Toyota's the first I've encountered that stipulated higher front pressure.

Higher inflations than spec have got to increase impacts on bearings and suspension components. How much, who knows, but food for thought.

My take: assume Toyota's erring on the side of comfort, and accordingly go slightly above spec, say 3~4 pounds at most.

 

A lot is made of tire pressures (on these forums, at least) but I'd like to inject some empirical data into the debate. The short story is tire pressures do not add much to lowering fuel consumption except when the tires are under-inflated. I'd lke to document now why I think that.

Having gone through a set of tires and paying careful attention to document the pressures and tire tread wear, I have gone back to almost stock pressure. I went through the gamut of 44F/42R –> 42F/40R –> 40F/38R –> 39F/37R –> 37F/35R –> 36F/34R. This is because I have demonstrated to myself that any amount of inflation above the placarded gives tread wear that indicates over-inflation.

Initially I thought 38F/36R was good until the evidence showed me it wasn't. And this continued until I'm at 36F/34R and this is more that it gives me a buffer to ensure that I don't end up under-inflated due to weather related atmospheric pressure changes. Under-inflated definitely hits your fuel economy.

Interestingly enough, I haven't noticed much improvement/degradation in terms of fuel economy as I have been dropping the pressures in percentage terms, until the tire pressure is less than 35F/33R.

I was disappointed that in running higher pressures led to my tires wearing out prematurely (despite my best efforts to monitor tire wear), and has cost me, maybe, 6 months of use, if not more. I calculated a 16% loss of tread life with the centers wearing out before the sides. So, what ever minimal fuel savings I may have made have more than been wiped out by the premature demise of my tires.

I feel I can confidently say:

• that higher tire pressures yield minimal fuel economy benefit;
• that higher pressures lead to tires wearing out sooner than they should.

If you think that you are getting hughly better fuel economy (and MPG being an exponential scale, exaggerates the benefit – use gal per 1000 miles in your calculations to see what I mean) then I would say it is mostly the placebo effect. Yes, you will get a better MPG, but at the expense of your tires.

Over-Inflated tires decreases fuel consumption by changing the tire footprint profile from flat to semi-circular, which means the tire is only making contact in the center. So, it makes sense as this give a lower resistance. It also makes the tire less safe – less grip (wet and dry), less stability/traction when cornering, less tire life, greater stopping distances, more waste (throwing out tires with still useful tread on the outside edges.

Even though it will not translate into other markets, in the interests of full disclosure the tyres were Bridgestone Ecopia PZX 195/60 R15 88H. Also of note, over the life time of the tires, even though I have changed tire pressure of the tires, I have had tank variations when the pressures were consistently 37F/35R from 4.0 L/100 km (best) to 5.3 L/100 km (worst) with an average of 4.63 L/100 km over the last 86,447 km. The biggest factor in the high/low is the driver. I can consistently get L/100 km in the low 4's whereas the other half is consistently in the high 4's/low 5's.

Even though I have documented empirical evidence, I'm sure there are those that will still disagree, but those that have an ear to listen, make of it what you will.

I realise this is an old thread and what others have said in the past might have been true when they made the claims, but tire technology has moved on at an incredible pace in the last 12 years. I just thought this was a good thread to throw in my data-points. I hope it helps someone make the decision a bit easier.

All the best.


Imperial measure equivalents:

	Metric	US Unit	Imperial Unit
1	4.00 L/100 km	58.8 MPG	70.6 MPG
2	4.63 L/100 km	50.8 MPG	61.0 MPG
3	5.30 L/100 km	44.4 MPG	53.3 MPG
4	86447 km	53716 miles	53716 miles

 

Dolj, thank you for the data! I was hoping someone would chime in with this sort of thing.
I might re-think my current tire pressures. The safety issues are important to me. So is fuel milage, but it tips the scale for me if the milage benefit from higher pressures is small. Mendel, that is also a good point about increased suspension/bearings wear. I want to drive my "C" for a long, long time.

Pardon my bumping an old thread; that seemed better than starting a new one for a topic that has been much discussed.

 

Actually my thoughts are evolving on tire inflation.
I think at highway speeds, wind resistance takes over and tire pressure effect is minimal but 1-2 MPG.
But slow driving (which we do more these days being retired) tire inflation probably is bigger factor.
We still drive the same speed in theory, but now its around town more.