mileage versus tire pressure

I think I would be a tad conservative trusting that your pressure guage is extremely accurate and that the tires, though spec'ed at 50 psi cold will REALLY be OK there. Could be the mfg sets the value low as compared to their "unsafe any higher" tests, 'just in case' but still

 

I'm likely to be flamed for this, but whatever...

Folks, there's a reason the tires have recommended and maximum pressures. They recommend you stick with what they recommend. If the recommended pressure is 35, and max is 48, then you don't want to go straight to 48. 40, maybe. But inflating to max pressure is really not a wise move.

When you drive, the air inside the tires gets warmer and the pressure goes up. As the tire ages, it starts to wear down. It's inevitable. If you're running at the maximum pressure and not the recommended pressure you're going to reduce the life of the tires, pure and simple.

 

What you are stating here is already considered when stating the maximum safe pressure of the tire by the mfr. Tire pressure is measured "cold." If the tire is inflated to the maximum safe rating mentioned on the sidewall, it will still safely handle the "hot" tire pressure.

 

I think I am speaking for a lot of folks when I say that I did not buy this car for the improved mileage. The mileage is a great talking point and gives me bragging rights. But it is only one of a great many selling points on this car. To compare it to a rattle-trap Geo Metro, or a dirty diesel Rabbit is a joke.

 

That is true, except the car's suspension was not designed to be optimal to that pressure. When you play at the edge like this, you must be aware of the compromises. Tire manufacturers provide a maximum cold pressure value for the specific tire, and NOT that tire's application to the Prius model. Things that are potentially lost by doing this are: tire life; wet or snow traction; and, general handling characteristics. Any deviations made from Toyota's recommendation needs to take all of this into consideration. Afterall, they have spent considerable effort coming up with recommended values. Tire sidewall flex is key to how a car grips the road and rides smoothly.

 

Drew,

Why not visit the local hardware store and see if they have some appropriately sized solid rubber tires.

:lol:

It is not opinion, it is fact. Sure, changes can be made, but it is reckless to do so without careful consideration of the tradeoffs.

Skidpad, anyone?

Paul

 

Keep in mind what else has been said in this thread about higher tire pressures... you are shortening the life of the tires. I remember seeing posts from classic owners who had to have tires replaced very early (under 20K miles even) and I wonder if these were the same people who were overinflating their tires.

 

OK, here's the preliminary results of my first weekend of pressure VS mpg testing. Please don't call the reaction to over pressure knee jerk. My comments were based on logic, researched data, and of course personal experience. There are other "experts" in the group that might disagree that's all. As a Mechanical Engineer I don't, and never will, "knee jerk". But instead I research the data and try to evaluate it in the most objective way I can. Then I try some real world experiments to valid or disprove what my research might suggest. I have nothing to gain by saying extra pressure is the wrong or right thing to do.

My qualifications are about 5 years working in a very reputable tire store back east. Radials were introduced while I was there and of course the state of the art has changed and improved since then. I also have a BSME that I received in 1980. I do not consider myself a "tire expert". An expert would be one that has worked in the industry, designed tires, is familar with the very non-linear and difficult to analyse behavior of tires, in my opinion. I've performed lots of FEA (finite element analysis) on rubbers and elasomers and believe me when I say that it's pretty complicated stuff even with the right tools. As stiff as a tread section on a tire can be, since it's not perfectly rigid, eventually at some internal pressure and vehicle load it will begin to deform into a somewhat curved shape due to the boundary conditions, pressure, and contact pressures. The debate is at what pressure will that begin to happen.

However many here might have long term experience with their cars and tires and I'm not minimizing that, I'm just saying while knowledgeable as they are, unless they have participated in the actual R&D effort designing radial tires I would not consider them experts...just knowledgable. If they have that expert experience, that's good too. I would love to have a tire design engineer step forward and add to this discussion.

So on to the data. I posted data yesterday that had a big mistake. That was the data for the 35/33 pressure run. As I found out last night I had not lowered the pressure so that was the 42/40 data. I posted that in the other tread on tire pressure. Those of you familar with Wayne's program know there are lots of factors that can affect gas mileage. The things not on his list like traffic, lights, hills can really affect a test like this. I've tried to normalize all the factors that I could, but the ambient temperture for the low pressure test was somewhat higher on average (5 deg). That was offset somewhat by the increased traffic today. I have not checked the computer data for the low pressure run with a fillup because it too warm here now and so will fillup tomorrow morning. But my 1st set of test data shows a 3% improvement in gas mileage with 42/40 VS 35/33. I did 3 round trip runs to work, 11 mi each way, 18 lights each way, 2 mi of freeway and lots of hills. I plan to repeat this next week, switching the days I do the low and high pressure test. Here's what I've learned so far:

42/40, temp range 55-63F, no appreciable wind
71 out, 41.1 back, 56.2 RT
78.8 out, 40.6 back, 59.7 RT
77.8 out, 37 back, 57.4 RT
Computer overall: 57.8
Actual: 53.8 (the trips to and from the gas station acted to lower the actual somewhat along with the "computer optimism".

35/33 temp range 59-70F, no appreciable wind
67.1 out, 42.1 back, 54.6 RT (caught 2 major lights, lost momentum)
77.5 out, 35.3 back, 56.4 RT
75.2 out, 41.4 back, 58.3 RT
Computer overall: 56.1
Actual: TBD tomorrow morning.

Qualified experts state that consistant gas mileage data is very very difficult to produce. There are too many variables to control that have a significant influence when looking for a couple of percent change. And when someone suddenly has say a 10 mpg improvement under an uncontrolled commute, I really question the vailidity of the immediate conclusion that it was the pressure in the tire that did all of it. It needs to be repeated over and over again and other variables carefully controlled. So I offer this data as preliminary. Since these runs take about 2 hours each day, I think I'm only going to repeat once next week. It's just too nice here now to spend weekends in my Prius doing a science experiment, as fun as it might be.

 

Oh I just realized that I could easily (well with some time) do a similar test to evaluate Mobil 1. I plan to go to that at 5K and I'm about 3700 now. So if I get to the point where thing looks reasonably repeatable I might do that. Luckily the temperatures are usually pretty moderate here so I might be able to minimize temperature effects. Although today did get warm fast and by 9 AM it was 70.