Tire pressure mpg tests - tire inflation

Max psi pressure and burst pressure are two different things. When you hit a pot hole going 60mph internal pressure in the tire will exceed 40-50-60-100 etc.. With that said, i'm running my NAH at max sidewall pressure and will increase prius to max after a few tanks logged.
27K miles on NAH at 44PSI
56K miles on toyota tacoma at 55PSI
Thanks to MSantos for taking the lead on real world PSI tests. The rest of us appreciate it.

 

Y'know that sounds reasonable given the impact, but I thought pressure was inversely proportional to the volume occupied by the gas. I imagine it would really be hard to reduce the total tire volume by more than about 25% without bending the wheel so much that the tire's condition wouldn't matter. So 35 PSI might go as high as 46 PSI for a fraction of a second, still way below the 200 PSI needed to burst the tire. For a tire at 50 PSI, it might pulse up to 62 PSI.

Those numbers might be wrong, but that's what I get.

The interesting fact is that so many people say they have been running on high PSI for years with no problems and improved performance. It would be intersting to get the real, inside story on this issue. More Polls???

 

I just increased mine from 40psi all round (dealer set and sidewall maximum) to 52/50. The difference in MPG was immediately noticeable. I don't have enough data yet to present a worthwhile analysis, but I could tell right away in how much easier it was to keep it well over 50mpg. The ride was noticeably harsher too, which may be the real reason for the OEM's recommendations being where they are. At any rate, I like it so far and intend to leave it for a while to see how it works. I'll report back if I blow one out

 

I think there may be an added advantage with 45 series tires and perhaps the 65s as well. If you really hit a pot hole hard and have lower tire pressures you may bend a wheel, with higher pressures such as you are running you may destroy the tire but the wheel has a better chance of surviving. Tires, of course, are much cheaper than wheels. Another poster on here had to buy a 17" wheel and I think he said it was $455 or something like that. I have no idea what the 15" wheels cost but I'm sure they are not cheap.

 

Best of luck, Rachaelseven, with the new PSI! I'm now at 42/40 up from 32/32 where my dealer had them. I am hoping to find time to make three 20-25 mi round trips (after a 10 min warm-up) over the same course on the same day: maybe at 50, 40, and 30 PSI to see what the MID says my MPG are for each trip.

This car is getting addictive, but in a fun way

- g

 

Ditto that! I wonder if this variation in dealer pressure setting explains why some people are saying they are having a hard time getting the EPA mileage rating and others are finding it so easy?

 

Interesting point. On the other hand, if the data I took today is close to correct, even a dealers' variation of 5 PSI is only going to change FE by about a third of an MPG. There *is* an improvement in MPG with increasing tire pressure, but it seems to be smaller than many people think. Of course, every little bit helps...

I started a new thread in the FE Forum, hoping to encourage more people to run conrolled PSI tests and to comment on those runs.

 

I bumped both front and rear up 4 PSI to 42/40 and after 60 miles I'm getting almost exactly what I was getting before.

UPDATE: Maybe not...see post #43: http://priuschat.com/forums/gen-iii...re-mpg-tests-tire-inflation-5.html#post920301

 

Could someone explain to me (a non-engineer) how it is possible that at a higher PSI, rolling resistance is reduced, i.e. less of the tire is on the road, and yet the tire is more resistant to hydroplaning on wet surfaces? And how a tire could have both less traction on dry and more on wet at the higher ratings? Seems counterintuitive.

I am interested in safety first and any MPG increase second. Don't care too much about ride comfort. What is the best PSI to get these Ecopias to stick to wet and dry (and eventually a little icy and snowy as I am in the Northeast) roads? Thanks for your input!

 

I, too, am not an engineer, but here's my understanding.

A larger contact patch giving better traction on a dry road is intuitive.

The reason a smaller contact patch gives better traction on wet roads (and snow covered roads) is related to hydroplaning and PSI--and not inside the tire! The smaller the contact patch, the less area the car's weight is spread over, and therefore the higher the pressure applied to remove the water between the tire and road.

 

Really??? That is mind blowing. I had been lowering the tire pressure to manufacturer's recommendations on my 05 for winter driving, assuming that I was taking a small hit on MPG for the sake of having more tire on the road and better traction in slush and snow and ice. Then I had been bumping it up a bit (maybe 5PSI over, not as extreme as some of you) for the rest of the year, assuming that it doesn't matter as much but that I am probably still decreasing wet traction with higher PSI. Did I have it wrong? Is everyone else (and I mean not you SOCAL folks but everyone else who drives in places with four seasons) leaving it bumped up all year round?

 

Why? Unfortunately, I am an engineer (though not of the materials or mechanical type), so this is not at all intuitive, as it violates the basic mechanics in my early training.

As for hydroplaning, I go on experience. My one car with a lower placard pressure, 26 psi instead of a traditional 32, was surprisingly susceptible to hydroplaning. Pushing up to 32 made it feel 'normal'. After that, I applied a selection bias, believing articles that supported my experience.

 

I think this is a mixed bag. Here's my opinion in laymans terms.

Higher psi normally means the tire will get more firm and have less bulge and thus less contact patch on the road. (I'm not sure with the rigid sidewalls of the 45 series 17" tires that the impact will be as great as on the 15" as they wont fles as much even with lower pressures.) If the tire doesn't bulge out it will have less contact patch from side to side and thus less rolling resistance for better mpg. They also will flex as you go around curves where as the harder tire will not flex and create a feeling of better handling at the expense of a firmer ride. There will be a limit to where the handling breaks over from increased performance to where they will break away easier. Most hypermilers are looking for the hard tire at the expense of the handling. For me a happy median will be as firm as I can get without killing th ride from harshness and providing me optimal grip in curves.

As for the hydroplaning, if the tire does not bulge out but has a more narrow tread width, it will be likely to push the water to the sides rather than to run up over the water causing the hydroplaning. Some wide tires have a tread designed to more effeciently move the water away from the tire. A balding tire will tyipcally hydroplane a lot easier than a new one due to the tread which acts as the channel for the water to escape. Still in the rain, there is a balance of determining how thin to go to reduce hydroplaning as compared to how much pressure to use to not reduce the contact patch significnatly enough to cause a loss of traction on a slick surface.

Hydroplaning and loss of traction on wet pavement are two different things but closely related. A 3000# car going around a curve with pumped up tires will lose control faster if the surface is simply wet than one where the tire has a wider patch. However if there is standing water (as you get during the storm) the wider tire will float over the water causeing a loss of control (hydroplaning).

As for snow and slush, Skaters use ice skates, not boots. The thinner blades are good for directional control. The thin skates also would cut through slush better. The boots would tend to float (or cause you to hydroplane). However the more tread on dry road that you have would increase stopping abilities. If you hit a patch of ice anything will slide. I believe the width, if kept to the limits between the OEM 15's and 17's won't make that much difference. The tread pattern may cause the most concern.

 

All good info. So being as most of us will not hop out of the car and change PSI when it starts to rain or snow or the sun comes out, if I overall care about the following in priority order:

1. Traction/safety on all surfaces
2. MPG
3. Handling/ride

does that mean I should stick to the typical average recommendation of this board (which seems to be to bump PSI up somewhere about 5 PSI above manufacturer's recommendations) year-round and regardless of conditions? Or are other people changing based on the season?

And the other question would be whether there is a point beyond which increased PSI does not equal increased traction in slippery conditions.

I guess my assumption would be that if higher PSI= higher chance of slipping on dry road but less chance of slipping on wet or snowy road, the former is less important and hazardous than the latter. Therefore I should go with the somewhat higher inflation. Make sense?

And one final thought: it would seem to me that if increasing the tire pressure results in a firmer, harder ride, that also means that more of the impact of bumps and jots are absorbed by the suspension system and less by the tires. So the factor that people may not be considering is whether, at significantly higher PSIs, you mayb be getting extra MPGs but ultimnately causing increased wear and tear on your venhicle's suspension...

 

I totally agree with this thought. That is why I will stick with the manufacture's suggested PSI (inside driver's door).

Peter

 

I found 40 psig to add considerable roughness to the ride, but 36 seems to be a good trade off for all considerations. That's just me though. I have the 17's which are already adding a rough ride to the equation.

 

If you wanna know what tire performs best in what condition, have a look at WRC rallycars. In snow and ice the tires are small and hard. On dry tarmac wide and a lot softer. With the Prius on 215/45R17 87W I'll go a tad higher than the recommended 33/32 psi in the manual (btw 36/35 for the 195/65R15 91H on the other models). Probably up to 38/37 to stay on the safe side, as I am no daredevil

 

Now, If we can just get rid of the tires and use just the rim....LOL

 

As for snow and slush - since we have many months of it around here - I would think a large patch area is desired. Just as you lower the pressure in your tires when you drive your Jeep in the sand dunes, I would think that you want as much tire and tread on the snow as possible.

 

The reason to lower your tire pressure off road is to float the tire on the given surface to get the most traction possible. On a road you actually want to cut through the surface to get down to the solid road where the best possible traction exists. As said in a previous post, look at World Rally Championship driving. They use very skinny high profile tires for the snow events.