Tire Pressure does make a difference!

back up to .../tpms/ in that URL and read the whole thing, then...
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<div class='quotetop'>QUOTE(molgrips @ Sep 4 2006, 06:04 PM) [snapback]314145[/snapback]</div>

Hi Molgrips,

My personal experiences was that I had very poor cross wind tracking, and in-turn handling from the car at the recommended pressures. At the same recommended pressures, I installed the BT Tech plate and the in-turn handling problem went away, and the cross wind tracking became more like other cars I have driven. No change in Tire Pressure.

Later on, I upped the tire pressure to 40/38, and saw improvement in fuel economy similar to Galaxee, except with my commute the MPG went up to 55-58, from 50-53 mpg.

I did experiment with the pressures before settling on the 40/38. I found if I got the pressure too high the handling got quirky. I think it was understeering. At the time, I did this the tires were warmed from the testing. So, I do not know what PSI this onset at. I attributed it to the patch starting to go convex, while the normal pressures result in apparently an concave pressure distribution across the patch.

After this, I bought a back-up battery/tire pump thing so I could change the pressure at home. Then I started setting the tires in the early morning, as dew point tends to stabilize air temperature, and the sun is not shining on the tires. Using the 1 PSI per 10 degree F rule, 40/38 at 70 F gives the tires a margin to be safe up to 100 F. Besides, there is the gauge accuracy to consider as well. Lately, I have been setting at 41/39 at 60 F in the morning.

 

I haven't done any mileage tests on the Prius to determine the effect of different tire pressures but I did do some tests on my previous car, a Honda Accord. I drove it for several thousand miles on the highway with the tires at 32 psi (psig actually) and then increased the pressures to 42 psi and rechecked the mileage. The weather was very close to the same, I went over the same highways at the same speeds, etc. The mileage increased by about 10% with the higher tire pressures.

An unrelated question, I notice most PC people maintain the 2 pound front/rear difference. Does it really make any difference? I have tried 42/40 and 42/42, I have tried 44/42 and I am currently running 44/44 and I can't tell that the 2 pound difference between front and rear matters.

 

i believe the 2psi difference is because the front is heavier than the rear

 

The weight differential between front and rear definitely makes a
difference, which in an unloaded car demands up to 3 or 4 psi tire
pressure difference to make up for it and get front and rear tires
turning at exactly the same rate. Two seems to be the adequate
compromise to allow for varying loads in the car and pretty much
land on either side of balanced.
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I'm only on my third tank of gas...my first tank gave me 30mpg, second 41 mpg....then I upped the tire pressure (of course, there are multiple variables). I know run at 40/38.

On my third tank now, I am at 44.7 mpg...although I have a few slightly longer trips in there and have been using electric only mode (don't have the switch..the car goes into electric only at times when it won't/can't go into glide)....

I do feel the car is a bit more "bumpy"...a harder ride...and sometimes, when I have managed to get into glide and hit a bump, the ICE comes on and I have to feather again to get into glide....but otherwise, I have a feeling that in the long run, I will keep the 40/38.

Thanks for the picture, hobbit! That makes me more certain I will keep the higher pressures.

terri

 

Regarding tire pressure. My Prius now has 1,200 miles, picked it up < 1 month ago. So, just for fun, decide to mesure the existing tire pressure based on all the discussions for and against higher ones.

Sitting in my garage, maybe 80 degrees, front is just over 40PSI, back is just over 39 psi.

So... Either the factory puts them at that, or, the dealer did. (or someone decided to add air to the tires while the car was unattanded I suppose). Either way, makes you wonder about the sticker on the door...

I find the ride quite good myself.

 

Ditto on that. I couldn't figure out why I wasn't getting the 55+ mpg I got late last summer and then noticed that the left/rear tire looked low. I put a guage on it and the tire pressure barely moved the scale. I pumped it back to the 40 psi I run in the rear tires and my mileage immediately jumped back up; last tank was 55.9 mpg. All the rest of the tires were where they should be +/- 1 psi.

The car is so light that there's little weight to cause a low tire to pancake. Check your air pressures at least once each month. Meanwhile I'm going to watch and see if I have a slow leak in that tire.



<div class='quotetop'>QUOTE(dulcimer @ Sep 4 2006, 09:57 AM) [snapback]313941[/snapback]</div>

 

<div class='quotetop'>QUOTE(Tempus @ Sep 4 2006, 09:26 PM) [snapback]314314[/snapback]</div>

I use 3.2/3.0 bar (46/44 psi) on the (european) standard Michelin Pilot Primacy tires (max 3.5 bar or 51 psi).
I think I have correct pressure because my tires wear very evenly, center and outer edges. After 47000 miles (75600 km) I still have 2.5 mm on the front and more than 5 mm profile on the rear tires, better than I ever had with previous cars.

 

<div class='quotetop'>QUOTE(dulcimer @ Sep 4 2006, 06:57 AM) [snapback]313941[/snapback]</div>

The higher you go, more bumps you will feel since they acts as shock absorbers too.

I run mine at 44 in front and 42 in back. .... I used to run 42 and 40 but went higher.

 

<div class='quotetop'>QUOTE(tumbleweed @ Sep 5 2006, 09:55 AM) [snapback]314519[/snapback]</div>

I keep the 2 pound difference, at 35/33 that's a 6% difference, but if you go up to 42/40, then it's a little under 5%. Probably still enough. I'm like you though, I took the car in to the dealer for the 5K, they told me they rotated the tires and checked the pressure, etc. I made a mental note to make sure they didn't drop it back to 35/33 (I keep it about 39/37), but with the rush to get ready for vacation I didn't do it until I drove to Minnesota. There I found out they simply rotated the tires, and I had 37 up front, 39 in back. I corrected it, but couldn't really tell the difference. Maybe if I was on a slalom course with smooth pavement I might notice...? Roads in northern Minnesota aren't known for being smooth, but the Wisconsin interstate was fine.

<div class='quotetop'>QUOTE(TonyPSchaefer @ Sep 15 2006, 12:44 PM)</div>

Hey, that's not fair! I have the same weather as you, and this last tank was bad! (About 48 mpg). Of course, I filled up on the coldest day of the week after the hot spell (we went from the 80's to the 60's in one day), so the gas was more dense...that's my excuse, at any rate, and I'm sticking to it. I'll see what happens on this tank and average the two together, maybe I really topped it off this time, but I don't think so.

 

<div class='quotetop'>QUOTE(TonyPSchaefer @ Sep 15 2006, 10:44 AM) [snapback]320168[/snapback]</div>

I thought about going higher.. but the hot summer weather around here made me go chicken.....