Max Side wall tires

[korbynlehr]

What am I looking for on the tires that will tell me the maximum air pressure? And what are the benefits to maxing it out and what are the cons to doing it?

[brucepick]

For the reasoning and benefits, see CleanMPG's "Articles" section.
In "Articles", see "Beating the EPA..." by Wayne Gerdes (Xcel) and "External Factors Affecting Mileage... " by tarabell: http://www.cleanmpg.com/forums/showthread.php?t=3168.
Both have good notes on tire pressure, plus other topics.

Briefly, higher tire pressure gives reduced rolling resistance, which helps your FE. Additional details in the two articles above. The "con" is that the ride gets stiffer. In my car I was able to increase about 50% above the car maker's recommendation. If your tires are currently well below max sidewall, I suggest you increase maybe 4-5 psi at a time. Get used to the higher pressure and see how the car handles, then increase again if within your allowed range.

Where to find the sidewall pressure spec:
On the sidewall, close to the metal rim edge. Fairly small embossed print beginning something like "Maximum load capacity xx00 lb. at xx kpa or xx psi". Kpa is the metric air pressure unit.

[korbynlehr]

Thanks for the info

[korbynlehr]

Okay this is something I thought of on the way home.

If I increase the pressure to the maximum the wall suggests this reduces the resistance won't this also reduce traction? So when it rains I will basically have less rubber on the road? Is this safe? What kind of increase are we really talking by increasing the tire pressure?

[tbaleno]

You will get better traction in the rain for exactly that reason. The tire will be cutting like a knive through the water. Rain, and dry roads, and handling are better with higher psi. Driving in snow is worse. Also, heat buildup is less with higer psi since the sidwalls don't heat up the tire by flexing as much.

So, other than snow, and harsh ride there are not many negatives.

[Dan]

If you want to sell yourself on the idea, find a deserted patch of dry road or parking lot where you can do some tests.

Now roll down your windows and take a predetermined 90º turn at a predetermined speed. Start out slow (5mph) then repeat the test, each time adding 1-2 mph to the entry speed of the turn. Stop the tests on the first turn that gives you ANY play (lateral movement) or when you hear the tires on the road. Not a SKREEECH, but listen for small chirps. This is the first sign of traction loss, and stop the tests at that speed.

Now record your tires PSI and the highest entry speed you were able to maintain without play or chirps from the tires.

Now add 5 PSI and repeat... You will be amazed!

I found this out by accident. I have the max entry speed on all my turns predetermined. Most are pretty low since they are in the city so there are other governing factors, but one (coming off the highway) is a "snowboard turn" and I take it as fast as I can. Normally this is about (35mph) and I usually get lots of chirps as I corner.

Then I went to HybridFest and for the MPG Challenge I increased my pressure by 10 PSI. When I was done with the challenge I went home and got back into my daily grind. The first time I hit the snowboard turn I was amazed. No chirps at all, felt like I was going 20mph instead of 35mph. At first I thought I must have hit the turn a little cool or with regen enabled, but same thing next few days. Finally I put it together... The 10 PSI had increased my maximum "snowboard turn" dry-road entry speed by a solid 5mph. Now I make it all the way to the light without pulsing up at all.

I'm a believer. But then again.. got no snow in Texas in August.

11011011

[pumaman]

Originally Posted by tbaleno

You will get better traction in the rain for exactly that reason. The tire will be cutting like a knive through the water. Rain, and dry roads, and handling are better with higher psi. Driving in snow is worse. Also, heat buildup is less with higer psi since the sidwalls don't heat up the tire by flexing as much.

So, other than snow, and harsh ride there are not many negatives.

For the same reason the higher pressure is better in rain, I'd have thought it would also be better in snow, at least until it's packed down solid. However with packed snow and ice the harder tires would act more like runners, and so you might want a bigger contact patch...

[hobbit]

I'm almost thinking there may be one small bit of non-supporting
evidence to emerge eventually, if I *ever* manage to find this
suspension klunk in the back of the Prius that happens when I
transition from regen to physical braking and shows up as a
newyork-taxicab rattle sometimes over bumps. But at 60K it
could be ordinary strut wear for all I know. The longterm
question to ask is, will I find any evidence that harder tires
for most of this car's life to date would be responsible for
faster suspension parts wear??
.
[Notes: it's *not* junk in the trunk or any loose interior parts,
there's nothing obviously amiss, and the car's handling/tracking
hasn't changed at all. I'm continuing to maintain the tires
at 50+ or thereabouts regardless.]
.
_H*

[korbynlehr]

So what what kind of mpg gain can be expected with raising the psi?

[tbaleno]

Depends on the vehicle. In my case in my honda civic hybrid I would give a rough estimate of about 6% minimum.