What is the optimum tyre pressure for fuel economy?

[TedV]

We can agree to disagree about moisture (not saying liquid water) in a tire. In the south we have 99.9% humidity on a daily basis. I've seen water coming out of compressed air lines.

http://www.tirerack.com/tires/tirete...jsp?techid=191
https://www.google.com/webhp?sourcei...e%20in%20tires

Lots of information on moisture in tires and the theories of using N2. There is moisture in the air we breath. So, it is almost pointless to go through the added expense of using N2 on a street car. Just check your tires pressures often. You never know when you will pick up a small air leak in a tire anyway.

[elhigh]

Based on Metro's graph - NICE, by the way - it's pretty obvious that there's a diminishing returns curve on that. Harder is better, but it means for a harsher ride. Someone mentioned giving the struts a harder life as a direct result of higher tire pressures, I can see why a lot of guys take pressure to mid-40s and stop.

[free radical]

Originally Posted by mada88

Hello everyone,

I used to own a car with 17 inch wheels and in the owners manuel it said that 38 PSI was the best pressure for economy.

I now own a car with 15 inch wheels and I have been over inflating all 4 tyres to 38 psi.

I 38 psi optimum for fuel economy? making the tyres have more rolling resistance with a higher psi?

What is the optimum for a 15 inch tyre?

I saw a myth busters episode and they said 30 psi was best.

Or does it depend on the wheel and tyre size?

Any help is much appreciated thank you

I'd use whatever is maximum pressure posted on the sidewall of the tire,using more would be probably improve mileage but also make the car ride harsh and reduce the tires life span,wear it out faster in the middle!

[user removed]

With my small compressor, if moisture was a serious concern, just run the compressor until the tank is full, shut it off, let it sit overnight, bleed the water out of the tank and you have moisture free air for your tires.

I used that procedure to paint cars and never had water in the lines, which will ruin an otherwise perfect paint job.

regards
mech

[thomason2wheels]

Back in the day when goodrear eagle gts and gatorbacks were the hot tires in scca showroom stock racing, racers with fwd cars were routinely running 50 plus psi cold pressures in their front tires to minimize the tire rolling over onto the sidewall and to minimize the chunking of the outer edge of the tread due to the extreme heat generated. If they could get away with that under racing conditions we can surely do so on the street. Having said that, I typically run pressures in the low 40s cold in both cars because having some compliance in the tire carcass to cope with potholes and such is easier on the tire and the cars suspension. Reference Fred Puhn's book "How to make your car handle" and you will learn a lot....

[PaulB2]

The pressure in tires is controlled by the combined gas law:

https://en.wikipedia.org/wiki/Combined_gas_law

The pressure will be proportional to the temperature in degrees Kelvin (same as degrees Celsius but starting at absolute zero). This assumes the tire volume does not change with changing conditions or temperatures, which I think is roughly true. There should be no difference between the case of 100% Nitrogen or 80/20 Nitrogen/Oxygen. Any small amount of water vapor will have no effect as long as it does not condense out. There is a lot of snake-oil hype about Nitrogen out there lately, if you ask me.

I agree that there are problems going too high. Handling can get bad. You will feel silly if your car ends up in the ditch, because any small savings you previously made by running high pressures will be consumed fixing the car. Also the ride just gets harsh. If it is harsh on your ass, it is harsh on the car components too.

[BrandonMods]

From my experience working with tires, the reason for using 100% nitrogen inflation was to reduce oxidation of the bead seat on the rims. I'm sure many of us here are familiar with bead leaks. That being said little to no oxygen present inside the tire would result in less oxidation of exposed aluminum or steel (iron) from consistent tire change (grinding of bead seat).

[ShinyMew]

The tyre placard on the driver's side door jamb for my Isuzu Trooper suggests a maximum 210 kPa (30 psi) for the front wheels and 230 kPa (33 psi) for the rear wheels.

To adjust the pressures I use an automatic tyre pump at a service station roughly 15 minutes down the road. It lets me set the desired pressure and inflates the tyre to that.

Based on Metro's MPG chart, it would seem to me that 40 psi is best (anymore is only go to net very marginal MPG gains at the increased risk of uneven tyre wear, blowouts and ride quality).

However, because the placard recommends different pressures for the front and rear of the car, will adjusting the tyres for equal pressure all around be imprudent? Also, as the tyres will have been moving for at least 15 minutes by the time I get to filling them, should I be aiming to fill them to slightly less than 40 psi? Is it better to fill them up when the outside temperature is cold or hot? I live in Victoria, Australia, where the mean minimum temperature for 2014 in my area was 10.4c and the mean maximum 19.9c (though our summers are known to have several days of 30-45c heat).

[CapriRacer]

Originally Posted by ShinyMew

The tyre placard on the driver's side door jamb for my Isuzu Trooper suggests a maximum 210 kPa (30 psi) for the front wheels and 230 kPa (33 psi) for the rear wheels.

To adjust the pressures I use an automatic tyre pump at a service station roughly 15 minutes down the road. It lets me set the desired pressure and inflates the tyre to that.

Based on Metro's MPG chart, it would seem to me that 40 psi is best (anymore is only go to net very marginal MPG gains at the increased risk of uneven tyre wear, blowouts and ride quality).

However, because the placard recommends different pressures for the front and rear of the car, will adjusting the tyres for equal pressure all around be imprudent? Also, as the tyres will have been moving for at least 15 minutes by the time I get to filling them, should I be aiming to fill them to slightly less than 40 psi? Is it better to fill them up when the outside temperature is cold or hot? I live in Victoria, Australia, where the mean minimum temperature for 2014 in my area was 10.4c and the mean maximum 19.9c (though our summers are known to have several days of 30-45c heat).

First, may I suggest that you postpone inflating your tires until the last thing before you return home. Inflate them to 5 psi above where you want them to be cold, carefully return home, then adjust the pressures in the morning when they are cold.

It doesn't matter if the outside temperature is hot or cold. What is important is the starting pressure when the tires have not been operated for a while - that is what we refer to when we talk about *cold* tires. You'll want to adjust the *cold* pressure as the ambient temperature goes up and down - about 1 psi for every 10°F (Let's see what this is in metric values: 0.12 bar for every 10°C)

And, it would be a good idea to maintain the pressure split front to rear to maintain the handling split front to rear.

[MobilOne]

You may put 100% nitrogen into your tires, but it probably won't remain that way. Reason is Dalton's law which tells that in a gas mixture (air), each gas acts independently. So, oxygen and other gases are slowly working their way into the tire and the nitrogen is slowly trying to work its way out until the pressure of each gas is the same inside the tire as it is outside the tire. Having said that, it seems to me that the permeability of the tire/ rim/ and stem would determine the final outcome.