Fill your tires. Reduce your CO2.

WHAH?!?! That is mind-blowing. Seriously. I'll be right back, I gotta go to the gas station.

 Thank you for bringing this point up!

Proper inflation of your cars tires means your tire is realizing the proper shape that is optimum for the operation of the vehicle.  Under inflated tires act an extra source of friction and requires the engine to work harder in order to keep the vehicle in motion or to start up from stop.  Under inflated tires, since they produce more friction, heat to a higher temperature and cause premature wear of the tire material.  Additionally, since they are under inflated, they are not shaped properly for the vehicle and will assume the stereotypical flat-bottomed circle appearance.  This extra tread patch means the tread, and possibly part of the side-wall, are in contact with the road-bed, increase the physical wear on the tire.


All-in-all, under inflated tires cost you lots more in the long run due to decreased tire life and decreased MPG.  Properly inflating your tires is not only good for the environment, but safer for the vehicle occupants and your wallet.  

Now, if you want to get into some really esoteric discussions on tire pressure as relating to vehicle MPG, then you can send me a PM.  I've read some interesting points on the hybrid forums....

All true, but also be careful of greatly over inflating. If you read the numbers on the side of your tire, they tell you the maximum load for the tire at its maximum pressure. So when does over-inflation start? It is not when you exceed the rated maximum pressure.

Your proper pressure is based on the load actually on the tire now. If you add extra load you need more pressure. But if you are carrying less weight on the tire, it should have lower pressure. You should not have tire maximum pressure with half the  weight on the tire.

If you go through the reasoning above, and apply it logically, you would be adding and releasing pressure every time you add weight to the vehicle or take weight off. Nobody is going to do that.

But your car manufacturer also provides upper limits to weight per wheel. You should limit the weight you place on the wheel based on lesser of tire maximum and vehicle maximum. But the pressure in your tires should not exceed that limit expressed as a fraction of maximum pressure.

If you have a tire that says max weight 2000 lbs at max press 40 PSI. That means you should never need more than 20 PSI if the weight on that tire must be less than 1000 pounds based on car manufacturer's  advice.

Well, if you are carrying more weight than your car manufacturer advises you must add more tire pressure, another PSI for every  50 pounds of load on the tire. If your tires lose a bit of pressure, add some to compensate.

But if you inflate those tires to 40 PSI when you are carrying only 1000 lbs, your tires will put too small a patch of rubber on the road. You will have the centre of the tire  wearing fast,  the tire tending to slip sideways on dry road, and to spin on any road. Your braking ability wil be compromised.

Tire manufacturers do evaluate how much of a rubber patch you need for safety.

It is not the minimum possible and it is not too much. The pressure you need is directly proportional to the weight carried to get that rubber patch.

You would get better gas mileage with steel tires on steel rails. But you are not on rails, and you need to retain traction too. You should not try to simulate steel tires on rails by overinflating, ie by incorrectly inflating to maximum rated pressure.

A critical rule is that your car will pull to one side and use a lot of extra fuel if you have a tire on one side harder than on the other. It pulls toward the softer tire. But even worse, when you brake it pulls far too hard toward the softer tire, it puts you into uncontrollable skids.

If you're a nerd like me, you can prove this by tracking your fuel mileage during hot weather vs cooler weather.  My tires gain about 3-5 psi when it gets warm, and I get about 3mpg more because of it.

Also, anybody who's raced bikes knows the incredible energy expenditure when you've got a low tire. 

Anyways, thanks for the post.  The data don't lie.