Best Way To Improve Gas Mileage?

Here is a question I received and answered on the now-defunct website

I had read in a book for better gas mileage and better tire wear that a person could look on the side wall of the tire and note the maximum tire pressure for maximum full load and then inflate the tires to that pressure when they are cold or unused for 4 hours. Naturally in snowy conditions you should drive slower. What is your opinion?

Hi, thanks for writing.  It’s an interesting thought, but not entirely correct.  First off, you always want to check your tire pressure and fill the tires when they’re cold, so they’ve got the timing correct.  But, to get the maximum fuel economy, you’ll want to fill each tire to the recommended PSI listed for the car, not the maximum for the tire itself.  
You’ll find the correct inflation numbers in either the Owner’s Manual or on the yellow sticker on the driver’s door jam.  When you check this, note that it may specify a different PSI for front tires than it does for rear tires.  If so, remember to check the tire pressure all around after rotating tires and adjust as necessary.
It’s been estimated that you’ll suffer about 1% loss in fuel economy for every 3PSI difference between the manufacturer specifications and your actual tire pressure.  That difference can also increase tire wear by 10%.
Note that that’s a really small amount of fuel loss.  Most people waste more fuel using the accelerator wrong (up to a 33% loss in fuel economy) than they could ever save by obsessively monitoring their tire pressure.  
As for snow, definitely decrease your speed.  Your stopping distances will be substantially longer when traction is compromised.  Some people believe that running your tires partially deflated during snow conditions improves traction.  I’m not sure how valid that is – winters here in the mid-Atlantic simply don’t get fierce enough to warrant me experimenting with a set of tires in that way – but it’s certainly going to cause extra wear on your tires.
My advice – keep your tires inflated according to your car’s specifications year round, check the pressure once a month or whenever you notice a tire seems to be low, and you should be fine.
If you really want to improve your fuel economy as much as possible, don’t speed.  Every 5mph increase over 50mph is like adding $0.25 to every gallon of gas you buy.
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