The EPA (Environmental Protection Agency) says that for every 2psi (pounds per square inch) of under inflation in your car’s tires, you lose 1% in fuel efficiency. In a recent study (not to exacting scientific method, mind you) students at Carnegie Mellon University found that out of the 81 cars parked in a nearby parking lot, only 1 car had all four tires inflated to the proper tire pressure. Further, they found that on average tires were under inflated by 20% – which translates to about 7 psi. If you go by the EPA’s assertion, the average person is losing 3.5% in fuel efficiency. According to the mathletes at Carnegie that works out to $432 for the average person…
Let’s apply that to me: I drive 30,000km per year. I get 600km to a tank. A tank costs $50. So if I apply a 3.5% decrease in fuel efficiency, I will only get 580km to a tank. So according to my math, I only save about $87. What are they smoking at Carnegie Mellon? Read about their study here – then do the math. It is flawed.
I decided to do some digging – I checked Edmunds.com which is a decent resource for car buying tips – they did an extensive review of various ways to save on fuel. They found a small difference as well.
But, I CAN tell you that by having properly inflated tires will probably save you $200 per year on tire wear! So if you factor that in, I would estimate that checking your tire pressures once per month and keeping them in spec will probably save you a couple hundred per year in fuel and tire costs, but not quite $432 in gas alone!
Side note: For every 5 degree increase/decrease in temperature your tire pressure will increase/decrease by 1 psi. So make sure to pay closer attention when there has been wild temperature swings (like change of season) – your tire pressure will have changed!