Tires

In my previous posts, I listed the factors that would increase or decrease your gas mileage. In this post you will learn how tires can affect your gas mileage. One of the first things that can be checked in 30 seconds, are your tires. Your tires are the only thing that come in contact with the ground. Having under-inflated tires will lower your gas mileage, it will cause your car to use power, waste more gas, have decreased stability at high speeds and not safe. Having properly inflated is safer and will save you gas. Let me give you an example, when a bike has under-inflated tires, it is harder to pedal and when the tires are inflated. its more easier to pedal. The same concept applies to your car. By inspecting your tires and having them properly inflated you are saving money.

My aerodynamic rims.  ; )

Personally, I over-inflate my tires by 10-PSI. It creates less rolling resistance but it also makes the car feel stiffer. I do not endorse over-inflating your tires because it can cause to tires to explode and void any warranties you may have for your tires. Do this at your risk, I am not responsible for anything that may result from having over-inflated tires. An increase of 5 psi, should not be considered as "bad".
The aerodynamics of the tires can be further increased  by having the wheel fully covered as shown in the picture. The last important thing is having tires with proper tire thread, they grip the road. A car that has no tire thread is dangerous because the tire may explode and they don't grip the road well. The expensive part is replacing all your tires. Whats your cars current PSI reading?