Gas Mileage vs Tire Size

Does the size of your tires effect your fuel economy? Larger tires have a greater diameter, and so for every revolution your car will be carried farther along the road. However, the larger diameter also means the torque provided by the engine will result in a lower propulsive force. The torque is the product of the tire radius and the force, so with a fixed torque provided by the engine, larger tires lead to smaller forces. With sufficiently large cartoon sized tires, the engine would not be able to even move the vehicle. So large tires may allow the engine to operate at lower RPMs during steady highway cruise, thus saving on gas. But at the same time, whenever you are accelerating the larger sized tires will force the engine to provide more torque then normal, thus forcing it to run at higher RPMs and burn more gas. Thus larger tires might help your fuel economy if you do a lot of highway driving on flat roads at constant speed. But if you are accelerating a lot, for example in city stop and go traffic or travelling on hilly highways you will probably use more gas.

Inflating Tires With Nitrogen

Riding on Nitrogen

Properly inflated tires can save on gas. Nitrogen may be a way to save on even more gas.

Fuel Economy Tips

This video gives a rundown on three of the most effective things you can do to improve your fuel economy. First and most important is your driving habits. Aggressive driving, idling, repeated slowing down and speeding up and plain old speeding will cause gas mileage to plummet. Second up is tire pressure and wheel alignment. Under inflated tires or a misalignment will generate extra friction your engine will have to spend gasoline on fighting. And lastly, maintaining your engine in good shape with tuneups will keep it in top efficiency.

Big Tires and Fuel Economy

This interview explains that bigger tires will not generally save on gas. You might get better fuel economy with bigger tires at constant highway speeds of 60 or 70 miles per hour, but that will be more than eaten away by worse gas mileage at lower speeds. Typical driving patterns will see the vehicle starting, stopping and operating too much at these lower speeds. So most probably, unless you do long highway hauls for a living, bigger tires will cost you extra gas.

Ride On Nitrogen Initiative

Nitrogen in your tires can provide some benefits. Nitrogen does not leak out of your tires as fast as oxygen, so filling tires with pure nitrogen can help pressure retention. This means if you forget to check your tire pressure regularly you are less likely to drive on underinflated tires. And with the busy schedules of today, most people have trouble regularly checking their tire pressure. Of course regular air is 78% nitrogen already, so you cannot expect gigantic changes. Only the 22% that is not nitrogen is leaking out faster.

The second advantage of nitrogen is that it is less temperature sensitive than pure air. The temperature will be lower when you first start driving than after some road time. That means at the start of your trip you will be effectively driving on underinflated tires. That costs gas and increases tire wear. Pure nitrogen tires will suffer less from this effect, reducing the cold start underinflation penalty.



The video explains how an American picture frame manufacturing company is encouraging their employees to fill their vehicle tires with pure nitrogen. It also talks about the benefits of using pure nitrogen. Aside from the gas mileage aspects of nitrogen, the video shows how far back Detroit has fallen. When the nation has to count on picture frame manufacturers to promote fuel economy measures what does that say about Detroit's ability to survive in the future?

Wheel Alignment is Not an Elective Service

Out of alignment wheels will decrease the gas mileage of your vehicle. They will also reduce the handling performance of the car, which makes it less safe to drive. And of course the lifetime of your tires will be shortened resulting in you spending more on replacement tires. When should you have the wheels aligned? Align your wheels after installing new tires or rotating the old ones. Have them aligned after replacing or having work done on the steering or suspension. Also go for an alignment after any collision repairs.

Keep your wheels in good alignment and you will save a percent or two on gas!

Lower the Rolling Resistance of Your Tires

The two external sources of resistance to your car's motion are air drag and tire friction. Using low rolling resistance tires can improve your gas mileage by up to 5%. In the past tires with lower rolling resistance usually wore out really quickly or had poor traction, especially on wet roads. Having to replace tires often will defeat any savings on gas with the increased tire cost. However, modern day technology is now able to make low rolling resistance tires without compromising wear or traction. One of the key innovations has been to reduce the weight of the tires and increase the stiffness only in the areas that flex the most.

Tire Rack did a series of tests on new generation tires. I was interested to notice that they used the ScanGauge II in the tests. Tests covered fuel economy, ride comfort, noise comfort, road handling, wet and dry braking and wet and dry cornering. The conclusion? Modern low rolling resistance tires will save on gas without giving up on handling, wear or comfort.

Resistance to Motion

Your car has to fight against friction to maintain its motion. There are two categories of friction: internal friction in the engine, the transmission and every other component where moving parts are found and external friction. Here is a breakdown of engine friction by subsystem. The external friction comes from rolling resistance in the tires and air drag.

The resistance of the tires depends primarily on the contact area with the road. Minimizing this area minimizes friction. In the picture you can see than underinflated tires have flat, square contact areas. The outer edges of the tires touch the road. Correctly inflated tires will not touch the road with the outer edges. Overinflated tires will have even less contact area and less friction but they will give you a bad, bumpy ride. It is even possible to damage the undercarriage of your vehicle with overinflated tires because they will not be helping to soak up shock and vibration.

The air resistance depends on the shape of your vehicle and the speed you are going. At low speeds the air resistance is low and increases slowly. At higher speeds the air resistance is large and increases rapidly. Low speed driving is dominated by tire resistance and high speed driving by air resistance. The tire resistance is constant, basically independent of speed. The chart above compares the tire and air resistance and shows their total.

Knowing the relative contributions of tires and aerodynamics helps understand what is happening. For example, there is a debate about whether running the air conditioner or opening the windows (and thus increasing air drag) is better for gas mileage. Since the open windows add to air resistance which is small at low speeds, we can tell that for low speeds windows down and air conditioner off saves on gas.

NItrogen in Your Tires

The picture shows liquid nitrogen being poured ... that super cold stuff you see in science demos. Air is about 80% nitrogen. Why would you want it in your tires, and how would you get it in there?

Both nitrogen and oxygen as found in the atmosphere form biatomic molecules. However nitrogen molecules are larger than oxygen molecules. The idea is this causes nitrogen to leak more slowly meaning you can go longer between tire refills without problems. Also because natural air has water in it and pure nitrogen does not, the nitrogen filled tires are effected less by heat. Finally, the oxygen in air can over time degrade the tires.

It is possible to find tire shops that offer pure nitrogen fills if you just look. To read more about nitrogen in your tires, follow the link!

Check Your Tire Pressure

Try to always keep your tires inflated to the recommended pressure. Under inflated tires will suffer premature wear and need to be replaced sooner. They will degrade the handling (and thus safety) of your vehicle. And of course, they will lower your fuel economy. This video clip from Edmunds shows how you can easily keep an eye on your own tire pressure. The most important lesson? You can't tell by looking if your tire pressure is slightly too low. You have to check the pressure using the right gauge.