I've always been interested in getting the most out of my vehicles from a fuel efficiency standpoint.
Tire Pressure and Gas Mileage
You read and hear about this all the time. Be sure to keep your tires properly inflated because low tire pressure will reduce your gas mileage. You also hear that inflating your tires to the maximum sidewall limit also improves gas mileage. At first thought, it intuitively makes sense that maxing out your tire pressure would work. After all, gas is basically energy for your car, and the more difficult it is to propel your vehicle, the more energy that will be used doing it. If you have ever pushed a wheelbarrow with a poorly inflated tire, you quickly realize it takes a lot more effort to push than if it had a fully inflated tire. More effort to propel your car should show up as higher gas consumption so it only seems right that miles per gallon would increase with firmer tires.
Recently, however, I came across a couple of articles on the internet that claimed tire pressure doesn’t have much of an effect on gas mileage. One article was by Edmunds and the other by Popular Mechanics.
Being an inquisitive person I wanted to test these claims for myself with my own car and with my own driving habits and routine.
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Another experiment was performed with my 2008 Subaru Forester to look at the effect of ethanol on gas mileage and to determine if it is better financially to use E10 or pure gasoline.
So, to test out the notion that higher tire pressure results in better gas mileage, I decided to run a little study to look at gas consumption for properly-inflated tires compared to over-inflated tires.
I drive a 2008 Subaru Forester with Michelin Primacy tires that are one and a half years old with 15,000 miles of wear. My plan is to drive several weeks at the Subaru manual recommended air pressure of 29 psi front/28 psi rear and several weeks at a pressure of 40 psi front/39 psi rear, just under the Michelin sidewall maximum of 44 psi, and plot the results as gas mileage versus tire pressure.
I will try to keep my driving patterns as consistent as possible. I will only use my car to drive back and forth to work and for errands on the weekend and no long trips. I drive the same 22-mile route to and from work every day. I do more driving on the weekend, but this is usually for errands such as dropping off the recycling, going grocery shopping, and visiting a few other shops and stores. My weekly mileage total usually falls somewhere between 150 and 200 miles. I buy gas from the same Marathon gas station and always use the same 87 octane grade. I don’t do any heavy hauling other than some groceries, so I’m not consuming gas towing items or carrying extra weight. My air conditioning, heater, and radio usage are also fairly consistent. I also consider myself a pretty mild-mannered driver which means I don’t accelerate rapidly from red lights, I don’t slam on the brakes at stop signs, and I tend to coast to stops. I don’t always follow the speed limit on interstates, but I don’t speed excessively either.
I will use the same dial indicator pressure gauge to check my pressure to avoid introducing measurement error of another gauge into the equation. I will also purchase gas at the same pump at the same station, pump #1 at the corner Marathon station. If you've never tried this, you don't know how challenging it can be to get your gas at the same pump at a busy filling station.
I will also try to continue doing this experiment for as many weeks as possible to add some statistical significance to the results.
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Table 1 and Figure 1 below show the results. I drove five weeks with properly inflated tires at 29 psi front/28 psi rear and averaged 24.2 miles per gallon. In comparison, with tires over-inflated to 40 psi front/39 psi rear I averaged 24.9 miles per gallon over five separate weeks. I checked the data with a two-sample t-test to see if there is any real statistical difference between the mpg results at the two different tire pressures. This test takes into account both the average and variation of the data to determine if there is truly a difference in the results.
The p-value for the test turned out to be 0.18, which means it can be said with 95% confidence that there is no statistical difference in the results and that there is no mpg benefit to driving with over-inflated tires. In other words, the averages are too close together, and there is too much spread in the individual data points to say with any confidence that higher tire pressure produces a change in mpg.
I must say that I am both a little disappointed and surprised by the outcome. Disappointed because I was hoping to be able to save a few dollars by over-inflating my tires and surprised because at first thought it seems firmer tires would result in better gas mileage.
Table 1 - Results of Tire Pressure-Gas Mileage Experiment
|Week||Tire Pressure (psi)||Distance Driven (miles)||Gas Consumed (gallons)||MPG|
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: The relationship is nonlinear but needs more variation in tire pressure?
Answer: I could only vary the tire pressure only so much to avoid putting myself into a dangerous situation with either too high or too low tire pressure.
© 2012 theframjak