Part 1 - Introduction - "What is Hypermiling?"
Part 2 - Test Hypotheses
Part 3 - Fun with air filters
Part 4 - We're talking tires
In Part 3, we talked about the gains in fuel efficiency by modifying the intake track on a 1994 Toyota Corolla for increased airflow. By doing this, we gained about 1mpg, for an average annual savings of 572 miles of fuel per year, which equates to 16.8 gallons. At $3.44 per gallon of 87 octane, that's a savings of $57.79 per year.
One of the tenets of Hypermiling is to increase air pressure in your tires to the maximum allowed pressure. But before you go out and do this, read on.
On the side of your tires, you'll find a "Max pressure" amount imprinted on the tire. This is the tire manufacturer's maximum rating for that tire. It may say, for example, "MAX pressure 50psi". Your vehicle manufacturer might recommend you inflate your tires to 32psi. The reason this is done is because the engineers have found a happy medium, or proper inflation pressure, for that particular vehicle.
Imagine your tires as a balloon. When you overinflate your tires, they react much like a balloon that has been blown up to a much larger size. It is very hard, but very brittle and is easy to pop. On the flip side, underinflated tires will heat up quickly, which causes a failure (blowout). These factors will increase exponentially as the vehicle's size and weight increase.
The other factor to consider with tire inflation is the shape of the tread. If you look at the front of the vehicle (and assuming you could see through the bumper), your tires will appear as approximate vertical rectangles. As tire pressure is increased, the top and bottom of the tires will bow out. This, in turn, means not all of the tire's tread is in contact with the road. This decreases rolling resistance (thus improving your mileage by making the car work less), but the decreased contact patch means less resistance for more important things like going around corners or braking. Especially in the rain. This also means the tires will wear out much faster.
Basically, if you want to play with tire pressures, do so at your own risk.
Knowing all this, I decided to go with the recommendations of Hypermilers and increase the tire pressure. I didn't have the nerve to go to the maximum pressure, so I increased the tire pressure from the Toyota-recommended 32psi to 38psi.
The result was a consistent 1.5mpg gain, which brought my average to almost 36 mpg. Annually, this calculates to 858 miles of fuel saved equalling 23.8 gallons per year which, at $3.44 per gallon, is an annual savings of $81.87.
Total savings with modded air intake and overinflated tires: $139.66
In the next installment, we'll delve into various driving techniques. You'll be surprised by what we came up with.