CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Gas mileage

In January of twenty ten we were starting a 2000 mile trip from the midwest to Arizona. I have an 08 toyota 4runner equipped with a dashboard device that displays the current MPG. Had the oil changed and filled the tires. I filled up at a gas station at our grocery store. Then I reset the MPG calculator to zero. I drove 99% of the first 300+ miles on interstate roads using cruse control and couldn’t get the MPG over 19.7. I filled up with a nationally known brand of gas, clicked off the cruse and immediately saw the MPG start to climb. When I refueled it was reading 21.3. The next day it climbed to 21.7. The only differences was while running on the first tank the outside temperature was cold, less than 20 degrees F, and I used the cruse control. My question is, can the brand of gas make this kind of difference in MPG or was it some other cause?

Colder temps make the fuel more dense and make more power. Cruise maintains a more constant speed usually increasing fuel milage. Cold temps lower tire pressures and cause poorer milage. The type of gas should not be a factor. I do not know how the computer calculates fuel milage, but I assume the cruise demanded speed was factored in and this caused the drop in fuel milage. Did you calculate your mileage at each fillup? Is the calculator correct?

Many areas require a minimum of 10% ethanol, and results in lower mpg.

I think it was the weather more than the gasoline. I’ve never noticed a difference in fuel mileage that was attributable to gasoline brand.

If the trip was on a windy day the wind alone could account for a few MPG. I drive back and forth across PA on a regular basis, and I almost always get slightly lower MPG going west, into the prevailing wind, than I do traveling east.

Unless you’re calculating mileage the old fashioned way (miles driven divided by gallons used) you’re only getting an estimate anyway. The mileage computer isn’t that accurate.

When you switched off the cruise you may have been driving in a more economical manner because you were watching the mileage.

There’s a component in the engine management system called a Mass Air Flow sensor. This measures the mass or density of the air entering the engine. The engine management system then adjusts the fuel mixture depending on the mass of the air of the air entering the engine

When the air is cold when it enters the engine it contains more mass or density. The engine management system detects this with the MAF sensor, and makes adjustments to the fuel delivery to the engine to compensate for the extra air entering the engine and the fuel mileage drops off.

Tester

A strong headwind will also affect the gasoline mileage. We have a 2003 Toyota 4Runner and low outside temperature and fighting a headwind both affect the gasoline mileage. Under cold conditions with a headwind, 19.7 mpg with a 4Runner is really pretty good.

The big issue is altitude. Air pressure effects the MAF. Also the fuel system is used to where you used to be. The first tank was a relearning curve for the fuel system. It is not the brand of fuel, but that you moved from one environment to another. The fuel in higher altitudes are also differently blended. They have the standard performance but altitude changes the performance. Temp is not as strong as air pressure in this equation.

On a trip like that, NOTHING is constant. Wind, temperature, average speed, gaining or losing altitude or driving in hilly country all effect fuel mileage. Unless your mileage meter actually measures fuel flow and computes it against distance traveled, then it too is a variable…

Thanks for all of the excellent feedback. I learned some things I hadn’t known and was reminded of some I forgot. I have chronic CRS…(Can’t Remember Stuff).
What puzzled me most was the sudden increase as soon as I refueled. I suspect the ethanol might be a factor.

Ethanal could be a factor. THe onboard milage calculator uses an algorithm with primarily inputs from the odometer, (how many miles you go), and the pulse width from the injectors (based upon the known fuel pressure, and the time the injectors are open, it comes pretty close to how much gas the engine is consuming). There maybe some other sensors that feed the computer (like the MAF) for the MPG calculator, but I checked on mine over several tanks and it was withing 2/10 of a MPG…so pretty accurate.