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Gas Guzzling Subaru

Q. I have a vehicle which I cannot seem to solve a problem.

Vehicle Information:

  • 1997 Subaru Legacy Outback Wagon
  • 2.5L with a 15.9 gallon fuel tank
  • Automatic Transmission
  • 21066 miles
  • Fuel Injection
  • ABS brakes all around
  • Power steering,Cruise control & Air conditioning
  • Rack & Pinion Steering

More Info: From the factory the tire size was P205/70 R15 95S the car as it stands right now has 215/60 R16 tires.

I have been keeping a record of mileage and fuel since buying this car a have consistently been getting 8 to 12 MPG. This car runs very well with no missing, no erratic behavior, no fumes or smells or spills in or around the car ever seen.

When I complain to the dealer, he puts it on his computer and says that their is nothing wrong, this includes the smog checks etc.

He said he took it out for a test drive after filling up, drove 40 miles returning to the dealership and put in 1.3 gallons, indicating phenomenal gas mileage. I have had this vehicle since April 1998 and never relieved this kind of mileage. However I did on one trip of 222 miles get an MPG of 14 MPG.

All he can do is tell me that I don't know how to calculate MPG (Ridiculous). The other thing, and I can't believe it could make that much difference is the fact I have larger tires than the factory recommends.

I am at my wits ends about this seeming attitude from the dealers personnel, and have not confronted them, any more than bring it in and saying that I am not receiving the mileage I think I should be getting with this vehicle. I don't know what to do now. Help!!!

Thanks

A. This reminds me of a joke I played on another Technician I used to work with. Every day I took one gallon of gas out of his tank without his knowledge. He went crazy trying to figure out why he was getting such poor mileage. I told him I could fix it in a couple of minutes. I went under his hood, played around with a few things and pronounced the problem fixed. The next day I started adding a gallon of gas to his tank everyday and he thought I was the greatest mechanic in the world because his mileage went up so much.

This is a very interesting question. Believe it or not, the problem is not with the engine or the tires directly. The problem is with your speedometer. The speedometer is calibrated based on the diameter of the tires on the car. In your case, the speedometer is working on the assumption that there are P205/70 R15 tires on the car. Now you put larger tires, and significantly larger at that, the speedometer is going to read lower than the actual vehicle speed. This in turn causes the odometer to read less than the actual mileage. The reason the dealer was able to come to a closer MPG is because he drove a shorter distance making the error smaller. The further you go, the larger the error. The reason you did so "well" on your 222 mile trip is highway mileage is always greater than city mileage.

Here's how to get an idea of how far your speedometer is off. Find a long, flat, stretch of road. Bring your car up to 60 MPH and set the cruise control (Making sure you are obeying the speed limit of course). Give it time to settle in and, using a stop watch, time how long it takes you to go exactly one mile by your speedometer. At 60 MPH it should take you exactly 60 seconds. A time of less than 60 seconds means your actual speed is higher than 60 MPH. A time of more than 60 seconds means your actual speed is slower than 60 MPH. If you can do 2 or 3 miles like this, that would give an even better idea. Just double or triple the time to 120 or 180 seconds.

To further check this, put the old tires and rims back on the car and drive it for a 1,000 miles or so. Check your MPG and I'm sure you will see your mileage improve significantly. A final note here, the larger tires themselves will lower your MPG a tad also.

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Additional Information provided courtesy of AllDATA and Warranty Direct
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