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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Stationary instrument cannot give instant mpgs

Bob Sikorsky The New York Time Syndicate

Dear Bob: You had a reader who complained about a sudden mpg drop in his 1999 Chevy. I have the same problem in a 2003 Saturn Vue with a four-cylinder engine and 36,000 miles.

About six months ago the gas mileage dropped suddenly, from 28-30 mpg on the highway to 21-23 mpg. I have had it back to the dealer, and he put it on an instrument that gives instant mpgs and it got - so they said - 30.2 mpg. But when I figure the mileage at the pump, it is still 21-23 mpg.

Does such a test, which they said indicated no trouble codes, actually show the problems you mentioned in your original answer?

K.McC., Cheney, Nev.

A: I don’t know what kind of test they could have performed on your vehicle to determine its “instant mpgs,” unless they hooked up an instant-mpg unit and took the vehicle out on the highway for an actual on-road test. I know of no stationary instrument that can give instant mpgs while the car itself is stationary.

The other possibility is that they hooked up their instant-mpg apparatus and then ran the vehicle on a dynamometer, which can simulate highway and city driving. That’s the instrument used in many of today’s emissions-check stations to simulate on-road conditions.

But, regardless of what alternative your dealership may employ, the best and surest fuel-economy figure is the one you get when you check your mileage at the pump. I have driven a test vehicle with an on-board computer that showed that I had averaged 25 mpg for a tank of gas — but, when I filled up, my actual mileage came out to be more in the range of 21 mpg.

The fact that a computer says you are getting X miles per gallon is no guarantee that you really are getting that many miles. Not every mileage computer is off base, of course, but the final judge should always be the real-world gas mileage you get through pumpside calculations.

I’d take your vehicle to another shop and get a second opinion. There is something more going on here than simply a difference in opinion on what fuel economy you are getting. I’d believe your mpg figures over the dealer’s, and insist that the shop investigate to find out what caused your sudden drop in gas mileage.

Dear Bob: I heartily concur with your recommendation to check the air in the spare, but disagree strongly about how easy it is to do.

The space-saver spares in my past Chevy, Ford and Plymouth vehicles have all had the valve stem “hidden” — one has to dismount the tire to check it, and I haven’t found a way to remount the tire so that it fits in the storage area with the valve stem situated where I can reach it. Even my Aerostar, with its under-vehicle spare, is hard to reach — perhaps because I’m rarely wearing “grubbies” at the service station.

I’d like to see manufacturers mount the tire where the valve stem is easy to reach, making it easy to maintain the tire.

Keep up the good advice. B.P., Spokane, Wash.

A: I’d like to see manufacturers do a great many simple, sensible things to make their vehicles easier to use and maintain. But, manufacturers being what they are {hellip} well, maybe one of these days.

In the meantime it’s still smart — even if it’s a royal pain, as it can often be — to check the air in the spare at frequent intervals. Better to find the spare with no air at a gas station where there is air available, than to find that it’s as flat as your other flat in some God-forsaken place with nary a pound of air to spare.

AMERICAN DRIVERS FIGHT BACK

Here’s a tip that will help you get better gas mileage while fighting terrorism and cutting our nation’s dependence on Mideastern oil:

The best way to learn economy driving is to install a vacuum gauge in your vehicle and let it be your teacher. The gauge is clearly marked to indicate when you are driving economically and when you are wasting gas.

It monitors engine vacuum, which varies with the amount of pressure you apply to the accelerator. Generally higher vacuum readings mean more miles per gallon, while lower readings mean that you are wasting gas.

Many new cars have vacuum gauges disguised as shift lights that come on to tell you the most economical time to shift gears. If your car doesn’t have one, a vacuum gauge can be purchased for around $20 and is relatively simple to install.

Many newer cars may not have a vacuum port where the gauge hose can be attached, however. Check to see whether your vehicle has an area to which a gauge can be hooked up before you purchase one.

An added bonus: A vacuum gauge can detect a host of engine ailments before they become serious. By correcting these immediately, you can save yourself costly bills later.