my 2005 lesabre has a low mpg around 10
 

HI, there,

I just bought a second hand 2005 Buick LeSabre Custom. The car has a pretty high mileage of 170k. But since the previous two owners mainly drive it on highway, I assume the engine is still not too bad.
And the car do run pretty well. The only problem is that the mpg is too low. I use the fuel info software to test it. And it shows a 10 mpg.
Is this normal considering the car has a pretty high mileage, or could it be something is wrong with the car? I check the smoke comes out of the tail pipe, it's white and clean.
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The car has to get better mileage than that ! There are many reasons for poor mileage some are EGR system malfunctioning, worn spark plugs, dirty air filter, tire pressure below spec, fuel leaks, injector problems, brakes dragging etc., see what I mean. Unless the car was seriously neglected the mileage ( 170k)isn't that bad. If you have no idea of what was done to it , you may have to start with a list and go from there. How about a manual check of the mileage? By that I mean fill it , mark down the mileage until half a tank is consumed then fill it again and compare gallons required to fill again against the miles driven. It's a good idea to check these things anyway.

Thanks for the the reply and advices, hanky.

Actually, I manually check the milage and it's almost the same as the result shown by software.
Do I need to take the car to a Buick dealer to give it a check or some regular car repair shop can do the check also?