Gas Milage


I have a 1985 VW GTI Golf gas. It was getting 35+ miles per gallon. It just dropped to 21.7 according to my computer console. I dont know what happened? I got a tune up, changed my fuel filter, plugs,etc… can you help me?


A faulty oxygen sensor can cause a drop in fuel economy without tripping the “Check Engine” light in a vehicle of that vintage. Was the drop sudden? Have you calculated your fuel economy the old fashioned way or are you just relying on the computer?


Besides the oxygen sensor, a bad thermostat could cause this type of mileage decrease. However, as NYBo said, you need to do some actual calculations (preferably over the course of 3 fill-ups), rather than relying on that device on your dashboard.


Did it show up before or after the tune-up? Those computer MPG calculations are usually inaccurate and often dead wrong. Check it the old fashion way for a couple of fill ups. If it really dropped that much you should have felt the difference in the way it drives. Have you been on the freeway lately? Have you had it up to 65mph? Did it still have power at 65 mph?


The drop was very sudden in a day. I’m not to sure even where the oxygen sensor is.Can you help thanks.


As a couple of others have suggested, don’t rely on the computer console for MPG. Fill the car, set the trip odometer (or write down the odometer reading), drive the car, fill the car. Divide the miles driven by the number of gallons used - and also as suggested, best to do it over several tanks of gas, not just one.

Do this before you go replacing parts that may or may not need to be replaced.

Also check your tire pressure.

If you want to find your oxygen sensor, they’re normally screw in electrical doohickeys on the exhaust manifold/pipe - you can check out autozone’s website ( where they have repair guides that often include diagrams with component locations.

The trick about the thermostat is that if it gets stuck open the car will run cooler than it is supposed to, so the car will run rich (less air/more fuel). The next time the car gets fully up to temperature check out the temp. gauge - if the gauge is lower than it normally has been in the past (mine is always rock solid at the same spot so it would be easy for me to tell) then a stuck thermostat is a good bet. And if it is, count yourself lucky - I’ll take stuck open over stuck closed any day!!


If you find that it is a stuck t-stat, don’t forget a NEW t-stat gasket! Some people I’ve known in the past have forgotten this very obvious bit of info to the engine’s detriment—and to the detriment of the owner’s pocketbook. Just a reminder.


A stuck-open thermostat makes a lot of sense, given the sudden onset of the problem.


Thanks for the info… Before I left Idaho now living in N.C. I had replaced my thermostat, can it get stuck that fast. Its a shame it was a private mechanic that did the work. Whats the cost for somthing like this and would I have to get a new one? Thanks for the help.


I don’t know - how long ago did you leave Idaho? And why was the thermostat replaced to begin with? And was the thermostat change around when the mpg indicator changed?

But before I even worried about any of that I would still do an actual mpg calculation based on miles driven / gallons used.


The thermostat was replaced becouse I was told that it needed to be becouse it was overheating. I replaced the thermostat last month. I would love to do a mpg check but however my mile indicator quit working before I bought the car in Idaho. I plan on selling the car within the year I have put alot of work into it just curious why my miles per gallon have decreased. If its the thermostat I’ll get it fixed if its getting it unstuck?


Also can you tell me if I can just unstick a stuck thermostat?


No. You replace it because a thermostat that is stuck in the open position is broken, in effect.

And, even if you could “unstick it”, a thermostat is one of the cheapest parts on a car and on many cars, the labor necessary to access it is far more expensive than the thermostat itself.