Sudden drop in gas mileage


My 2000 Chev. Blazer was getting 250 miles to a tank. I check it all the time - compulsive. Suddenly I am getting not even 200! It seems to be less with each tank even. The gauge seems to be fine until it hits the half tank mark, then it will suddenly drop and the rest of the tank is gone in one day. I only drive to and from work and a few errands - maybe 20-30 miles a day. This started to happen after moving almost 600 miles and pulling a U-Haul typle trailer - it’s a 6 cyl. It ran hot during this trip -well it was June! It was well maintained - had a recent tune up and oil changed right before the trip. Could this be an electrical problem? Or what? I can’t let this go on and I like the Blazer. Don’t want to get rid of it.


The most likely problem is a flaky fuel gauge, the rest of your vehicle behaving normally, as ever.

It’s time to start calculating your mpg the correct way, by recording gallons purchased over a period of time and writing down the mileage that appears on the odometer.

Too many people think it is sound to determine fuel economy by miles per tank. This may be a simple method but there are far too many ways to get an erroneous result.

For now, at least, I will suggest there’s nothing wrong or different with your vehicle’s fuel economy.


The gas gauge on my 2000 Blazer quit working in April, it hovers between 3/4 and full most of the time. Since the sending unit is part of the fuel pump assembly, I’m not going to get it fixed. I calculate my mpg (17.5 to 18.0 in the summer) every 3 tanks of gas. I can drive about 250 to 270 miles before I refuel. I second the previous poster’s suggestion about calculating mileage based on miles and fuel consumed.

Ed B.


No, no! I HAVE been calculating by the odometer reading and gallons per tank. I do this religiously every tank and that is why I was able to catch this right away. I fill up and then use the entire tank each time. I have been doing this for over a year. The gauge and mileage is way off from what it has been operating as before, and this sudden change occurred immediately after the long trip pulling a full trailer.


[b]Since you admit that the engine got hot, there are two things to check for a sudden drop in fuel mileage. The thermostat and the coolant temp sensor.

If the thermostat body got distorted from running the engine hot, the thermostat may be stuck open. When this happens, the engine isn’t able to come up to full operating temperature.

The coolant temp sensor for the computer. If this sensor was damaged while running the engine hot, it may not be working correctly. And could be telling the computer that the engine doesn’t come up to operating temperature.

I’ve seen either of these components fail after an engine has been run hot or overheated.