Sudden drop in mileage

2004 LeSabre. After 80,000 miles with 32 mpg at speed limit (70 or 75) I put on new Michilan tires and an oil change then mpg dropped to 27 mpg. Noticed gap in air filter connect so put in new air filter and corrected that. Also have tried rebooting the computers by disconnecting battery for 1/2 hour. Have checked mileage on 5 trips 1600-2200 miles each. My calculations confirm the car’s average and instentaneous mpg. What should I try next? On a recent 1600 mile trip I would be getting 27 for a few hundred miles then for the next few hundred I would be getting 33-34, so averaged 30. So seems inconsistent.

Possibly the new tires have a much higher rolling resistance than the old tires. I assume you’ve made sure they’re inflated correctly?

Did you start using E10 gas when this started?

Sometimes resetting the computer can hurt your fuel economy until it relearns the settings it had before you reset it.

Measuring fuel economy is hard to do accurately. If you calculate one tank full of fuel use, you don’t really know you got 27 MPG. You know you got about 27 MPG. Unless you use the same pump all the time, at the same temperature each time, and you always drive the same route in the same weather, you can’t accurately attribute any single cause. Perhaps the new tires contribute to the change. Perhaps the new tires allow you to drive a little more aggressively. Perhaps your old tires were over-inflated, or the new tires are under-inflated. Perhaps the warmer weather has you running the air conditioner more than you were before you bought the tires.

Since we don’t live life in a laboratory, narrowing this down to a single cause is just about impossible. I recommend you continue to drive to see if it gets better after the tires have some wear. If it goes away, it proves it was nothing to worry about.

Has the car been running cooler than normal? Check the thermostat and coolant temp sensor.

Do make sure that the tires are properly inflated, as noted.

Check from time to time for brakes that seem to be dragging.

Now that you spent some time driving around with a gap in the intake/air filter, clean your MAF sensor.

This car is rated for 18 MPG city and 27 MPG highway. It seems you’re getting the kind of mileage your car is supposed to be getting. The tires could have a higher rolling resistance. You could be using the A/C more, could be E10 gas, likely on a 1600 mile trip where your mileage went up and down every few hundred miles. (or every other tank of fuel).

It is common to get mileage variations on trips when you may be going with or against the wind, also your worn tires with reduced diameter due to wear would cause your odometer to record more miles that you went. Now your tires are full diameter again and you are recording less miles for the same trip.

“I put on new Michilan tires and an oil change then mpg dropped to 27 mpg.”

Were you using your odometer to track miles traveled?

Your old tires were worn and had less thread. Less thread = smaller diameter = your odometer thought you were traveling further than you really were. (and the speedometer read faster than you were going) You thought you were getting better mileage than you really were.
New tires = more thread = bigger diameter = you are now traveling further, so more gas is required. You are also traveling faster, so again more gas is needed.

I was just thinking the same thing. The rear tire on my motorcycle is getting pretty worn out. (I am having it replaced on Saturday.) I have noticed when the speedometer reads 40 MPH when I pass one of those unmanned radar signs, the display on the sign reads 36 or 37 MPH.

Most vehicles have their speedometers miss-calibrated on the high side. They do this to protect you (and them) from a speeding ticket and to allow for some error.