Bad Fuel Millage after switching spark plugs and coil socket

Hello everyone! I was hoping someone could help me out with a quick question. I recently had my spark plugs and ignition coil socket replaced and noticed once I had everything put back together that my fuel mileage started to decrease significantly. I thought it was possibly adjusting to having those items replaced but I’m not sure? I have not ran my car on the highway yet but noticed the bad mileage fluctuating up and down driving around locally? Is this normal? I’ve only driven my car twice for about 10 miles since I changed everything out?

I’m not sure you’ve driven far enough (20 miles or so) to really be able to calculate fuel mileage. You need to fill the tank, reset the odometer, then calculate the mileage the next time you fill up dividing gallons to fill up by miles driven.

I try to use original style/brand plugs. Make sure they’re gapped correctly. Make sure you didn’t crack the porcelain when tightening them. If you are ok on all of those, I doubt a plug change has affected fuel mileage negatively. But, you can’t calculate fuel mileage very well by watching the gas hand over a 20 mile period.


1952 Studebakers have a habit of doing that.

We know nothing about your car, make - model - mileage

Also 10 miles is not anywhere enough to get a good mileage calculation. Assuming the car runs normally without misfires or hesitation I would run 50 to 100 miles and re run the calculations, use miles and gallons of gas do not use the calculation the display gives you.

It’s a Chevy Cruze.



1 Like

I take it you are going by the fuel economy readout in the instrument panel.

You get the worst fuel economy while the engine is warming up. Driving 10 miles is barely far enough to get the engine fully warmed up.


As a follow on: I get about 10 mpg for a few miles when I first start out in my 2017 Accord. After that, the gas mileage steadily increases. For a 40 minute ride to work, mostly highways, the mileage goes up to 40 mpg. OP, take a half hour or longer ride and see what your readout says.


As others have said there is no way to get a MPG indication in 10 miles … the car is barely warmed up even in the summer in 10 miles and the transmission will n ot be warmed up in 10 miles either .

1 Like

The year make and model is a Chevy Cruze 2015 with about 125000 miles on it. I started it again this morning and it sorta struggled to turn on like it did if it needed new spark plugs? A misfire I’m assuming?

I think you need to return to the shop that did the work . Struggle to start sounds more like a battery problem .

1 Like