20-30% mpg loss..

Changed my spark plugs last week. Went from 15mpg to 18mpg.

 

Fuel trims will tell a lot like b_r_o said.

But also a good point above, make sure you are calculating mpg correctly (and consistently). Trip A for accurate mileage, fill up till the pump clicks (no topping off), and do the math out. Not being condescending either, I've read about people getting terrible "mileage' because they were guessing at miles or not doing the math right

 

I hate when people complain about winter fuel destroying their fuel economy. I knew it wasn't much but had to look it up. Your "couple %" is dead on According to amsoil at 1.7%.

https://www.google.com/amp/s/blog.amsoil.com/the-difference-between-winter-and-summer-blend-gas/amp/

 

i really think we need better definition than '20-30%'. i mean, when i was driving 2 miles to work in a previous car with a 3.0L v6, the cars mpg readout averaged 7-9mpg. that same car easily got 20mpg on the highway. one single highway trip would bump me into the 13-15mpg range for a while. there was absolutely nothing wrong with the car, just the short in-town low speed trips murdered my average mpg.

if 20-30% works out to be somewhere in the range of 1-5mpg average, that can easily be explained by a number of personal/environmental factors. different routes, longer idling at lights/trains, or simply nuances to your driving style.