I have a '90 rex, d15b2. When I bought it I got 42ish mpg on the highway with 35ish in town. I put a k&n intake on it, and some new plugs... mpg was relatively unchanged.

However, recently I got a new exhaust, cat-back around 2" going to a magnaflow muffler. I also did a rear disc swap from a teg around the same time. After this I am now getting 25mpg (mainly city).

Any ideas as to why this might be? I'm pretty sure that my rear brakes aren't dragging or anything. I dunno if my cat magically clogged at the same time as i was doing all this or what...

 

hows the fuel filter? What are the disc brakes weight compaired to the old ones? More airflow more fuel. ???IDK

 

your car is not rated at that by the epa, check fueleconomy.gov

then again, mileage does vary, but every car I've measured is very close to EPA test results.

Also, my guess is the bigger exhaust made the engine more efficient at a higher rpm/load, therefore less efficient at a lower rpm/load (which you are at more of the time). That would lead you to less gas mileage and more power, is it more powerful now?

 

Thanks, I'll check the fuel filter just in case. Yeah, it does seem to have a bit more power. I was thinking easier airflow in/out = less stress on the engine = even better mileage but apparently that didn't work out. Thanks again for your replies.