Aftermarket rims and mpgs
 

I recently bought my first Honda, a 2005 Civic EX. It came with 18" rims on it, how does that effect gas mileage, and would I get better mileage with the stock size rims?

Im gonna guess...Yes.
Because 18 inch rims are usually A LOT heavier than 14's or 15's (which is the original size for your civic...Right?)

you would in theory get better gas mileage, but its not a noticable difference, if you get say 30 now, your not going to get 40 swapping rims lol if you like the rims keep em, if you need better gas mileage do a tuneup with NGK plugs, may just need a tuneup

stock rims were 15's, I did do a tuneup about halfway through my first tank of gas and did notice quite a difference. But I was getting real bad mileage so my next tank I will see the difference with the tuneup.

Your MPG and speedometer will be off with rims that big and heavy. I went with 17s on my EK when it came with 14's stock. Huge difference in acceleration and stop time between the two sizes.

Yeah. It's mainly due to the rims and tires. I went from 14" stock wheels to 16" wheels. The tires were 155/60/14 for stock and now I have 205/40/16. I don't know about gas mileage but it knocks off my speedometer at high speeds. Say my speedometer reads 80MPH.. I'm really going 75. D:

As long as the overall diameter is the same, your speedometer accuracy will not change. I went from 14s to 16s (205/45/16) that ended up being the same weight and there was no change to either speedometer accuracy or gas mileage. But that's because I made sure that the new wheels were fairly light (16 lbs). There are many wheels out there that are 20+ lbs - way too heavy for a 4 cylinder car.