Well after I installed the headers I was able to get 17 when driving kinda hard, 18 to 19 when driving normally, and 20+ when driving it easliy. For some reason I'm getting like 13 to 14. If anyone remembers, I had another post fairly recently talking about how my car was stalling and how I was having some weird charging system problems. Now, the charging seems to be fine, but my gas mileage gets worse and worse every time. The spark plug wires must have 3 or 4 k miles max on them, the cap and rotor may have 13k or 14, and the plugs may have 3 to 4k on them as well. This just kinda came on. My throttle is sticking (always has) so I'll be installing another throttle body, readjusting the iac, and the tps (I just adjusted the tps and set it at .50-.51 or so). Does anyone have any ideas? Oh yeah it's a 1986 Iroc-Z 305 tpi

Oh yeah and I also just replaced my air filters and I cleaned the maf, but that didn't help. The o2 sensor has no more than 7-10k miles on it. It runs just as good as it always has and there are no codes. The ses light isn't on.

This is gonna be a hard one isn't it

Could have something to do along with losing back pressure possibly on the exhaust system.

This might sound dumb but it just happened to me..is your fuel tank leaking?

You think that maybe I could have another leak somewhere? The whole intermediate pipe is rotted lol (one of my next projects), but it's always been that way and the headers brought the gas mileage up from like 15 or 16 to like 18 average (for thousands of miles), so they couldn't be causing it. I think im losing plenty of back pressure, but unless I have something else that happened in the exhaust system all of the sudden, I'm not sure.

I haven't noticed any leaking, but I'll definitely check. That's scary.

It's funny because I'm not noticing a lack in power at all, but my gas mileage sucks. The only thing I've done is put on 18" wheels (reproduction chrome irocs; my old rims were cracked), BUT they are the same size in diameter...well, within like 3/16 of an inch. I used some online program to check the change in the speedometer reading with these tires and it said that when my speedometer says 65, I'm really doing 65.18 .

Edit: Wait....that couldn't be it right? If that was the problem, wouldn't it be saying I went much farther than I really did, rather than say I didn't go as far as I really did?