my Maxima hates me. It's a 94 GXE (or maybe GLE) with 88k on it. I've had it since 99. In the past couple of weeks, the RPMs have been rough when idling and accelerating. Gas mileage has been horrible (usually in the mid 20s, now about 15). Also, black smoke comes out of the exhaust sometimes when I'm accelerating, and I think the speedometer may be off a few miles now. I've had the fuel injector system cleaned and new sparkplugs put in, but I think the problem is still around. As I was driving home from school (a 220 mile trip) the first half of the trip was getting horrible mileage, like 50 miles on the first quarter of the tank. But just above the halftank point, the mileage and engine started behaving itself again. Any ideas?
Thanks

 

02 sensor, fuel filter, oil change, throttle body cleaning, tune up, etc. lots of stuff is possible
pull the codes and see if anything comes up
some of it may be cheap and some stuff is expensive, but don't just go switching stuff for the sake of it.

Also, you can't judge gas milage by looking at the gas guage. It's not linear. My car also gets about 50 miles on the first quarter, but hangs at around 1/2 a tank for 150 miles, and then starts falling again.

If you want to measure gas mileage, you need to divide total miles drive by number of gallons to fill the tank again. It also depends greatly on your driving speed, style, air pressure, and weight just to mention a few. For most people, I think 300-350 miles/tank or more in combined city driving is typical.