Not Getting my MPG & Gauge Problem

Ok my nissan 2002 se-r is only getting about 16 miles a gallon in the city.
I dont have to much knowledge about cars. When i bought my car it came with several aftermarket parts. I have a Injen Technology Cold Air Intake, a GReddy Oil Pressure Monitor (I think, it looks like a silver canister), an Exaust System with a GReddy Muffler (Looks more like a Can). I believe I should be hitting 24 in the city. A friend told me that my air intake is not combusting correctly which is why i am not getting my mpg. is this true? or is it something else. does anyone know what is causing my car to guzzle gas like a v8.

Seperate question do those fuel additives really clean fuel injectors etc?
Another seperate question, i have gauges believed are called inglos and they do not light up and night the bulb is prob broke but what i want to know is that my orange needles on the gauges move in intervals and jerk rather than a constant flow, do i have to buy new gauges? is the motor broken? or is it repairable.

I apologize in advance for all my long questions.

i get 38mpg on the highway and closer to 30 in the city, but it depends how you drive, ive gotten as low as 16 in my car but i was flogging the shit out of it, the throttle body coolant bypass mod gets a lot of gas mileage, look on b15sentra.net for the writeup it takes like 5 min

Your gagues could be related (ECM issues), but I'd check the small things first. Air filters, tire inflation, O2 sensor (check engine should be on), etc.

Does the car run well, or does it jump around alot, like your gagues say?

sounds to me like a case study in why not to buy a modded car. id take it into a mechanic if you dont know cars well. id bet the previous owner fucked something up. and dont do the coolant bypass if you live in a state where it gets cold. especially if you have to get it emissions tested. the coolant running through the TB and intake helps with cold start driveability and emissions. ive heard your TB plate can freeze shut and your car wont start, its never happened to me but i was probably just lucky.

your throttle will only freeze closed if you have a wax pellet type idle air control valve, (i.e cable throttle) the spec uses a servo in the throttle body for its drive by wire idle control so it doesnt freeze

Never buy engine cleaners or crap like that. Most of the time they do more damage than help. I have heard not to use fuel injector cleaner on specs.

dont use injector cleaner that you put in the tank on any car with less than 150k on it. even then, there is a better way. at some shops you can get a fuel injector service that pressurizes the cleaner through the fuel system separately from the fuel pump, and it includes an intake cleaner as well. even that i wouldnt use unless you have over 150k on your motor and know for a fact that your injectors are clogged. fuel injectors on modern cars get dirty very rarely. a lot less often than most people think.

yeah agreed, no injector cleaner