Everything affects fuel economy.
Air flow meter
Catalytic converter(s)
Spark plugs
Plug wires
O2 sensor(s)
Air filter
Idle actuator etc.
If you have a mis-fire, start with that repair. I generally find when you have a smooth idle and then a limp for a second or two it's plugs and wires. Not a guarantee, just a place to start.
How hard is it to get your car on a gas analyzer?
How many miles on the O2 sensor and catalytic converters?
Last air filter replacement?
Need lots more info on this one.

 

Well, i got this car from this one guy for $1000 its a 95 s420 with 148k miles clean title! needs new throttle body. i dont know how everything else is. but i want to install the throttle body and see if it still like that.cuz when i drive the car the MPG shows its getting about 15-20 but actualy i get about 5-7 mpg! i did 50 miles on $20 (about 6.5 gallons)

 

o yeh and something when i start the car or hit the gas it starts to smell like gasoline really bad. and also it cant pass smog cuz the air and fuel mixture is not right cuz of the throttle body.

 

o and whats a gas analyzer?

 

If you smell raw fuel maybe you should find the source of the leak.
As you know a leak will lower fuel economy as well.
The analyzer just checks the fuel/air ratio coming out the pipe. See just how rich you really are.

 

hey guys, well today i got my throttle body and when i put it in, the car started perfectly ASR light went off and the throttle worked perfectly, the car ran for about 30 sec, then there was a little clicking noise and the ASR light come on and throttle didn't work again, turned the car off and started it again ASR light off started up just fine and throttle worked then again 20 seconds later light comes on and throttle don't work, so i took the battery terminal off waited for a few min and put it back on and know everytime i start the car the ASR light is on and throttle dont work. has anyone had this problem? what can it be??