so I have an 06 evo IX RS with 21,000 miles on it... I used to get exactly 200 miles per tank on city driving, and now suddenly I'm getting 150-160... what could this be?? thanks!!

 

have you changed your driving style at all? 200 miles per tank is really low. i get about 290-300 miles a tank. that are your mods?

 

i get about 250 without tune.

 

I don't know how you can do over 220mi with a full tank. The only reason is if you are driving on the highway with no boost....

For the gas mileage, I would check the air filtrer (unlikely) and look for any boost leaks (<== most likely your problem).

 

I only get 140 per tank

Use to be 240 without a tune, after a tune 160-190 but the past month it's now been 140 per tank.

 

damn thats like 10mpg. i average about 18-19 city

 

There has to be something wrong with your car to get such low gas mileage. Even if you are constantly on the boost it shouldn't be that low.

 

Try the o2 sensor...

 

It's a good idea as well
Shouldn't he have a CEL if the O2 is bad?

 

Boost leak?

 

no mods at all, fully stock - should I just take it to mitsu service and explain the situation and see what they come up with? I honestly don't know much about cars to diagnose or check myself...

 

No mods, 21,000mi. You can go the dealer to have them check what's wrong

One very simple way to check if you don't have a (big) boost leak is to check when you hit full boost if you see a black cloud of unburnt gas.

 

wow that's bad. i get 230soemthing for city drive, if i do city and highway i get like 260-270.

 

o2 sensor might have crapped out.........

 

even with boost i get 240-250 per tank. Right now i just drove 2 hr highway miles. 260 miles and still have 1/4 tank left. Cant wait till i get tune to gain gas milage