When I bought my car it would get 21 MPG. If I stepped on it and drove like a maniac I'd get 18-19 MPG.

Then I started the mods:

First batch:

-Stillen CAI w/ Z-Tube
-9 Wire Grounding Kit
-Motordyne 5/16th Spacer
-XYZ w/ Straight
-Motordyne Shockwave TI Gamma

ECU reset

Gas milage: 11 MPG

So I remove the Shockwave TI and install the Motordyne VQ Exhaust -

ECU reset

Gas milage: 16-17 MPG

-Upgrade from 20" Sporza D5s and stock brakes to 20" Volk GT-C + Brembo w/ Slotted rotors & Steel Braided Brake lines
-Swap 3.3 VLSD Pumpkin for a 3.5 VLSD
-Warranty replaced Compression rods and end links

ECU reset

Gas milage: 15-16.5 MPG

Install Tein Flex Coilovers w/ EDFC and SPC Front & Rear Camber with new Front Yokohama S-Drive tires & had alignment done by Evasive Motorsports.
(Rear tires are 285/30/20, Fronts are 255/30/20) Car is lowered.
-Installed Resonator module for XYZ pipe and removed Straight

Gas milage 13-14.5 MPG


I always use Premium 91 Octane. That's the highest in CA.

I'm not driving crazy or anything....often times I'm below the speed limit. I don't jump out at every stop light or anything so it's not a case of lead in my foot.......

Does an aftermarket suspension negatively affect gas milage?

I've read that the intake mods should help gas milage but that hasn't been the case. How much does an exhaust system affect gas milage? I wonder if having the Cat-Back system without test pipes is the reason for the exhaust changing my gas milage......

Anybody know? Or is this just a matter of me needing to get my car tuned?

 

i have the z tube and pop charger and 19" chrome wheels on a 04 coupe 6 speed and i been doing this mpg stuff for the last month and the best i got was 390 to a full tank and 23.5 mpg so something is wrong there but im running 93 octane

 

Your intake and lowered stance has very little to do with the drop in gas mileage. Consider you changed the differential gear and are using 20" wheels and big azz tires did you really expect to get good gas mileage? Those rims probably weigh close to 28+ lbs each, bigger tires weigh considerably more too, you haven't done anything to increase mileage....just hurt it! Oh, and I quote: you quite often drive the speed limit! My guess is you're listening to your engine/exhaust and don't really know how fast you're driving. This is very common with modded vehicles where your brain is listening to sounds and gas mileage goes in the toilet....think about it!
Gary

 

Mine in city ~17-18 mpg; 50/50 ~20 mpg; hwy ~24-25 mpg on Cali crap 91.

Air filter clean? MAF sensor clean? Plugs have how many miles? Try a basic tune-up.

My 20's with 285/30's rear and 245/35's front didn't affect my mpg's from oem set-up.
My cat-back exhaust didn't either.

 

I have most the mods you do, and I dont calculate gas milage, but noticed a drop in MPG after instaling the motordyne 3/8 spacer and osiris reflash, dyno tune.... so obviously I knew I was going to gain alot of horsepower, sooo I dont worry about the gas milage... did you buy a 3.5 liter engine, mod it, and expect to improve your 21 MPG???