So since gas has been so pricey recently I wanted to see how having a tune and modifications on your vehicle affect your gas mileage. I was talking to Chris, who recently had his car tuned by Eurocharged, and he told me that his gas mileage increased with a bump in horsepower to boot. For some reason I thought a tune would decrease gas mileage.

What I want to know is how does a tune, mods, or both affects our gas mileage? I was trying to convince my wife that we both need tunes to improve our gas mileage, but she knew what I was really trying to do.

the tune will not affect gas mileage at all since they only make changes to WOT maps. When cruising, the computer maintains a constant 14.7:1 a/f ratio.

You may notice a difference if you spend most of your driving under WOT.

I was able to achieve 26.6 mpg at 65 with cruise control on after Jerry did my latest tune. Before that the best I saw was around 23-24.

This is just simply not true.

You will see that many cars, including other makes, improve fuel efficiency from a tune. The improvements are most dramatic in forced induction cars..

It's unlikely the tune made the difference. As I said, the computer has to maintain 14.7:1 af ratio when cruising. Any leaner and you will melt pistons, overheat, and have high NX which will ruin the catylitic converter. Not to mention will not pass emissions.

I get about 24 mpg cruising as always. (75 mph ish)

I'm not tying to start a debate, just trying to explain how an ECU works. When cruising, your computer ignores the programmed map (which is what a tune alters) and it uses the oxygen sensors to regulate fuel delivery.

Best I see with my non tuned C55, and only mods are secondaries and resonator deleted is about 21 on no traffic FL Turnpike at about 80 mph, I find I get worse fuel mileage at 65 usually around 18.5 mpg. Mike what size tires are you running. I have been told the 235s up front and the 265s out back are my biggest issue for fuel economy.

I have always had 225 35 18 in front and 265 35 18 in back. However, this pasw sunday I replaced my rears with 255 35 18 (all they had in stock) and the tire is notably smaller in diameter. I prefer the look of the meatier 265.

with a larger tire, you will travel farther with every revolution but the computer won't account for that. And it's a pretty small difference.

really 225/35 in front? that is a much small diameter tire compared to the rear 265/35. That is even smaller than stock which is 225/40. I would think that would mess with your ESP/Antilock/ etc. Your tires would be spinning at totally different speeds. and it is not really the diameter that affects fuel mileage other than the added weight of a taller tire, it is the width, additional tire patch on the ground and in the air causing additional drag. 20mm's is a good bit extra tire patch compared to stock.

Mods like crank and supercharger pulleys will reduce MPG and a good tune will increase MPG. I've heard reports from people on here (and at tuning events) that that their MPG went up between one and two MPG post tune. When I put my C3 pulley on, my MPG went down about two to two and a half MPG (I don't have a tune right now).

Totally stock C55 with 45,000 miles. Summer tires on (225/40/18 fronts, 255/35/18 rears), averaging 18.5 - 19.5 mpg, have seen highs of 24.3 mpg on long trips (except for one trip where I kept it on cruise at 65-68mph the whole time - which is very unusual for me! - and averaged 25.5), city driving averaging 16 mpg (approx.), typical mixed highway/city cruising I see 21.3 - 22.3 mpg.

been avg 18.4 ish for the past couple thousand miles on the 32 with longer trips. honestly can't tell you it makes any difference before and after the tune and mods. it all depends on your driving condition, habit, and tires. my 350 avg around 20 (25-26 hwy around 70mph) before and after the tune. 21+ when I had still had those stock 17" touring tires. summer and wider 18" tires = expected lower mpg.

Right now I'm averaging close to 20 mpg hwy with a crank pulley, no tune, and 265/18 tires in the rear. I remember getting better gas mileage, before I replace my intercooler pump and I think my city mpg is a lot lower due to the tires. Once I get my headers on, I'll get a tune and see how much that improves gas mileage.

My wife's C55 gets 19 mpg mixed and around 20 straight hwy, I'll see about getting her a tune soon as well.

I have 245/40x18 front and 265/35x18 rear and this morning on my daily 50 mile commute to work I got 23.6mpg. No highway just back roads all the way.

Looking like the supposed MPG gains from tune don't apply to us people with larger crank, or smaller S/C pulley. I average 18MPG city, and about 20MPG highway. City can easily drop as low as 13MPG if I drive aggressively... Figure I could save some gas by opening up my ISO and running higher IAT?

I saved gas in my SRT-10 by unplugging 5 of the fuel injectors. the V-10 is actually 2 inline 5 CYL engines and it runs balanced and smooth with 1/2 a motor. 12mpg hwy on 10, and 21 mpg on 5. the problem is, it's slow as a turd like that. Someone really needs to build an aftermarket cylinder kill computer that can be programmed to kill cylinders in a sequence that allows the motor to run smooth and balance the killed cylinders evenly. that would be super.

If my memory serves me right, I averaged around the same mpg before and after all my mods.

When the car was untuned for a week, mpg drops by 2, but the tune brought it right up.

I averaged around 17.5 mpg, 30/70 highway/city with heavy 19 inch wheels.

[QUOTE=Zeebrock;4671009]I have 245/40x18 front and 265/35x18 rear and this morning on my daily 50 mile commute to work I got 23.6mpg. No highway just back roads all the way.

Yeah I will have to take a pic of mine.

Sadly not as impressive, ussually shows 36 mph average over 48 miles with 18.3 being average mpg

If I may chime in, I get around 30-32 on the highway with my c55, going around 70.
It gets better mileage then my 4 cylinder turbo on the highway

I think this statement is wrong

I think a vital question here is tire pressure as well... higher pressures yields better mpg, so maybe that should be taken into account/mentioned as well

What are you doing that I'm not??!

You need a tune up, clean out your MAF sensor, change your oil and inflate your tires.
I get 24 MPG at 70 mph with 235/ 275 sticky tires.
Course on the race track, I drop down to 7 mpg at WOT.

well i guess i might need to do that this weekend i used to get about 26mpg when i lived in washington ever since i moved to cally i have not seen better than 19mpg on straight highway perhaps a tune might help

How do you fit those tires on the stock rims in the pic? Also don't the 245 rubs badly on the inner fenders on the front? I tried the 245 and had to move down to 235 for that reason.