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Exhaust size + gas mileage

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bhop

15+ Year Contributor
1,462
4
Aug 26, 2004
Los Angeles, California
I'm wondering if exhaust size could have any effect on gas mileage?

First off, I have a RRE downpipe, high flow cat (3") and 3" Apexi N1.

About a week and a half ago, I got my car back from the shop, but it had an exhaust leak which was a little loud, so I stuck in my exhaust silencer (which obviously narrows down the exhaust exit), just until I could get the leak fixed, which I did this past weekend. I took the silencer out today, and it seemed like my gas needle moved a lot farther than usual, compared to when the silencer was in, so i'm curious if it's just my imagination, or is it possible that an exhaust that doesn't flow as well will give you better mileage?
 
Pre-turboback, I'd average 27mpg highway @ 70mph.

Post 3" turboback, I average 36mpg highway @ 70mph.
 
I must have some other issues going on.. i'm only getting around 200 miles between fillups. I know stop and go traffic is bad for gas (i do live in l.a.), but this seems worse than usual.
 
Typically opening your exhaust will improve mileage, all other things (lead foot) the same. I notice 1-2 mpg difference with an open exhaust VS stock.
 
suicidal2af said:
Pre-turboback, I'd average 27mpg highway @ 70mph.

Post 3" turboback, I average 36mpg highway @ 70mph.

holy shit, I was going to wait a little while before I bought an exhaust, but you just convinced me to buy one right now. I hope you're not BS'ing.
 
An exhaust system will improve milage because the motor has to work less because of the reduced backpressure. I don't know about 9mpg increase but I have seen 4 or 5. In either case it will improve.
 
With the 3.5" and all the other crap in my profile I seem to average 19-20 MPG. Mixed city and semi-highway with tuning/spirited pulls here and there.
 
95blackGsTurbo said:
An exhaust system will improve milage because the motor has to work less because of the reduced backpressure. I don't know about 9mpg increase but I have seen 4 or 5. In either case it will improve.

Keep in mind I removed a 10 year old cat, too.

If you run with a cat, your mileage may vary.
 
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