im just wondering if my high flow cats are affecting my mpg for my car. i only get 10-12 mpg per gallon on local or highway and thats not even driving aggressively. do anyone have an idea why this is happening?

 

HFC should theoretically improve your mileage as they increase the engines efficiency, that’s to say as long as you can keep your right foot out of the pedal. If you can't that extra efficiency at moving air will require more fuel to be combined with it...

 

A high flow cat would not make your vehicle double it's fuel consumption. Maybe you're only using 3 gears?

 

I have the same problem. Rarely do I see 12, most of the time its 10-11. Ive only had the car for about a month and I thought there was something wrong with it. The only mod I have is KN dropins. I do not drive aggressive at all. I would love to get up to around 17mpg.

I have a 08 5AT with 29k mi, running 89 or 91 octane.

 

^ part of your problem there is you should be running 93. engines are designed for the premium octane gas

 

also try pumping up your tires
i have an 08 G37S Coupe 6MT,

went from 19mpg to 21+mpg after tire pressure check: 25ish psi -> 37psi

 

91.

8 char

 

do your have high flow cats?

and yes my tire pressure are perfect. i only run 93 octane and its a 08 g37s coupe at

 

I have had HFC's for a while on my car, and they affected MPG in no way whatsoever. Just sound and drone. You might be confusing your HFC's with an mileage effect coming from another issue. If all's done right, you shouldn't have any noticeable MPG changes.

 

Nope, only KN drop ins. Ive tried switch from 89 to 91 octane and I only saw about .3 mpg improvement. My tire pressure is fine all around 37.

 

In Australia the basic/cheapest fuel is 93octane with premium being 95 octane and the highest (that I put in my 370gt) 98 octane (BP Ultimate or Shell Vpower). Is there some sort of conversion that should be done or is the fuel in the US just lower quality?

 

There is a formula. The octane rating in Aus is the RON (research octane number). In the US it is shown as the average of the RON and the MON (Motor Octane Number) The actual fuel is not that different in octane.

 

I added Cats as my first mod, but I of course ended up losing mileage overall since I was accelerating more to hear it. So do be careful, but it will improve your mileage and give you some extra torque. I would say that the HFC are one of the most restrictive parts of the exhaust

 

Thanks for clearing that up. I have been told on numerous occasions that Japanese fuel is higher quality than Australian fuel so I found it hard to believe that the US fuel would be of a lower quality.

 

i spoke to a few shops and according to them, they said i need to get my computer remapped to accept the new cats. the computer is reading that too much free air is passing tho, so its dumping more fuel into the motor.