I have a problem. My Jeep gets about 12 miles per gallon. I have a custom intake on which has a cone filter no cold air box it just rests on my fender.
Is this normal or do I have LFS (Lead Foot Syndrome). Will I get better gas mileage if I put my stock air box on?

 

Thats about right around town in 2wd. I get that as an average, 20 on highway, in 4wd about 9mpg. If you want good milage get a toyota prius, I think they may be having a sale now...Just dont plan on stopping..

 

check the obvious spark plugs,wires, cap rotor. check for fuel leaks and then check the mechanical fan clutch. mine was bad hurting the mpg but i also have a small fuel leak at the plastic fuel line at the intake theres also one by the drivers front fender by the rocker check those get back.

 

I would rather get 5 miles per gallon than drives a prius. If I had one of those I would probably sell it and buy another jeep. I definitly wont buy one though. I got better mileage before installing the intake though.

 

Likely, the intake is suffering from heat soak with out fresh air coming from outside the engine compartment. After the engine warms up the intake air temperature will increase, causing less dense (hot) air to be drawn into the intake. This would be increased if it is a nice shiny metal intake pipe instead of the stock "plastic" intake. If you still have it, you could try installing the stock air box or mocking up some duct work to get some fresh air to the filter.