will having just a straight pipe with cat, no muffler hurt the gas mileage on a diesel(no back pressure)? My Gas mileage sucks right now. I know I have 35" tires with 3.71 gears, but this is insane.

 

diesels don't need back pressure. Therefore open her up and let her breath.

 

Dodges don't right now, but Fords, and Chevy's do do to them having EGR valves that need the backpressure.

What's killing your millage is your 35 inch tires, not your lack of muffler. I had some 33's on mine for 2000 miles and they caused me to loose about 1.5 to 2mpg. I'm sure your 35's are hurting you more than that. Put your stockers back on for a few miles and see if there is an increase.

Hope this helps.

 

I have 35's and 4.10's on my truck and average around 16mpg. This added about 1-2 mpg over the stockers. I haven't ran the muffler since the truck was new. Haven't had any problems.

 

No Muffler? No problem!

 

Just do it! You won't regret it!