View Single Post
Old 04-27-2008, 02:22 PM   #1 (permalink)
EcoModding Lurker
Join Date: Apr 2008
Posts: 1
Thanks: 0
Thanked 0 Times in 0 Posts
mpg + jeep - thinking cap help?

Hello I stubbled on to this forum by accident (or destiny?) while searching for some information. I was wondering if anyone could help. I have a 2003 Jeep: 4 liter engine, 33" tires, 3.73 gears and the 42rle auto trans in it. The OD gear in it is a stratospheric .69. (third and OD has lock capabilities)

What I need to figure out is what gearing should I run in the axles to achieve the most fuel economy? I understand that I have the aerodynamics of a brick, but right now on the highway the RPM's spin about 1700 rpm @ 65. Anything slower the engine lugs (forget about passing or small inclines on the highway)

options : 4.56 gears - 2058 RPM or 4.88 gears 2228 @ 65. Jeep people swear that 4.88 is better choice (with this transmissions od of .69) but I fear that fuel efficiency is not taken into consideration (this is my daily driver). If I understand fuel economy has many facets one of them is piston speed. An old book written by a guy named Taylor stated that fuel efficiency is maximum when piston speed is at 1000 to 1400 ft/min or 16.4- 19.8 ft/s. That being said, 2000 rpm is the edge of this calculation with the piston moving in the 4L at about 19. But then again Taylor wrote this book soon after WW2 and since then we have synthetic oils and fuel injection - so I dont know if the PS ranges still apply to my 4L engine.

Which one of these gears would be a better choice (40 %city 60% highway)? I am so confused... Right now I get about 17.49 MPG in the highway @ 65-66mph and since regearing is $$$ I dont want regrets with a decrease in fuel economy.

Any thoughts or opinions would be greatly appreciated!


  Reply With Quote