Gearing and Tires on Jeep 6.2

[HH]

I am contemplating my tire and gear choice for my Grand Cherokee Diesel conversion. The 6.2 is NA and I am looking more for mpg. The Jeep currently has 3.73 gears.

My thoughts are to use a 215/85/R16 (60 psi) tire which is tall (30.4"), skinny and will have less rolling resistance than a softer and wider tire. My gear choices are 3.07, 3.55, and 3.73. 3.42 and 3.31 gears are NOT available for the Dana 35 or that would be my choice, of course. I have to repair the front differential anyway, so I will change over both. I am leaning toward the 3.07s. With the 6.2 installed it should weigh about 4000 lbs or less. It has a 700R4 with a manual TCC lock up.

Here is how the engine rpm should compare if my math is right:

3.07 gear - 1866 rpm at 55mph no OD, 1325 rpm at 55mph in OD, and 1566 rpm at 65 mph in OD.

3.55 gear - 2158 rpm at 55mph no OD, 1532 rpm at 55mph in OD, and 1811 rpm at 65 mph in OD.

3.73 gear - 2268 rpm at 55 mph no OD, 1610 rpm at 55 mph in OD, and 1903 rpm at 65 mph in OD.

Thoughts and ideas welcome, will the 3.07 gears be my best choice?

[76m880]

3.07 will be fine, the Fullsize k5 blazers had the 3.08 gears and they were fine

[diaric]

its important to know what your normal driving speed will be and if your gonna do any towing. your rpm goal should be around 1800 rpm in OD. get way to low and it will lug, burn more fuel and slow down

[AKMark]

Leave the 3.73's in there. My 83 Burb with 3.41's got 20 mpg and my 91 with 3.73's got 22-24.

Both had 31" A/T tires.

Just make sure you get the lockup functioning otherwise bad things happen.

[arveetek]

I vote for 3.07 as well. That's an excellent gear for fuel economy. Like was mentioned, the Blazers and 1/2 tons had 3.08's and they had great fuel economy.

Casey

[Edahall]

I vote for 3.08 if your object is excellent fuel economy. I have those gears in my 82 Suburban and I'm using 30x9.50 15 in. tires. My highway fuel economy is in the upper 20's and sometimes I'll break 30 mpg if I'm really easy and there is no head wind. I also tow a 5000 lb trailer and these gears allow me to pull at 55 mph in 2nd gear.

[Ebrown]

My blazer has 6.2, 700r4,np208 and 3.73 31X10.50X15 and i'm getting around 25+mpg, I would go with 3.73 gear.