ok guys im starting a new thread here to get as many opinions and experiences i can
heres the deal:
i drive a 2006 F250 crew cab lariat 4x4 6.0 Powerstroke.....im currently running stock gears which are 3.73's.... im wanting to upgrade tires to 37x12.5x17... it is a daily driver... very light towing if any
WHAT SIZE GEARS SHOULD I GO TO FOR BEST OVERALL (MPG's, AND POWER) PERFORMANCE?

 

For gas powered (5.4L and 6.8L) Super Duties, my general rule is that for every 2" increase in tire height, there should be an increase in gear size.

Example 1: If your GAS truck has 33" tires, 3.73 gears, and you are happy with the current performance, then use the following guideline to maintain the stock performance:

33" Tires - 3.73 Gears
35" Tires - 4.10 Gears
37" Tires - 4.30 Gears
40" Tires - 4.56 or 4.88 Gears

Example 2: If your GAS truck has 33" tires, 3.73 gears, and you are NOT happy with the current performance, then use the following guideline to enhance the stock performance:

33" Tires - 4.10 Gears
35" Tires - 4.30 Gears
37" Tires - 4.56 Gears
40" Tires - 4.88 or 5.13 Gears

My 2005 F350 5.4L has 4.10 OEM gears. I would like to have 4.30 gears with this current configuration. So, for me, I would do use the following guideline:

33" Tires - 4.30 Gears
35" Tires - 4.56 Gears
37" Tires - 4.88 Gears
40" Tires - 5.13 Gears

However, you have the 6.0L diesel engine with huge TQ at low RPM's. Also, the diesel engine does not have the longer RPM band as the gas engines. In short, your diesel engine doesn't have the same need for higher gears as the gas engines.

Example 3: If your DIESEL truck has 33" tires, 3.73 gears, and you are happy with the current performance, then use the following guideline to maintain the stock performance:

33" Tires - 3.73 Gears
35" Tires - 3.73 or 4.10 Gears
37" Tires - 4.10 or 4.30 Gears
40" Tires - 4.30 or 4.56 Gears

Example 4: If your DIESEL truck has 33" tires, 3.73 gears, and you are NOT happy with the current performance, then use the following guideline to enhance the stock performance:

33" Tires - 4.10 Gears
35" Tires - 4.10 or 4.30 Gears
37" Tires - 4.30 or 4.56 Gears
40" Tires - 4.56 or 4.88 Gears

It's a little more subjective with diesel engines. Your other thread listed a ton of upgrades for your 6.0L engine which will add HP/TQ to the already powerful diesel engine. I would wait until all of your engine, tire, and lift kit upgrades are installed before committing to a change in axle gears... it may not be necessary.

 

yea im thinking the gear swap will be the last thing IF i decide to do it