Join Date: Nov 2011
Location: Houston, TX
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Need advice RE: gear ratios & MPG
New to this forum, though I have been lurking here for a few months. New to diesel and mostly new to Ford trucks as well. I have read through the forum and done some searching, both in the forum and using google, but I haven't really found an answer to my specific question. Sorry in advance for the lengthy post but it boils down to this: I want to regear my axles and need help deciding what gear ratio will give me adequate towing power while maximizing my MPG. 3.31 or 3.55?
The truck is a 2000 F250 ext cab short bed, 4x4, with a 6spd, 7.3 and 108K miles. Tires are 285/75-16 load range D at max psi. No gauges, no upgrades, no mods, all stock. The truck runs very well but the gearing is too low for me, I turn about 2200 @70 MPH in 6th. At 65 I'm just below 2000, say 1950 or so. I haven't pulled the diff cover to count teeth but plugging numbers into several online gear ratio calculators seems to indicate that I have 4.10 gears. I don't tow a lot and I would love to drop the RPMs to get better fuel economy, but this is where I need advice.
My concern is that I will either get my 6th gear speeds too low so that I end up bogging the engine OR that my 5th gear speeds will be too high and if I do need to tow something heavy I will be burning LOTS of fuel and listening to my engine rev. Where is the sweet spot for best MPG while still having power on tap?
3.55s will give me 1850 @70 in 6th and 2350 @65 in 5th.
3.31s will give me 1700 @70 in 6th and 2200 @65 in 5th.
3.31s sounds like what I want but I don't want to bog the engine down and make it struggle all the time. I live at sea level, I don't drive in the mountains and because insurance and fuel are expensive I tend to drive a lot slower than I used to. Will 3.31s lug the engine too much and hurt my ability to tow moderate loads? Would it make the engine struggle in around town driving?
Last week I had occasion to drive from Houston to New Orleans and back a few days later. All highway, 60 or 65 the whole way using cruise control and over 741 miles it produced a hand calculated 19.96 mpg. Pretty good for a truck this size but I am sure it would do even better if I could drop the RPMs.
Other random things:
The main reason i want to regear is because both diffs are open and the occasions I need 4x4 I really need all 4 wheels spinning. I want to add lockers or limited slips and it makes sense to regear at the same time.
It doesn't make sense to sell the truck for another one setup more to my liking, this one has low miles for the year which would be hard to find again and body rust which would badly hurt the resale value.
I do plan on adding some performance parts later on (exhaust, intake, chip and whatnot) but better off-road traction and lower RPMs are what will benefit me most right now.
Thanks in advance for any and all comments, advice & help! Well, any and all constructive comments, advice & help...