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Old 11-13-2011, 12:42 PM
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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...
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Old 11-13-2011, 07:33 PM
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For the amount you'd spend to re-gear (around $2000), the fuel savings won't be worth it. Also, I think your speedo is off. Have you checked it with GPS? Your truck should have 3.73s, and with 285 tires, 2000 RPM will put you right around 70-72 MPH. What is the axle code on the sticker in your driver's door jam? Or, there should be a tag hanging off a bolt on the rear diff cover that will tell you your gear ratio.
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Old 11-17-2011, 04:09 PM
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I checked my speed with an old GPS handheld and it was showing that at 65 on the speedo I was actually doing about 68. I had a face palm moment when I realized that 285 tires are not stock size. I knew that, I had read it on the tag inside the door and in other posts, but my brain processed and then ignored it. D'oh. AND I found the tag on the diff and after flaking off a bunch of rust I can read something that looks like #.73. So going down to 3.55s seems kinda pointless and after a bit more research I find that nobody makes 3.31s for the front Dana 50.

SO, it seems that I am going to get my speedo corrected and keep the gears I got. Thanks for the tip about the GPS Zaairman, that was a really easy way of checking the accuracy of the speedo.
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Old 11-17-2011, 04:17 PM
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Originally Posted by EddoHOW View Post
I had a face palm moment when I realized that 285 tires are not stock size. I knew that, I had read it on the tag inside the door and in other posts, but my brain processed and then ignored it. D'oh.
I've done that before when ordering SEVEN new tires. "Door tag says 235/85R16. I have 235/85R16s. I'm gonna order seven 215/85R16s." : tard:: doh:
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Old 11-17-2011, 04:22 PM
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welcome to the org!

I would also say that you are getting some pretty awesome MPG already. I don't idle much and don't ever tow... really don't haul much more than the wife, kids and dog and all I can muster is what you see in my signature.

I am happy with what I get with it, I just got rid of a V10 Ford F250 that never averaged more than 7.5 towing a 4k lb generator (!!) and 9.5 towing nothing. ... I now have a 2012 Dodge 2500 w/hemi (I know, it sucks) and it doesn't average more than 11.5 with a lot of highway, small stock tires and about 1500 lbs of cargo. I haven't towed but it's going to be awful. Luckily work pays for the gas.

Good luck and enjoy the pages here. I forget if you are pretty much stock or not but you can do a lot to gain some here and there - diy intake, freer flowing exhaust (BTM or straight pipe is easy and cheap), making sure your caliper pins are greased and not siezed, getting a chip, switching to synthetics, etc. That will get you more functionality, durability, fun and MPG.
Enjoy -
Scott
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Old 11-18-2011, 05:57 PM
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I would leave the gearing alone and go with a slightly taller tire. There are calculators that you can use to figure out the best tire size to reduce your RPM's. Plus it will be alot cheaper in the long run.
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Old 11-18-2011, 08:56 PM
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Originally Posted by outlaw8950 View Post
I would leave the gearing alone and go with a slightly taller tire. There are calculators that you can use to figure out the best tire size to reduce your RPM's. Plus it will be alot cheaper in the long run.
I agree. I get 12.75 mpg, so stop complaining!
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Old 11-18-2011, 09:24 PM
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I feel good knowing i can get a SOLID 19mpg on the highway @ 70 Running 285-75-16's @ 65psi
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Old 11-19-2011, 09:28 AM
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Originally Posted by Young-SC-Owner View Post
I feel good knowing i can get a SOLID 19mpg on the highway @ 70 Running 285-75-16's @ 65psi
I've nailed 22mpg before at 70psi tires, stock 31's.
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