69 F100 rear end gear with 3 speed
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69 F100 rear end gear with 3 speed
I know this has probabl been asked but I ccouldn't find it. I have a 69 F100 with an inline 300 and 3 on the tree. It has 235/75 15 tires on it. I am trying to figure out what gear I need to put in the rear end. Right now it has a 3.70 in it and it is turning 2800 rpm's while driving 60mph. I would like to get the rpm's down so it doesn't suck the gas out quicker than I can get to a station. I would like to be able to drive 60 - 65 but not lose all of my take off power. Everyone I talk to tells me to get rid of the 3 and put a C6 in but I just can't bring myself to it. I like the 3 on the tree and it is getting to where you never see them anymore. Thanks
Lead Driver
Joined: Apr 2011
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From: El Dorado, Arkansas
An automatic will consume more fuel than a standard.
Trading any non-overdriven transmission for another non-overdriven transmission (standard or automatic) will also not give any net gain. All non-overdriven transmissions will be 1:1 in high gear.
A C-6 will also consume more horse power to run than a C-4.
Trading any non-overdriven transmission for another non-overdriven transmission (standard or automatic) will also not give any net gain. All non-overdriven transmissions will be 1:1 in high gear.
A C-6 will also consume more horse power to run than a C-4.
Hotshot
Joined: Mar 2013
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From: Phoenix, Az.
IMHO to change enough rear gear to get the highway RPM's down noticeably will net a noticeable loss in take off power. I would suggest doing all you can with what you already have and try and make it a better fuel miser. COMPLETE tune up. Correct tire pressure. Maybe rebuild carb. Read your spark plugs. See if it's running more rich than necessary.
Lead Driver
Joined: Apr 2011
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From: El Dorado, Arkansas
An overdrive will give you the most bang for your buck.
You can keep your rear end ratio for pulling/acceleration and on the highway, the overdrive will not penalize you with high RPMs and excessive fuel consumption.
Final drive equivalent, in overdrive with 3.70 rear end gears, would be about 2.52:1. --with an overdrive transmission in overdrive and with 3.70 gears, it would be like the rear end has 2.52:1 gears.
If the overdrive ratio of the transmission was .68:1 and you multiply this by the actual rear end ratio, this is what the equivalent ratio would be.
.68 x 3.70 = 2.516
You can keep your rear end ratio for pulling/acceleration and on the highway, the overdrive will not penalize you with high RPMs and excessive fuel consumption.
Final drive equivalent, in overdrive with 3.70 rear end gears, would be about 2.52:1. --with an overdrive transmission in overdrive and with 3.70 gears, it would be like the rear end has 2.52:1 gears.
If the overdrive ratio of the transmission was .68:1 and you multiply this by the actual rear end ratio, this is what the equivalent ratio would be.
.68 x 3.70 = 2.516
Hotshot
Joined: Mar 2013
Posts: 14,255
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From: Phoenix, Az.
I am not disagreeing with Ultra but the reason I didn't mention it is because at 60MPH and in overdrive a stock 300 I6 would be a gutless wonder. IMHO you'd have your foot farther in to the carb to keep speed. These trucks are as aerodynamic as a brick. So no net gain in MPG. Just my .02.
Fleet Owner
Joined: Dec 2009
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From: NorCal
I am not disagreeing with Ultra but the reason I didn't mention it is because at 60MPH and in overdrive a stock 300 I6 would be a gutless wonder. IMHO you'd have your foot farther in to the carb to keep speed. These trucks are as aerodynamic as a brick. So no net gain in MPG. Just my .02.
These rigs aren't as aerodynamic as a barn door... but rather, the entire barn!
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Thanks for the suggestions, but I would really like to keep the 3 on the tree. I just didn't know if changing to a 3.50 or 3.25 would help or if it would hurt the take off to much. I mainly drive on back roads but I do get on the Hwy about once a week or so. I am driving this truck everyday when I am not in my work van. So any little bit would help.
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Tuned
Joined: Mar 2010
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I agree with JEFFFAFA.
I have a 1967 F100 with a 240 and a 3 on the tree and I have done some gear swapping. Your truck will obviously be more powerful with the 300 but I suspect you will still have similar results. My truck came factory with 3.70 gears--as I suspect most with that engine & transmission combination did (or all of them). I have experimented with two different gear sets, first a 3.00 and now a 3.25. Both sets of gears provided a very small gas mileage improvement at highway speed but the truck lost most of its limited accelerating power at that speed. Also, with those gear sets and the low gear in the 3 on the tree transmission it takes a lot of revving and feathering of the clutch to get it rolling from a dead stop. It is embarrassing when the light turns green in city traffic. I am going back to the original 3.70 gears. When Ford built those trucks they obviously "did the math" and picked the best gear ratio to match that engine and transmission combination for all types of driving.
I have a 1967 F100 with a 240 and a 3 on the tree and I have done some gear swapping. Your truck will obviously be more powerful with the 300 but I suspect you will still have similar results. My truck came factory with 3.70 gears--as I suspect most with that engine & transmission combination did (or all of them). I have experimented with two different gear sets, first a 3.00 and now a 3.25. Both sets of gears provided a very small gas mileage improvement at highway speed but the truck lost most of its limited accelerating power at that speed. Also, with those gear sets and the low gear in the 3 on the tree transmission it takes a lot of revving and feathering of the clutch to get it rolling from a dead stop. It is embarrassing when the light turns green in city traffic. I am going back to the original 3.70 gears. When Ford built those trucks they obviously "did the math" and picked the best gear ratio to match that engine and transmission combination for all types of driving.
Tuned
Joined: Mar 2010
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That C-4 does seem to make a difference. My father had a 69 with 302, C-4, and 3.25 gears. It drove very nicely with adequate power and managed to get gas mileage in the high teens. That was really why I tried the 3.25's in my 67 with the 240 and 3 on the tree--for the most part it turned out to be a failed experiment. Of course, the 302 also has more power than the 240 which deserved some consideration.
With a 300 and the 3 on the tree, the 3.50 gears would likely be a good match.
With a 300 and the 3 on the tree, the 3.50 gears would likely be a good match.
Lead Driver
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From: On the Edge of the Desert
I was running a 3.25 with C-4 behind my 351C. It was a pretty good overall ratio behind the big engine. But I wanted more power and less rpm. So I went overdrive with 4.11. Final ratio comes out to 2.75. Cruises a lot better and pulls like never before!
3.50 behind the 300 is usually a good choice.
3.50 behind the 300 is usually a good choice.
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