2.75 gears
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Fleet Mechanic
Joined: Oct 2007
Posts: 1,972
Likes: 103
2.75 gears
I bought a set of 2.75 gears out of a mid-70s F100 that Im planning on installing in my 66 F100 352/3 spd LWB 2WD. Both gear sets are 28 spline tapered axles. I currently have a 3.25 gear.
My goal is to lower highway rpms and increase mileage, mostly lower rpms since my 66 is a brick.
I don't tow or haul heavy loads
I don't really want to change the tranny, but I put PS in out of a 75 F100 and I really don't want to waste a good 3 speed power steering column.
So for those of you with a similar combo, how is the drivability in town and out on the road?
I know that Im going to lose low end and Im estimating that with a 235/75R15 tire I will be turning about 2250rpms at 70 mph.
Will I be able to maintain 70 mph in hilly country or will I bog down and lose speed? I cant downshift into second at 65-70 mph.
In town at 40-45 mph will I be in second or third? Right now Im in third at 35-45 mph?
Lots of nay sayers on the 2.75 gear set but Im not looking for low end performance, Im looking for drivability and economy.
My goal is to lower highway rpms and increase mileage, mostly lower rpms since my 66 is a brick.
I don't tow or haul heavy loads
I don't really want to change the tranny, but I put PS in out of a 75 F100 and I really don't want to waste a good 3 speed power steering column.
So for those of you with a similar combo, how is the drivability in town and out on the road?
I know that Im going to lose low end and Im estimating that with a 235/75R15 tire I will be turning about 2250rpms at 70 mph.
Will I be able to maintain 70 mph in hilly country or will I bog down and lose speed? I cant downshift into second at 65-70 mph.
In town at 40-45 mph will I be in second or third? Right now Im in third at 35-45 mph?
Lots of nay sayers on the 2.75 gear set but Im not looking for low end performance, Im looking for drivability and economy.
Cargo Master
Joined: Apr 2009
Posts: 3,038
Likes: 4
From: South west Idaho
I had a 79 f-100 with that rear but it had a 302 with c/4 trany , so it would be different. A little slow on leaving the stop light and i never kept in that way long. Changed it out to a 460 / c/6 and 3.50 gears.
The 73 f-100 i have now came from factory with a 360 , 3 speed standard and 3.25 rear gears . I never used the 360 and changed it to a 302. What i never liked was if i went in to third under 40 it would bog down . There was a big gap from second to third it seemed like . Now with your 360 it may be better and i don't think you will have a bad problem on hills. Depends on the hills , out here where i live we have more than just hills, we have mountains.
I just recently put a c/4 and 3.50 rear gears in it. Performance is better and hopefully the MPGs will be okay.
.
The 73 f-100 i have now came from factory with a 360 , 3 speed standard and 3.25 rear gears . I never used the 360 and changed it to a 302. What i never liked was if i went in to third under 40 it would bog down . There was a big gap from second to third it seemed like . Now with your 360 it may be better and i don't think you will have a bad problem on hills. Depends on the hills , out here where i live we have more than just hills, we have mountains.
I just recently put a c/4 and 3.50 rear gears in it. Performance is better and hopefully the MPGs will be okay.
.
Elder User
Joined: Nov 2004
Posts: 874
Likes: 0
From: Ashland, KY
When I got my 79 F100 2wd with a 302/C6, it had 2.75 gear in it. Highway was good but it would downshift on a slight grade. Around home cruising it would spark knock at 45-55mph bc of the lower rpm's. I could never get the spark knock to go away. I ended up changing the gears out to 3.50. It changed the whole truck and I enjoy it alot more know. With you having a 352/3sp the 2.75 gear isnt going to do the same to your truck as mine. With the 3.25 gear a 4sp would help you out alot but you said you didnt want to change any of that so the next best thing is what your doing, changing out to 2.75. Keep us informed how your truck does.
Mountain Pass
Joined: Dec 2008
Posts: 101
Likes: 0
From: Scotts Valley, CA
I had a '66 F100 LWB with a 302 out of an '88 F150 and at T-19 behind it. The engine was stock except for long tube headers, and a carb conversion (600 Holley). It had 4.10s when I got it, and changed it to 2.75 or 2.80 (can't remember; it was 20 years ago), along with 31" rear tires. Worked great; I cruised that thing at 75 all the time.
My current '76 has a 300 with a 3 speed, and 3.00 rear. I'm thinking about taller rear gears for this one, too. I say, go for it!
My current '76 has a 300 with a 3 speed, and 3.00 rear. I'm thinking about taller rear gears for this one, too. I say, go for it!
Elder User
Joined: Jul 2004
Posts: 911
Likes: 2
From: Northern Virginia, USA
I also have a 76 F100 with a 300, granny low, 3.00 open diff and stock tires. Low end is fine for me, even with a load, so I think the 10% gearing difference from 2.75s would be just fine. I know I wish for lower rpms on the highway. Gearing change is equivalent to upgrading to a 31" tire, without the loss in braking capability. Go for it.
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