underdrives
Thread Starter
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Elder User
Joined: Jul 2005
Posts: 969
Likes: 1
From: North Texas
underdrives
I have a 77 F150 with a 400, C6 and 2.75 rear gear. The engine has a 4v carb, intake and performer cam, so I don't want lose bottom end for more performance. I am wondering if a underdrive is the least expensive way to decrease 1/4 mile time. I've seen them for $2500, which seems cheaper than a E40D trans and 3.89 gear swap. Any suggestions?
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
The best bang for you buck is going to be a gear swap. It's a fifth of the cost of the underdrive you're looking at, and it will do the job. It sounds like the gear you are looking at is very close to what you want.
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Elder User
Joined: Jul 2005
Posts: 969
Likes: 1
From: North Texas
With a 3.89 I'd run 3150 rpm at 70mph. That seems a little high, and I spend a lot of time on the highway. It's 30-60 minutes of 55-70mph road to greenville, plano, mckinney, or anywhere else I'd work. Top speed, gear limited, would be about 105... that's acceptable in a truck. That's why I'm looking at a underdrive. Low gears for towing, drag racing, and high gear for highway. Is there a better/cheaper way to get both acceleration and low rpm highway cruising?
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
What about swapping third members? Since you have a 9", it's very easy to keep a third member laying around already set up.
When you have highway gears, swap in the 2.75 third member. You want to drag, pull, etc. drop in a 4.10 third member.
Do a search here - a lot of guys go this route. With a little practice, you can do the jobs pretty quickly.
When you have highway gears, swap in the 2.75 third member. You want to drag, pull, etc. drop in a 4.10 third member.
Do a search here - a lot of guys go this route. With a little practice, you can do the jobs pretty quickly.
Thread Starter
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Elder User
Joined: Jul 2005
Posts: 969
Likes: 1
From: North Texas
That's an interesting thought. Let's see, differential removal about 10 bolts, partially pull the axles - 4 bolts on each retainer ring and a slide hammer, pop in the new diff, thread the needle with the axles, bolt it all down, and replace the gear oil. I do believe that's the low cost option.
