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Changing to 3.90 rear gear

906 Views 4 Replies 4 Participants Last post by  Billy177
I'm thinking of changing the rear gears in my car to 3.90 from 3.42 for alittle more fun :D but more so to make 5th gear a little more useful. I put a TKO-600 w/.64 OD in my car and with using a vacuum gauge I installed, it seems like the car wants to run with the RPM at atleast 2000, so 5th gear isn't useful until around 65mph with my setup, any lower and I get better reading on the vacuum gauge in 4th gear. I figure with 3.90 gears I could use 5th gear a little more since the speed limit here is 65 mph anyways. Would this be a correct assumption? I was thinking 4.11 may even be better, but the car is a street car so it will have street tires and I'll probably start to have some real traction problems I'm thinking. Also, will this 1st gear still be useful? I ask this because I've heard of people with numerically high rear gears that say 1st gear for them is almost useless since it goes by so fast. Also, I don't rev the car past 5500rpm for now and the tires now are 25.85" in diameter.

I'm guessing my cam is the main reason the engine wants to stay above 200rpm. What that be correct?
Here are the specs I have on my cam: .454"/.480" 272/284, I forgot where I read it, but I believe the rpm range for the cam is listed at 1,800-5,600RPM.

Thanks in advance
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· Premium Member
6,858 Posts
Going to the 3.90 will give you 2100rpm at 65 compared to 1850 with the 3.42's. 4.11's will give you a little over 2200rpm.
I think the 3.90 will be perfect for what your looking for.

· Registered
193 Posts
I had 3.90's in my 10 bolt before that blew 5th gear was really nice and would pull suprisingly hard for being .64 however 1st and 2nd gear were completely useless and I would have to ease into 3rd gear to get that to hook with street tires 275/60/15. I have 3.70's now they are better, 4th gear is perfect for driving around on the back roads and 5th is perfect for the highway
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