68HulkCharger
Active Member
I’m thinking of going with some 3.91s to improve acceleration on my 68 charger with 383/727. It currently has 2.76s. First, is it worth it and second, any idea on cost for a shop install?
Gear Change
- Thread starter 68HulkCharger
- Start date
It will definitely improve your acceleration! But, we need more info on your driving style: lots of highway miles, concerns about fuel mileage, concerns about shortened engine life? A 3.55 or 3.23 may be a better choice.
Mike
Mike
68HulkCharger
Active Member
Should have included those details. Mostly cruising with limited highway driving. No concern with fuel economy or engine life. I’m running a 27” tire.
jt4406
Well-Known Member
Next time you drive your car keep it in 2nd gear. With your present 2.71 gear the 1.45 2nd gear ratio puts your overall gear ratio at 4.0, almost identical to what you will have in 3rd with the 3.91 you're thinking about. Drive it for a petty good trip (at least half hour), THEN you decide if you want your car that way. Just my .02 jess
I agree with 493 Mike. 3.55 to 1 would be a better choice with 27 inch tire.
BSB67
Well-Known Member
What a good response.
68HulkCharger
Active Member
Thanks for the feedback. I’ll have to try driving in 2nd and decide from there.
355 is what you want you will be happy.
451Mopar
Well-Known Member
As mentioned, the 3.55:1 ratio is a good ratio for what your looking for. Works well with most OD transmissions too.
Switch the rear tires to a 275-60-15, that’s a 28 inch tall tire. That’ll help with the 3.91’s. It’ll still boogie outta the hole, but cruising won’t be as high rpms.
If you have all the parts, the shop is doing just labor, I’d expect it to be around 3-400. They supply parts and labor, 8-1k.
Perhaps you have some buddies that can do it for you? Then it’s just beer.......after they’re done.
If you have all the parts, the shop is doing just labor, I’d expect it to be around 3-400. They supply parts and labor, 8-1k.
Perhaps you have some buddies that can do it for you? Then it’s just beer.......after they’re done.
Around here I'd guess about $800 for a shop to make the change, with you providing the gear set. I had a Coronet with stroker big block and 3:91's behind a 4 speed. A blast to drive around town. Just driving it from the twin cities area to Rochester would drive me crazy and I had 28" tires on the back. I strongly agree the 3:55 would be the best compromise.
68HulkCharger
Active Member
Appreciate all of the great info. Sounds like 3.55s will be the ticket.
machphil
Member
I had a '64 Dodge 330 former state police car with a 383 as well as a '68 Charger R/T and a '68 GTX. I tried the 3.91 and driving on the interstate was mind numbing. I went back to the 3.23. The '64 had 137 mph top end with the 3.23. I had a '93 Cobra that I wanted to bump up from the 3.08 and after my experience with the 3.55 in my '89 GT five speed I decided to stay with the 3.27. I ran the numbers and the 3.27 would also allow me to hit 140 in high gear before going into overdrive. I like the 3.23 and 3.27 gears unless you go to a taller tire.
3.55
ran 3.54's in my Satellite. Had 4.10's in it after 5 miles on the highway it was old. 3500 rpm an getting passed by everything . $250 error.
ran 3.54's in my Satellite. Had 4.10's in it after 5 miles on the highway it was old. 3500 rpm an getting passed by everything . $250 error.
3.55 is a nice all-around street performance ratio. I run 3.73s with 28" tires and it cruises fine at 65mph on the freeway. More than that and it gets a bit annoying, but that's more the loud exhaust than pushing the engine. RPMs around 3000 at those speeds which is fine...I don't go on the freeway much at all though. If you want mild RPMs at those speeds with good acceleration performance you need a good overdrive. Unfortunately it's a major affair with old Mopars, in cost, labor, or both..
Tim Ziegler
Deceased, But not forgotton
There is a drag strip in Eau Claire and Brainerd (hint hint). When my 75 360 Duster was new (500 miles) it got Hooker race headers and 4.10 gears. My priority was 1/4 mile but frequently took freeway trips through S.Dak and Wis.. Not a problem. The car had G-60-14 rear tire
racinguy25
Active Member
Great responses. I have a 68 GTX and am thinking about changing the rear gears and this thread is just what I needed to read, 355s for me now. Not sure what is in it now, but at 2500 rpm= 50 mph. Same ratio at 3500=70mph. I think that is 411 gears, is that right? Do you have to go to dealer for new speedometer gear or Speedway or the like have them?
AJ/FormS
Well-Known Member
With 4.10s and 27" tires;
50=2550 @ zero-slip, and
70=3570 @ zero slip.
So it looks like you nailed it.
As for speed-o gears, there are a few guys on sister-site FABO that have them
tanglewoodHemis
Active Member
Back in the early 60s I was driving my 60 Ply convert with a 361 4-barrel & Torqueflite. I had 14 inch wheels, relatively small tires by today’s standard. I replace the stock rear with 4.56. Then I changed to a 4.89. Objective is to hit top speed in the 1/4 mile traps. Yes, I drove it to school and to the strip. Gas was 25 cents per gal. No problem, or so I thought at the time.
Got to decide....do you want to win at the strip....or get good gas mileage?
In the late 60s I drove a street Hemi 4-sp with 4.10 gear.
Got to decide....do you want to win at the strip....or get good gas mileage?
In the late 60s I drove a street Hemi 4-sp with 4.10 gear.
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