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Old 07-21-2003, 11:24 PM   #1 (permalink)
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I'm ordering my new rear end gears this week (Trak-Loc unit . . . $450 rebuilt), and have my choice of gears, and only want to do this once . . . .

And, so I've asked here on the VMF, and checked out a few calculators online, all with slightly varying results ::

Anyway, THIS online RPM at MPH Calculator returns results I'm happy with (my tire diameter is 24.9 inches, 4th gear trans ratio is 1:1).

What I'm looking for is "What rear-end ratio is going to give me good 'get-up-and-go', but still allow me to drive at 70 MPH on the freeway, and not let the engine sustain high RPM's for extended periods of time"??

I've got a 3.25 now. I was thinking of going with a 3.00, but now I'm thinking of going with a 3.50 (the "calculator" shows me at 3300 RPM at 70 MPH with the 3.50, which doesn't seem too detrimental for sustained periods of time).

What do you guys think?? (I know I've asked this before) But . . . 3.00 gears?, or 3.5 gears?, or stay with the 3.25?? Anyone with more experience with rear end gears than me?? (I have none :p)

Thanks,

Rick
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Old 07-21-2003, 11:29 PM   #2 (permalink)
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3.50 is pretty steep for tire diameter you are using. But if you don't like it you can get some taller tires when you need them. Easier changing tires than buying new gears Gary
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Old 07-21-2003, 11:32 PM   #3 (permalink)
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Rick
It depends on how much highway driving your planning on doing and what you concider to be high rpms. 3300 will kill the gas mph but give you good get up and go. If thats not a concideration on the gas mileage then go with the 3:50s you'll love them.
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Old 07-21-2003, 11:54 PM   #4 (permalink)
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Gas milage is not an issue at all . . . All I'm looking for is to be able to cruise on the freeway for, perhaps, hours at a time at a sustained engine RPM, that's not going to be detrimental to my motor, and still get all the 'Umph" I can out of the ol' 390.

So, I guess what I'm asking is: Is cruising at somewhere between 3,300 and 3,500 RPM's for long periods of time going to harm my engine??

Rick
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Old 07-22-2003, 12:14 AM   #5 (permalink)
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Personally I would go with the 3.50 and go to a 26.1 inch tire suck as a 235/60r15 would be. It would drop your rpm to around 31 at 70 and if you wanted to go with some 255/60r15 which is a 27 inch tire it would further drop em down to a more than tolerable 3000 rpm which I have driven that in mine b4 I put the overdrive in with 2.70 gears in it for longer than 3 hours at a time and not hurt anything. Not even run hot. But I was going a little faster than 70. I actually have a set of 3.50 gears that will be here on friday and hopefully I will have them in soon after and let you know. More so the swap to an AOD is so simple it took me a weekend to do it and get all the parts to change out that if you wanted to go steeper than 3.50 think about puttin an AOD in there. Just my .02 ~Heath
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Old 07-22-2003, 12:14 AM   #6 (permalink)
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Well I'm running 3.55s with 24" tires and I'm happy. I've made a few 300 mile trips to Eureka/Crescent City taching 4k (80mph) most of the way through mountain highway. No problems. . .

How ol' is the 390? 3-3.5k isn't high IMO, since my engine really likes the 3-7k range. But if you're losing oil now it'll be more so if you consistently tach high rpms. . .

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Old 07-22-2003, 12:33 AM   #7 (permalink)
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The ol' 390 has been rebuilt, probably less than 15K miles ago (by the PO). Of course, it's torquey from idle up, but has a fairly narrow HP range from 3200 RPM to 5800 RPM (yeah, it really craps out around 5800 :p)

Doesn't burn oil at all . . . it's just that, jeez . . . at 3200 RPMS, it sounds as if it's 'winding-out', even though the HP is just starting to 'kick-in', and so, I guess I've just got to learn that, even though it sounds like it's reving high, doesn't mean it actually is (relatively speaking).

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Old 07-22-2003, 01:18 AM   #8 (permalink)
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If you only want to do it once, order ~3.70-3.89 gears and swap in a 5 speed OD trans. Everything else will be a compromise. 3.00 or 3.25 (or even 3.50) gears have little value in terms of performance. The total cost for a conversion is reasonably low, and complete kits are available. The 5 spd trans in my '66 has absolutely transformed the way the car drives. Blast through the bottom 4 gears, then shift to 5th for 60mph @ 2000 rpm. There is NOTHING that you can do to your car for the $ that will make that much difference in performance and drivability.
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Old 07-22-2003, 03:28 AM   #9 (permalink)
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I guess I'm going to have to check the accuracy of my tach.

Assumming my tach is accurate (Sunpro), with my 3.50 gears (9") and 225/60R15's, I'm turning about 3.2k at around 75mph in Overdrive. I can't image you're really turning 2k at 60mph with ~3.70-.389 gears, even with an overdrive.

Before I put the 3.50's in back I would turn 2.2k at 60mph with the 2.79 gears in overdrive.

I would also have to disagree that 3.50 gears have no value in terms of performance. No, I'm not going to beat a similar engine with 4.11's in it, but I have run a 14.33, with my old, way too lean carb, and the track lock not working (spun the tire through the first half of 1st gear). Now with my richer carb, and soon as I get the track lock fixed, I'm sure I'll be in the 13's, which, IMHO, is not to shabby for a daily driver.
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Old 07-22-2003, 09:37 AM   #10 (permalink)
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3.50's are a good all around ratio.....I have 3.25's right now and this Cleveland will light 295's from a roll on command.
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Old 07-22-2003, 11:38 AM   #11 (permalink)
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"Before I put the 3.50's in back I would turn 2.2k at 60mph with the 2.79 gears in overdrive"

This sounds too high. My 89 with 3.27's turns about 1900 rpm at 60 mph. With 2.79's you should be turning around 1500
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Old 07-22-2003, 12:12 PM   #12 (permalink)
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johnpro,

The formula for RPM is:
MPH * gear ratio * 336 / Tire diameter

For my '66 with 225*60*15 tires
MPH = 60
gear ratio = 2.45 (3.89 gears * .63 OD)
Tire diameter = 25.5 mounted on the car
RPM @ 60mph = 1936

You mentioned that you run 3200rpm in OD @ 75mph with 3.50 gears. As I recall, an AOD has a ~ .67 OD, if that's what you are running, your true RPM at 75mph should be in the neighborhood of 2300.
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Old 07-22-2003, 01:13 PM   #13 (permalink)
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I am using 3.55:1 rear gears in Lottie, and at 75 miles per hour, she is turning right around 3,200 or 3,300 RPM.

As far as being detrimental? As long as you drive it at steady pace (as in not flooring the pedal and then returning to normal speed again and again) then you should be OK. I've been driving Lottie on the freeway at 75 to 80 miles per hour for over 10,000 miles now and it's been fine.

Remember, these motors are often put into boats with few bottom end modifications, and boats only run in one gear... Sometimes up to 4,000 RPM for sustained periods.

I know what you 're saying about the way the motor sounds at 3,200 RPM though. Remember, some of that sound you're hearing is your mean, big-block exhaust too.
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Old 07-22-2003, 04:25 PM   #14 (permalink)
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I've got a small block in mine, but with a 4.11, I am driving at around 3400RPM at 60MPH. No problems yet (~50K on this rear-end with a lot of "easy" 65MPH highway miles), but I do burn oil... the 1970 302 has unknown miles, but was put in about 100k ago. Car total is 430k miles. :: I am planning to put in a 5spd though as I don't like to rev for long periods. If you stick with the 4spd, I'd not go over 3.55, though.
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Old 07-22-2003, 04:50 PM   #15 (permalink)
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Crapping out around 5800 doesn't sound too bad for a 390, IMO. That'll put your top end around 115-120mph with the 3.55s. The best cure for that 'winding out' sound you're hearing is to turn up the stereo. ::
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