Newbie..needs a little help... - Impala Tech
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post #1 of 5 (permalink) Old 11-11-2011, 03:48 PM Thread Starter
Join Date: Nov 2011
Location: Florida
Posts: 13
Newbie..needs a little help...

First, I just want say hello and Thank You to everyone in advance for helping me. I just purchased a sweet resto mod 1969 Impala. She has a 383 stroker with a turbo 400, 2500 stall with the electronic passing gear, 12 bolt eaton posi, 4 wheel disk, CPP 2" drop spindles, 18" and 20" Billet Specialties wheels, a/c, 2.5" exhaust with Hooker jet hot coated headers inside and out and 3.08 gears. I drove the car from Nebraska to Orlando about 1,500 miles.....she ran real easy with them 3.08's, not one cliche. But now that I'm home, I was toying with the idea of possibly changing up the gears. With the info I have provided...what would you guys recommend as to changing the 3.08 gears. Would you go with 3.42 or 3.55 gears. With the 20" tires in the rear along with the Turbo 400/2500 stall on a 3,800 lb car, do you think I should leave the 3.08's in. I'm just looking to get a little more giddy-up out the hole. Thanks again and any info would be very helpful and greatly appreciated.
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post #2 of 5 (permalink) Old 11-11-2011, 07:04 PM
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Welcome to the Team Jay!

The wheels have twenty inch rims but the tires out side diameter are about 26 to 28 inches tall depending upon the tire you choose. The TH400 is a heavy duty three speed with a final drive ratio running at 1:1. This is the limiting factor on the choice of rear gear as you do not want to be constantly over reving the motor.

With a taller gear such as a 3.55 (installed in most cars and light trucks as a towing gear) you can still run on the road without hurting anything other than your gas mileage. Most three quarter ton four by fours have gear ratios of 4.10 to 4.56 to get a heavy truck moving in deep muck. But if you have ever ridden in one you will note that they are noisy and get about 8 to 11 miles to a gallon of gas with the motor always sounding like it is over reving. Most people think that a 3.73 gear is the optimal choice being half way between the 4.10 and the 3.55 gear.

I have 4.56 gears in my Impala but I also have the overdrive version of the TH400. It's final drive is 1:0.7 so that the motor spins at two thirds of the speed at which it would with a TH400. The Over Drive transmission having four gears in the transmission instead of two has a steeper first gear to get a car moving faster.

Big Dave
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post #3 of 5 (permalink) Old 12-15-2011, 03:45 PM
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Join Date: Nov 2010
Location: Spokane, Washington USA
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Big Dave makes some great points. To answer your question more simply... it depends on what you are willing to deal with. The 3.55 or even higher will give you better get up, but will make the engine work harder on the highway. The 3.42 might not be enough of a change from the 3.08's.
Without an overdrive tranny, you can't have your cake and eat it too.

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post #4 of 5 (permalink) Old 12-15-2011, 05:45 PM
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If I were in your shoes - I'd probably do the 3.55's. I'm assuming that you either don't plan on putting 8000 highway miles on it in a year, or that you wouldn't be concerned if the motor is turning (guessing here) 2600-2700 on your highway drives at 65+.

If you search 'gear ratio calculator' on the 'Net, you can play around with whatever RPM you are comfortable with on the freeway, and at what MPH, then the calculator will tell you what gear you need to achieve that. (also based upon your tire height)

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post #5 of 5 (permalink) Old 12-24-2011, 12:55 PM
Join Date: Nov 2011
Location: Laingsburg Michigan
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I have 4:10 's in 4 cars and like it the most . How often do you cruise very far any ways? With 4:10's in the cars, they hover around low 3grand to high 2 grand at highway speeds, and when you punch it , it doesn't have to wind up much to get the grunt, lol. Who cares about mileage in classic cars any way's,lol.
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