Rear End ratio help

[Fluid Power]

I need some help determining the correct rear-end ratio for the car. The rear tire diameter is 26 inches, The car is running a Tremec with 2.95 first and .63 5th. the motor is a 383 415/hp with 400 ftlbs of torque. I am interested in good acceleration while knocking down good mpg for the 58 mile one way to work. Any suggestions would be great. Before the manual, I ran a th350 tranny with 3.73 and was somewhat happy with the results.



Darren

[chicane67]

I'd stick with the 3.73's or maybe 3.42's if you do any distance driving.

[dhardison]

Darren, I have the same trans in my Nova with a 3.42 rear gear and 255/45/17 (26" tall). On the highway I run 2100rpm@70mph in 5th and pull down 17-18mpg.

Take a look at the bottom of this page and enter your info. This is in NO WAY an endorsement of Keisler, they just have a good rpm calculator on their site. According to this calculator 3.73s would only increase your 70mph rpm by an additional 300rpm.

http://www.keislerauto.com/gm/transm...evy2_68-74.asp

HTH,
Dan

[SHANE 73Z]

I run the The TKO600 and 3.73s in my car. I am currently running a 350 that doesnt really start making power until 2500 rpm or so. Unless your 383 is a real torque pig down low (under 2000) I wouldnt run anything shallower than the 3.73. It just makes the engine dog too much in 5th.

Take care,
Shane

[CAMAROBOY69]

I agree with them. The 3.73 would be the perfect gear for your car.

[Fluid Power]

The motor makes good torque down low, basically a flat torque curve from 1500 - 4000 @ 400 ft lbs. Thanks for the reponse guys. Looks like the 3.73 (well in this case a 3.70 since I now have a 9 inch).

Darren

[CarlC]

With the kind of daily mileage you are doing, I'd consider something numerically lower than a 3.73:1.

You can easily support a 3:30-3:40:1 gear if you have 400 lb-ft at 1750. You will likely have to downshift for any spirited passing.

At least with the 9" you have a lot of options. 12-bolts are more limiting. I'd throw a 3:42 in mine in a heartbeat if they were readily available.

[SHANE 73Z]

I gotta agree with Carl. With that much grunt down low I would be looking at a 3.42 gear also. With the 3.73s and that much torque you are not going to like the wheelspin (uhhhh...I mean 1st) gear.

Shane

[ProdigyCustoms]

Fluid Private messaged asking me to review the post for my opinion, so here goes.
With a 3.42 you have the best all around. You have an effective starting line ratio of 10 to 1, which for reference, is lower then a 69 Z28 or 70 LS6 Chevelle (9 to 1) with a 4.10 (Muncie had 2.20 1st ratio), so you will have great low end grunt still, but not so much that you over power the tires all the time. Any lower gear may make the car to torquey to launch. It also would force a shift 20 feet from the traffic light, you probably would end up leaving in second often when you feel lazy.
On the other side of the coin, I like the 3.42s on the highway because at 60MPH you will tach 1670, and at 70MPH you will tach 1950. I suspect for your motor to be 400+hp, the cam duration is around 225 to [email protected], which means it would actually still lope at anything lower then 2000RPM or so, and loping and chucking at cruise speed is miserable, you could end up finding yourself in 4th gear a lot. I also suspect like most of us, you probably cruise at 77MPH more often then 70, so you will be a couple hundred north of 2000 at cruise. Based on the cam specs, (if the cam is over [email protected]), I may change my mind and recommend a 3.55, and unless the cam was over 240@50, anything lower then a 3.55.

[Fluid Power]

The motor was built by Golen Engines in N.H. Great guy to work with.

Here are the CAM specs

Intake is 224 @ .050 lift = .477
Exhaust is 230 @.050 lift = .480

Looking at the Dyno chart, it makes 400ftlbs at 2250 rpm. not 1750 like I had previously mentioned.

More input is welcome! Thanks!

Darren

[ProdigyCustoms]

Looks like my guess of 225 to 230 was pretty damn close. I stay with everything in my previous post. You will be happy.