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mikes55
05-26-2015, 09:48 AM
any suggestions on which gear to install in rear end. I have a small block with Muncie with the 2.20 1st gear witl 31.00 hoosier pro street radials in rear. thinking 3.55 or 3.73. looking for cruising rpm but would want it not to be too difficult when taking off in 1st. any suggestions?

rockytopper R.I.P 5-13-2017
05-26-2015, 12:08 PM
You can use this calculator to compare http://www.grimmjeeper.com/gears.html.

According to it with a 31 inch tall tire your rpm @ 70 mph in 4th will be 3.08 = 2350 rpm , 3.55 = 2700 rpm ,3.70 = 2800 rpm, 411=3100 rpm

chevynut
05-27-2015, 08:57 AM
any suggestions on which gear to install in rear end. I have a small block with Muncie with the 2.20 1st gear witl 31.00 hoosier pro street radials in rear. thinking 3.55 or 3.73. looking for cruising rpm but would want it not to be too difficult when taking off in 1st. any suggestions?

What do you mean by "would want it not to be too difficult when taking off in 1st"? What rear gear do you have in the car now, and do you like the way it drives? What engine are you running and how much torque does it have at lower RPMs? Do you have an example of a car that you think accelerates from a stop like you want yours to? Do you have a example of one you think it hard to take off?

Without having an overdrive transmission, your stoplight to stoplight performance and your highway cruising RPM is going to be a big tradeoff.

I personally think an M21 or M22 isn't the best choice of the Muncies to use because of the higher gear ratio in first gear. I think a 2.52 or 2.56 ratio M20 first gear is better. You compound the problem with a large rear tire, but it does drop your RPM at cruise speeds.

I've always felt that you should shoot for an overall first gear ratio in the 9-12 to 1 range to get decent city driving performance. Here's some examples of real-life cars...The 2011 Camaro SS has a 3.01 first gear and a 3.45 rear axle. That's a 10.38 overall ratio. An early Viper has a 2.66 first gear and a 3.07 rear gear for an overall ratio of 8.16 which seems a bit too high to me especially in a 3400 pound car with 465 ft-lb of torque. The newer Vipers are about 8.06 to 1. But keep in mind they have a 600 ft-lb V10 in them, and only about a 25" diameter tire. My Porsche has a 3.82 first gear and a 3.44 final drive for a total of 13.14 and it only weighs 2700 pounds so it really gets going quick, but winds out pretty fast in first.

My Nomad has a 2.66 first gear and currently has a 3.45 rear gear for an overall ratio of 9.18. I plan to go with a 3.73 or 4.10 rear gear so that will change to 9.92 or 10.91 overall ratio. I also have over 500 ft-lb of torque from 2000 RPM and up and a 26" rear tire. I haven't driven the car so I can't say what it's like in real life.

With your large rear tires I would want to have at least 10:1 overall ratio, I think. Unfortunately with a 2.20 first gear you would have to run a 4.54 rear gear to get that. Even a 4.11 rear gear would only get you to 9.04. Again, the tradeoff is high RPM on the highway. This problem is the reason most modern cars have overdrive.

markm
05-27-2015, 09:19 AM
Got to go with Cnut on this really like my 55s 2.87 x 3.54 [10.15] or my 67 SS350s 2.56 x 4.10 [10.50] both have 275/60/15s at about 27.5 tall. 55 has a TKO 600 and 67 has a BW Super T-10.