Hello, I am looking for some help on gear selection. I am moving to an 8.5" 10 bolt posi rear and can't decide on a rear gear. I was thinking either a a 3.42 or 3.73 would be optimal. I don't plan on racing the car. I would like the car to pull hard if I put my foot into it, but do not want it to suffer from excessive RPM's at highway speeds. The car does not have a lockup converter so there is some slippage in OD. The car is a 70 Skylark, it's a non AC car weighing in @ approximately 3700 lbs. Here is the set up: Enging - 455, bored .30 over - 10:1 compression - SP1 intake - TA Performance Stage 1 bowled heads - Alumuminun roller rockers 1.6 ratio - Comp Cam Duration 268/268, Lift .469/.469 - Holly 750 double pumper - HEI Ignition - Long tube headers - 2 1/2" exhaust with x crossover Transmisson Art Carr built 200R4 2600-2800 Stall, 10" non-lockup Converter Current gear 8.2" 10 bolt 2.56 gear open end Current Tires: 27" 295x50x15 I Plan to move to a 295/55-15 tire after the gear change This tire is 27.8" tall Appreciate all help Regards, Jim
With a 2004R, the OD gear is .67 3.42 X .67 = 2.29 final drive 3.73 X .67 = 2.50 final drive 336/tire diameter X final drive X MPH = RPM (no converter slip) 336/27.8 X 2.29 X 70 = 1937 RPM 336/27.8 X 2.50 X 70 = 2115 RPM Add 200 RPM for converter slip. So, for the 3.42's, 2137 RPM @ 70 MPH, and for the 3.73's, 2315 RPM @ 70 MPH.
With the non lockup converter you will probably be ok with either. If you had lockup then the 3.73 would be the way to go.
My gs has a 3.33 posi in it. Pulls like a bitch and chirps all gears. Hiway at cruise rpms a wee bit high fuel economy not great. I wouldn't go any higher.
With the numbers posted above you would likely be happiest with the 3:42 with a 455. With that 8.5 axle you could probably locate a G80 coded Chevy van rear with all the right guts for cheap. The van rears are not valuable and have the same 8.5 carrier. You would be looking for a GU6 code on the service label for 3:42 gears or a GT4 code if you want 3:73,the G80 is a posi carrier code.
That's the advantage of an overdrive.You can run more gear in the back,and the overdrive will drop the rpms down for a better cruising rpm at highway speeds. I run a 3:73 with no overdrive,a TH400,with a 28" tall tire,and it is 2800rpm at 55-60mph. The overdrive will drop that rpm down to a more desirable range. For the overdrive guys: The customers that I have had with a small block will order a 3:90 or 4:10. The big block guys usually get a 3:42 or 3:73. Since you have the taller 28" tire,I would go with the 3:73,but that is just my opinion.
I'd be interested to know if anyone is running 3.42s withOut overdrive, and how they like it. I have a stock 350/th350, looking to go from 3.08 to 3.42. My concern is that it seems most people with shorter gears fall into one of two categories. They either have an OD transmission, Or they practically never drive the car faster than 70mph for any real length of time. Obviously an OD transmission would be ideal, but strictly speaking 3 speed automatics, can anyone shed some light? Is there anyone out there, who has this setup, who doesn't think it's that big of an issue to cruise at 70mph+?
