Have a 73GS non stage with TH400, non A/C with posi. Does anyone know standard and optional gear ratios that would have been available? Steve
Thanks Jim. Anyone convert from 2.73 or 3.23 to 3.55 or 3.73? If yes, how much of an improvement in performance did you see? Was highway driving still manageable? Steve
You need to skip at least two ratios to really notice any improvement. Example: if you have a 2:73,and move up to a 3:08,you will hardly notice anything,but if you go from a 2:73 to a 3:23 or better yet 3:42,you will feel it.
Steve, someone posted this chart, I'll post it here. Gears are multipliers of torque. Look up your torque increase to answer the question above. Manageable means different things to different people. What's manageable to you? 336/tire height X rear gear X MPH = RPM without converter slip. Add 150-200 RPM for converter slip. As an example, a Stock GS with 3.42 gears and stock height tires (26.6") at 60 MPH 336/26.6 X 3.42 X 60 = 2592 RPM, add in converter slip and you are at 2742 2792 RPM
Thanks Brian and Larry...good info with the chart and formula. What does the 336 designate? For me, manageable is being able to do 60-65mph on highway @less than 3k rpm, preferably 2200-2500 or so
https://books.google.com/books?id=N...vation of 336 in rpm formula for cars&f=false You might as well stay with the 2.73 then. Do the math, you are looking at about 2400 @ 65 MPH.
Thanks for the link Larry. Actually, was playing with the #'s and I will be running 28's so that changes things somewhat. Looks like 2200-2500 is pretty low, even with 28"s so I suppose I need to ascertain what a good tradeoff will be to improve performance, match stall correctly while keeping revs "reasonable", whatever that is, considering the amount of highway driving I actually will be doing.
I run 28" tires as well. I have 3.73's out back, but I also run the Gear Vendors Overdrive on the back of my THM400. That is a .78 OD gear. It drops me down to 2.91, the 28" tires get me down to 2.76. 60 MPH is about 2200 RPM, 70 is a bit over 2500. 3.08 is probably what you want.
Larry, The Gear Vendors Overdrive is interesting and seems best of both worlds...looks a little pricey but I will investigate further. Can I ask what stall converter you run? Steve
I had a 10" 3400 converter from Ultimate Converters. I just swapped that converter out for a 9.5" unit from Jim Weise. With the weather the way it is, it will probably be at least a week before I can test the new one out. The new converter is supposed to be 3000-3200 with a positive light throttle feel. If I am happy with it, the 10" will be for sale.
Tks. I've already been in touch with JW and will go with his 9.5 unit. I know he matches the unit with the car using engine specs, weight, type of use and other variables, including gear ratio so that is what originally prompted the question on ratios for a 73GS.
I had a friend (he passed last year) who owned a beautiful 66 impala 2dr ht with a 454 chevy. He added the Gear Vendors OD and loved it.
