Honestly I would probably go even lower. At least to a 3.55. You may not have enough power to pull those higher ratio's except for going downhill!
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| Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? https://www.slantsix.org/forum/viewtopic.php?t=61637 |
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| Author: | Walter Lee Jackson [ Sat Nov 25, 2017 11:09 pm ] |
| Post subject: | Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
I have a 1963 Dodge Dart GT convertible with a 1987 a999 transmission with electronic lockup torque converter (shifted via an Imperial Services conversion of the push button gear selector). Attached to the automatic is a Gear Vendors overdrive ahead of an 8 3/4 rear axle. The overdrive ratio of the Gear Vendors unit is .78. What should I choose as the gear ratio for the 8 3/4 for the best fuel economy and the least amount of problems: 3.23, 2.93, or 2.76 (which is 2.5194, 2.2854, or 2.1528 respectively)? I don't know what the stall is for the torque converter. Should this be a concern? tia --Walt Jackson |
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| Author: | Rick Covalt [ Sun Nov 26, 2017 5:03 am ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Honestly I would probably go even lower. At least to a 3.55. You may not have enough power to pull those higher ratio's except for going downhill!
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| Author: | SlantSixDan [ Sun Nov 26, 2017 10:35 am ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Wow, that's an interesting (and very expensive) drivetrain setup! You are the first person I've seen report actually getting the Imperial Services pushbutton conversion; mostly I see "I wanted to order it, I tried to order it, but it kept not coming and not coming and not coming and eventually I gave up", which is sad, so I'm glad to hear it actually exists. When did you get it? The math is pretty simple, as you already k now multiply a rear axle ratio by the overdrive ratio (0.78) to get the effective final-drive ratio. So a 3.23 rear axle nets you a 2.52 final drive, a 3.55 rear axle nets you a 2.77 final drive, or a 3.91 rear axle nets you a 3.05 final drive. The 2.93 and 2.76 rear axle are not practicable (overdrive will be unusable; much too tall and you'll never pull it unless you're headed down a tall mountain). What size tires are you running? What are the specs and health of your engine? Where do you live (altitude above sea level)? What kind of driving do you do (up mountains, along flat roads, mostly city, mostly highway...)? This info is needed to pick the right rear axle ratio. |
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| Author: | afastcuda1970 [ Sun Nov 26, 2017 2:03 pm ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
My FD had 2.93's with a .73 OD manual trans. In my opinion they were a little too tall. I blew out the 7 1/4 rear end and replaced it with an 8 3/4 with 2.76 gears. Needless to say they were way too tall. I changed them to 3.23's and they were better. I then went to 3.55's and that's where the car seemed happy. My recommendation would be 3.55's. I am in Dallas, TX at less than 500 ft above sea level and very flat land. |
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| Author: | Walter Lee Jackson [ Sun Nov 26, 2017 11:56 pm ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Dan, I got the Imperial Services conversion and kit about 15 years ago. Same with the Gear Vendors. The project's just been sitting all these years while life got in the way. Sometime in the next year I'll finish it off (I hope!). I don't know if you can buy the conversion directly from Imperial Services anymore. I think you have to work through Summit Racing. At the time, I had to send my push button gear selector back to Imperial to convert to a one cable setup. This is similar to the system used on the 1965 Dodge D100 pickup that my brother owns. The turnaround at that time was three weeks. Thanks for your advice. I haven't settled on the tires yet because I haven't decided whether to keep Viper caliper/Baer 13" rotor kit and the Baer caliper/13" rotor on the rear. I really like the Dart hubcaps. My latest idea is that I could change the offset of the 13" wheel and use the 4 piston Kelsey Hayes system I took off the car. You might remember that I once had wheel adaptors to pull the 13" wheels out one inch to clear those calipers. (Of course, each week I was checking the system with a torque wrench after your warnings! The engine is rebuilt and never started. The plan was to get it started and then convert to multipoint fuel injection using an offy manifold. The head has the oversized valves. Now, I do have two aluminum block engines that could be rebuilt!! I'm afraid I've dreamed about this car more than I've worked on it in the last 15 years. I live in Phoenix where the elevation is about 1100 feet above sea level. The topography is fairly flat unless I go to the northern part of the State. The plan was to put around town but mostly use the car for long trips. The A999 transmission has that lower first gear, so I figured it would offset some of the taller final drive ration. You are always helpful and extremely knowledgeable. If we could only put you to work solving the rest of the world's problems!!! (vbg). --Walt Jackson |
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| Author: | Dart270 [ Mon Nov 27, 2017 8:07 am ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Hi Walt, That sounds like a super cool setup. I have wanted to try that combo with LU 904 and GV OD for a while. Not sure I'll get to it... I vote for 3.23s with your setup. I have run 2.20-2.50 final ratio with good success NA. If you go to a turbo motor, you could likely run 2.93s due to the higher torque. Happy building and driving! Lou |
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| Author: | SlantSixDan [ Mon Nov 27, 2017 9:48 am ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Yup, I'm voting for a 3.23 rear axle, too. I'm voting against 13" wheels, though, with both hands and both feet. With the rest of your setup being what it is, that's well over the border out of silly, past unwise, and well into dangerous. |
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| Author: | ceej [ Mon Nov 27, 2017 8:48 pm ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
You want your engine to cruise above stall, otherwise you'll build heat in the converter. 2 or 3 hundred rpm above Stall at normal cruise rpm. CJ |
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| Author: | Dart270 [ Tue Nov 28, 2017 5:21 am ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Yes, 13" wheels/tires are crazy in this day. He has lockup, so no worries with cruise stall. Lou |
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| Author: | Walter Lee Jackson [ Tue Nov 28, 2017 8:44 am ] |
| Post subject: | Re: Best Final Drive Ratio For Rear Axle With Gear Vendors Overdrive? |
Thanks you all for your generous comments. This has greatly helped me. By the way, the Viper brake kit was from Andy F. --Walt Jackson |
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