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Author Topic: Overdrive Trans Ratio  (Read 901 times)
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1986 TMC 102A3 6V92TA

« on: June 03, 2009, 07:32:28 AM »

Hello everyone,

I'm a newbie.  I was reading a thread about tranmissions ratios from May 7, 2009 and thought someone would be able to help me too.  I have a 1990 MCI Classic (not a highway bus - not a city bus or an RTS - a Suburban Commuter).  It has the 6V92TA DDEC 2, Rockwell 4.56 rear with Allison VR731 trans (three speeds plus overdrive - also has the retarder).  My tires are Good Year G391E - 295/80R 22.5.  I believe they are 40.6" but not sure.  I haven't installed a tach yet and I'm trying to determine engine rpm at 60, 65 & 70mph.  I've also been trying to find out what rpm is best for the engine and what rpm to stay away from.  It will accelerate to 70plus without a problem if I'm not watching the speedo but I don't want to damage anything.  I'd like to know its limitations so I can drive it accordingly.
I'd appreciate some help.

BTW, I believe the my trans ratios are 1st = 1.77:1,   2nd = 1.21:1,   3rd = 0.88:1,   overdrive = ?  Thanks.


Aim high but look out below
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« Reply #1 on: June 03, 2009, 08:21:52 AM »

Rick- I just went to the Goodyear commercial tire site and did not see the G391E-maybe a special bus tire not offered to the public, or an older design not made anymore.  Check the date on the side of the tire.
295/80R22.5 is a wider version of the 11R-22.5 for better weight carrying.  I would advise that when you need new tires, just go with 11R22.5's since they are a very popular tire size with truckers, compared to the very rare 295/80R22.5.  It is a 502rpm tire.
You got the ratios of the V731 correct.  The normal shift pattern is 1st converter, 2nd converter, 2nd lockup, 3rd lockup-so it feels like it is shifting like a four speed, but it is a 3 spd.  The .88 is the overdrive.
So based on the 502 rpm tire, 4.56 rear end ratio, and .88 overdrive, your engine speeds are 1846 @ 55; 2014 @ 60; 2182 @ 65; 2350 @ 70.  With this gearing and tires, I would cruise at 55 with no faster than 65 for any length of time.  92 series likes to be cruised in the 1600-1800 range.
If you're going to keep the bus for many years, I'd suggest changing to the 4.1 ratio.  This will give you 1660 @ 55; 1811 @ 60; 1962 @ 65; 2113 @ 70; 2264 @ 75.
For faster gearing, change the tires and wheels to 11R-24.5's that spin at 476rpm-these are the biggest 75mph tires you can get and are readily available on the road.  With 4.1 ratio you would have 1574 @ 55; 1717 @ 60; 1860 @ 65; 2003 @ 70; 2146 @ 75.
My AMGeneral (the original model to your bus) has 11R-24.5 with 4.56 for gearing of 1750 @ 55; 1910 @ 60; 2069 @ 65; 2228 @ 70; 2387 @ 75.  So changing the tires and wheels to 11R-24.5's would be a cheap way to get better speed.  Changing the rear end ratio can cost upwards of $2,000.00.  Good Luck, TomC

Tom & Donna Christman. '77 AMGeneral 10240B; 8V-71TATAIC V730.
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