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.
Rick
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