Clarke Echols (216.17.134.97)
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Votes: 0 (Vote!) | | Posted on Tuesday, July 03, 2001 - 8:45 pm: | |
The optimum ratio is that which gives you your normal preferred cruising speed at an engine speed of 1600-1800 RPM. This is hard to accomplish without an overdrive transmission.
2100 RPM at 71 mph indicates your tires are turning 563 RPM and the running circumference is 476 revolutions per mile. That is pretty close to the 483 revs/mile you get from 11R24.5 tires commonly used on large trucks, so you can't improve much on tire size.
At 60 mph you are running 1775 rpm. If you change to a 3.55 rear-end ratio, you'd run
about 1690 rpm for the same 60 mph which would get you to about 74 mph at 2100 RPM. If you could find something in the 3.33 range, your top speed would be about 79 mph, and you'd be running 1585 RPM at 60 mph or 66 at 1750 RPM.
If you have a DDEC-controlled 6V92, your fuel consumption at 1600 RPM would be about 10% less at 1600-1700 RPM than at 2100. If you have a straight-mechanically injected engine, it may be somewhat different.
I would guesstimate that if you dropped your ratio to about 3.55, you'd see a 5% fuel savings, and perhaps 10% at 3.33 if you can get a ratio in that area. As for hill climbing, you'll still be
OK because with an HT-740, if you drop to 3rd gear, you have a good hill-climbing gear with a speed range from about 42-55 mph where you get maximum HP from the engine. A 3.55 ratio puts that in the 38-53 mph range. With a 35,000 pound bus, you'll get good results until you hit 6% or heavier, then the 3.33 ratio will start knocking you to second gear a bit before a 3.55 would.
If you are using a non-Allison-HT740, your results may differ, but this will put you in the
ballpark. |