Carecraft wrote wrote:
There is a major problem with the Carecraft example. I shift my gears by the rpm the engine is going and not by the vehicle speed.
He assumed in both cases the car was in the same gear. He did not use the transmission for what it is intended.
Here is a neat site to use to calculate rpm's vehicle speeds etc with different transmissions and gear ratios.
http://www.grimmjeeper.com/gears.html
I used my 05 Duramax with the Allison 1000 for my example. The 05 has a peak hp at 3000 rpm and peak torque at 1600 rpm. These would be ideal shift points. Max rpm is 3250 for the 05 Duramax.
I assumed 30 inch tires and a 3.08 rear end for one case and same tires and a 4.10 rear end for the second case. I will accelerate from 40 to 65 mph. A typical speed change when getting on a freeway and sometimes passing a slow truck when hauling my 5th wheel.
With the 3.08 I would be in 3rd gear going 40 mph at 1956 rpm and accelerate to 65 mph where I would be turning 3162 rpm.
Now with the 4.10 I would be in 4th gear going 40 mph at 1837 rpm and accelerate to 65 mph where I would be turning 2935 rpm.
Interesting the 3.08 gives me a change of 1216 rpm and the 4.1 gives me a change of 1148 rpm. Almost the same except the 3.08 is greater and Carecraft claims this will result in a shorter acceleration time.
In almost every post in this topic the transmission is not taken into account. Combustion engines have a transmission to keep the engine in its power band if used properly.
In my example my 05 Duramax would perform almost the same with both rear ends with the exception when both are in 1 st gear or both in 5th gear. But if the transmission is allowed to do its job performance in vehicle speeds between 25 and 90 mph will almost be the same.
