Cummins question.

[mountainexplorer]

Most Dodge Cummins trucks in... say the early 90's, had 3.42 gears. They probably came stock with tires that measure an equivilent height or near to a "rated" 33" tall tire.

So, say you had the same motor/tranny/T-case setup as the Dodge came with factory, only with 4.10 gears and rated 33" tires. Would fuel efficiency increase by going to a rated 35' tall tire? (by knocking the gear ratio back closer to stock)

 

[joez]

I *think* the really high ratio's were only available in trucks equiped auto tranny's. I may be wrong, but this is what i seem to remember.

Would fuel mileage increase? Depends. Peak torque comes between 1400 and 1800rpm, and the more you can keep the motor at its peak torque rating, the better fuel economy you would get. So yes, economy would go back up a little with getting the motor back in its peak torque range at a higher speed, which in theory will up your mpg numbers. Whether it would work like that in reality, i dont know, but on paper it should work.

With an overdrive, 4.10's and true 35's, you will be at 1800 RPM's almost right at 65mph, which would be perfect for economy.

 

[mosesburb]

Joez is right. 1700 rpm is the sweet spot for mileage with the Cummins. So long as you are content with 65 being an economical cruise speed you should be set. I, however, wasn't. I decided to go with 3.55's, with overdrive in the NV4500 and 35's so that I could go around 75ish (been a long time since I did the math, so I don't remember exactly), and I will have a set of street tires (LTX's) for long road trips with no wheeling involved, so they will be shorter, thus raising cruise rpm a bit, so I went as high (low numerically) as I could. I have a doubler and a exhaust brake for compression braking off road (don't care for the pucker factor much anymore) to negate the ill effects of too tall of a gear. With 640 ft/lbs, acceleration will not be an issue /forums/images/graemlins/rolleyes.gif.

 

[mountainexplorer]

Thanks guys. /forums/images/graemlins/grin.gif

I just wasn't sure. I know in my gas powered truck, going from 33's and 4.10s to 35's helped me be able to cruise down the road faster, but the ability to maintain speeds on hills with heavy towing decreased.

I suppose a Cummins greater torque mixed with the different gearing options in the 5spd as well, pretty much allows the 35's with 4.10s without too many downsides.