rear end ratios

http://www.roadranger.com/rr/index.htm

Snoop around this website, they used to have a page that you could put in your parameters and it would give you the information that you are looking for. I thought I had the exact page saved in my favorites but I couldn't find it. The page that you are looking for will have you put in tire size and transmission ratio etc. etc. If I can find the page I will post it here, if you find it post it here also, a lot of guys ask these questions regularly.

 

that is the link

http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

 

That is exactly the page, Thanks car11220

 

Fill in the parameters with the correct information and it wiil give you the information that you are looking for, assuming you know how many RPM's that you want to be running at cruise speed. The basic concept you already understand if I read your post right. Each engine has an optimum cruise speed, (rpm). You were asking the difference in rpms between the 3:55 versus the 3:36 if I remember right.

 

Maybe this will clear up the "pulling" aspect people have pondered over for years. Lower ratios (say 4:56 or something) are better suited for heavy duty action such as a dump truck or low boy pulling out from a dead stop in soft earth or sand etc. It keeps one from "slipping" the clutch and starts moving the load with less strain ,but back to the pulling part. Take two identical trucks with the same engine ,horsepower ,transmission etc, and the only difference is differential ratio. When the engine is "maxed out" climbing a hill one truck might pull the hill at oh say 37 miles per hour in seventh gear ,and the other truck maybe doing 37 miles per hour in sixth gear. These are just examples mind you ,but ultimately you only run out of pulling power when you run out of gears.

 

Most fleets today opt for either a 3.55-58 rear end with a .73-.74 O/D top gear or 2.64 rear end with a direct drive top gear. Both combinations give roughly the same cruise rpms and final drive ratio and the best balance between efficiency and pulling power. However, theoretically the 2.64 rear end/direct drive will be more efficient due to the reduced losses and wear in the transmission by not running the final drive through a set of gears.

That being said, if you mostly ran, say, the Rocky mountain states pulling lots of passes, you migh be better off with a lower rear end (3.70-4.10) for reduced downshifts when climbing hills. On the flipside, if you are a bullhauler in the plains states, you might be better served with a higher rear end (3.36-3.08 ) for lower cruising RPM on the flatlands, ideal when your pushing 80 mph, lol!