What do truck specs mean really

The 3:55 ratio means that the drive shaft will turn 3.55 times for every single revolution the drive tires turn.

 

The higher the number (like 4:11) means the engine will turn at a higher RPM, than say a 3:55, in any given gear.

If you wanted a 100 MPH truck, you might opt for a 3:08. If you wanted to climb mountains easier, you might opt for a 4:56. These are extreemes for example. Most trucks are closer to the 3:55 you mentioned

Higher numerical number (short) = higher rpm

Lower numerical number (tall) = lower rpm

 

And higher rpm at cruising speed of course translates into decreased fuel mileage. So if you're on a dedicated run pulling whatever through Nebraska where you can stand on a soup can and see 50mi in every direction, a lower number may be preferable. Likewise, if you're in Oregon and run logging trucks that go up and down hills all day long, the higher number means the truck won't have to work as hard and you won't go broke putting fuel in the thing. Most trucks are somewhere in the middle since they do a lot of both.

 

Actually it means that the ring gear will turn 3 times while pinnion will turn 55 times

 

I hope you are just messin with this guy.

It actually means that the ring gear has 3.55 times as many teeth as the pinion. That means that the output shaft, in this case the axle, will rotate one turn for every 3.55 rotations of the input shaft, which in this example is the driveline.

If you want the ring gear to turn 3 times the pinion will turn 10.65 times (3.55 X 3)

 

There is a 9 page Auto Shift thread going here...
http://www.thetruckersreport.com/tr...om-new-drivers/82608-autoshift-questions.html

 

Also, the tire size has alot to do with r&p.....a low pro 22.5 will need a lower # gear...as 11r24.5 will like the high #.