I did a little homework and tried to develop a chart that would show how the different transmissions ratios 1:1 -- 1.25:1, tire sizes, rear axle ratios, and tire sizes come together to see how changes effect the drivabilaty of a combination. I did this in the mid eighties for Class 8 trucks and it really helped pick the correct combination of trans, engine, rear ends, and tire sizes when the speed limits were changing, along with newer diesal engines, wierd overdrive ratios, different tire sizes etc. The speed limits were changing from 55 to 65 then to 70mph while the tires were going from 11:00 24.5 to Lo pro 22.5. It was kind of like shooting at a moving target and to top it all off fuel prices were going up so dropping rpms at a given speed was very important.
I set out to find out what kind of power we had been getting to the ground for known combinations of drivetrain, and then using a new set of charts and tables what the changes we were making were doing to our crusing road speed and available torque.
What we finally devised was a table that showed USABLE TORQUE to the ground and another table that shows road speed at different rpms for the same eng/trans/tires/Rear axle ratio. The results were interesting because one could change everything on the truck and see how it affected the operating rpm at different speeds, and how it was going to effect the usable power.
Here is a example: Torque within a usable range of speeds
I like to use 2000 rpm and 65mph as a reasonable fuel economy mode and which will still pull well around town. In this case the 3.42 with a 700r4 has 580 ft lbs of torque in OD and a 200hp 350. My truck now has 627ftlbs available in 3rd gear with a 130hp engine but it is having to turn too high a revs. A change to 3:42, 700r4, 200hp means it will have more power and better fuel economy. Kieth
265/75/15-----230-----250------292-------350------350
----------------120hp--130hp-----150hp-----200hp----250hp
3.08
3---or--4spd--478ft#--518ft#---597ft#----697ft#---871ft#
OD-1.25-------358ft#--388ft#---448ft#----523ft#---653ft#
____________________________________________________________
3.42
1:1-----------506------548------632-------736------921
1.25 to 1-----398------431------498-------580------725
____________________________________________________________
3.73
1:1-----------579------627------724-------979-------1059
1.25to 1------434------470------543-------734--------791
___________________________________________________________
4.11
1:1-----------638------691------925-------930-------1162
1.25 to 1-----478------518------694-------697--------872
After Looking at this chart which shows torque not HP, you can compare it to what you have now and other powertrain combos that you know work pretty well. None of this is supposed to represent actual numbers since no losses are taken into account but it does give one a way to compare these variables and have some idea how they will perform compared to other combinations. Kieth
