Posted by Traveler on December 24, 01 at 13:59:30:
I'm cross-posting some valuable info from another forum. The post originator "Marty" is not our own Marty but a great guy Fat Boy and I met at the Diesel Page Rendesvous. He has a nice camping rig they tow cross country. Marty's boys "twin towers 5' 16" " are like talking Helm manuals even though they are teenagers.
"A little present for those getting for xmas/buying new trucks. Hopefully some info to make sure you buy the your truck, with the correct HP, torque, gear ratio etc to pull as fast, slow, and steep as you want.
A present I found for myself, the other day was a spec book for MDT’s. This info should help those with pickup/trailers combo’s, as well as type A and C motor homes buyers.
This is a long post!
Assuming you have a vehicle(s) with a total frontal area 91 sq ft you need minimum HP to pull GVWR/GCWR at MPH. These number’s can very by 30% less to 36% more for a very aero dynamic rig vs. very non aero dynamic. A car hauler was used for non aero dynamic rig.
GVW 55 60 65 70 Additional HP per 1% grade at given mph
15K 97 122 152 187 24 27 29 31
20K 102 128 159 194 33 36 39 42
25K 107 134 165 201 41 44 48 52
If you want to pull 20K at 60 MPH up a 4% grade, 128 + (4 x 36) = 272 HP minimum. Assuming no head wind etc.
To figure out gradability, i.e. how steep a hill you can start on, I knew a formula existed, but trying to find it was hard. I found it, very confusing, but I did find a chart showing overall gear reduction needed to go up a hill with X ft lbs of torque. This uses a 10R20 tire with a dynamic (loaded) radius of 20.2”. A typical pickup tire of say 235-85/265-75-16 or a 225-70-19.5 has a dynamic radius of 15-16”, so you would reduce the figures by the % difference, or about 25%.
525 ft lbs 15% 25% 35%
20K 10 17 21
25K 13 22 26
450
20K 12 20 24
25K 15 25 30
350
20K 16 26 31
25K 20 33 39
To figure out your overall top gear ratio for best mpg, you need RPM (typically max torque rpm) that you want engine going at X mph.
Gear Ratio = rpm x Dynamic radius inches
168 x mph
If you want to go 60 mph @1900rpms with a 15” diam tire = 1900 x 15 = 2.83 - 1
168 x 60
This would be in Direct drive, with a .71 (typical O/D) = 3.77 rear axle gear ratio
To get first gear to for gradability needed overall low ratio needed/ gear ratio = first gear ratio needed
So if you have a 450 ft lb torque motor, 20K gcwr, 20% grad ability, 2.83 gears w DD tranny. You would need a first gear of 7.07 x 25% reduction for smaller tires = 5.3-1 first gear
Or with an O/D tranny and 3.77 gears = 3.98 – 1 first gear
While I probably confused some of you. For those I have not, this should help some get a truck for their needs instead of looking at some brochure that states said rig has a GCWR of X lbs, yet you can not go up a certain hill as fast as you want! Or what has happened to me once with a 16K GCWR truck, I could barely go up a 35% hill at 10k, much less sitting at the bottom at 16K burning up an auto tranny going no where! And that sales rep said my 454 th400 w/ 4.10 gears would go anywhere! Have I got some news for him! How many of us actually have the same needs/wants as the engineers at the factory! Much less “MY” needs and wants vs. “YOU”!
Merry Christmas and have a Happy New Year in your correctly powered “NEW” vehicle!
Marty
ps
I have not gone into figuring out the correct GVWR/GCWR of needed rig, as I personally feel this is covered in many other threads. But I have never seen this info covered or touched!
96 K35 Crewcab 6.5L TD
92 "Motel" Prowler 24C for "6" "