This Horsepower Calculator has been modified from the original (see reference below) . Some of the constants used in the original formula were changed to yield results consistant with the information published by Cat Diesel in their document "Understanding RV Performance (CAT) LEGT5364.pdf".
See notes below for explanation, history, and links to the Excel spreadsheets. Download the version 3 spreadsheet. Ver 3 of the horsepower worksheet was created and shared by rv.net forum member SteveRankin. Credit to Steve and the others that have contributed.
Note: All variables are expecting positive integers with no punctuation. No error checking is done.
Enter the width of the widest part of the vehicle Reminder
Minimum Ground Clearance (in inches):
Enter the ground clearance for the vehicle that sits the closest to the ground
Desired Speed (mph):
Enter the speed to maintain
Anticipated headwind (mph):
Note: Tailwind does not help like headwinds hurt.
Anticipated grade (steepness) of the road:
%
For level ground, enter 0
Total combined weight (in pounds, tow vehicle and trailer):
Elevation (in feet):
Elevaton Factor:
%
0% for turbo-charged or super-charged diesel engines, 2% for gas and non turbo-charged or super-charged diesel engines.
Horsepower needed to combat:
Rolling Resistance:
(Speed x combined weight x (6.75 + (0.074 x Speed))) / 375,000
Air Resistance:
(Frontal area x Speed x Speed x Speed x drag coefficient) / 375 Special Note
Grade Resistance:
(Frontal area x Speed x Speed x Speed x drag coefficient) / 375 Special Note
Elevation:
(Elevation / 1000) * Elevation factor
Total horsepower required.:
The Total Horsepower calculated here is merely an estimate. The calculation does not take into account horsepower that may be lost due to the use of certain vehicle accessories (such as air conditioning, power steering, etc). If you know the horsepower output of your engine at its peak torque output, you can estimate the approximate speed at which you should be able to climb a particular grade with the engine rpm at the peak torque rpm. (You will need a horsepower curve chart and a torque curve chart for your engine to do this.) Enter the figures for total combined weight and for the grade. Then simply change the speed until the calculated total horsepower number is close to your engine’s output. Additional information can be found in the Notes column. Assumptions: 15% drivetrain efficiency loss.
This calculator is based on a spreadsheet sent to me by rv.net Forums member smkettner. The original spreadsheet was created by a couple of unknown forum members. If they contact me, I will give them full credit on this page. This link to GM provides additional information and formulas that may be helpful in understanding the calculations. Please understand that I simply converted the spreadsheet into a web application. I do not have any additional information other than what is included in the spreadsheet.