Ron4RNevada
New member
I did this a few years ago just to work out the differences for driving at various speeds on a straight, flat, level highway at constant speeds and no wind. Note 1: I did not make any calculations for varying tire sizes, tread patterns or rubber durometers. Just some food for thought. Note 2: No I'm not going to bother with these inertia calcs for getting tires to initially turn from a deaq stop; but you guys go ahead with it and I'll follow along. This is a much more interesting thread than all of the TRD Pro "MODS", "CLUNKS"; "COLORS"; WHEELS; and "BLING" threads.......
opcorn:
- - - - - - - -
Some Calculations of Horsepower Required for a Medium Sized Full Time 4wd Vehicle at Various Highway Speeds.
All dimensions, measurements and calculations are for a 2004 Land Rover Discovery S with the following characteristics and modifications:
No external add-on devices such as roof -racks, lights, a-bars or winches.
Truck lifted 5 inches (3 inch spring lift and fitted with 265/75-16 tires).
Assumed a Cd (coefficient of drag) of .42 (based on typical pick-up with top shell).
Calculated fA (frontal area) of 42 ft² (6 ft wide x 7 ft tall).
Assumed a weight of 4600 pounds.
Assumed that the ratio between rear wheel horsepower and flywheel horsepower for a full time four wheel drive vehicle would be about 5:4 (multiply bhp at flywheel by 1.25 for bhp at wheels).
Calculations:
*1 - Rolling Resistance = 4600 lb x .015 = 69 lb. I actually measured this and it came out very close to 65 lb using the bathroom scale to push the truck in neutral, brake off on a flat surface. Then with the added resistance of transmission, transfer box and differential gears; 69 lb should be pretty close!
*2 – Air Resistance = fA x Cd x .00256 x mph² = 42 x .42 x .00256 x 4900 (70 mph).
10 mph = 5 lb
20 mph = 20 lb
30 mph = 45 lb
40 mph = 80 lb
50 mph = 125 lb
60 mph = 180 lb
70 mph = 221 lb
80 mph = 320 lb
*3 – Total Drag = Air Resistance + Rolling Resistance (69 lb).
10 mph = 5 lb + 69 lb = 74 lb
20 mph = 20 lb + 69 lb = 89 lb
30 mph = 45 lb + 69 lb = 114 lb
40 mph = 80 lb + 69 lb = 149 lb
50 mph = 125 lb + 69 lb = 194 lb
60 mph = 180 lb + 69 lb = 249 lb
70 mph = 221 lb + 69 lb = 290 lb
80 mph = 320 lb + 69 lb = 389
*4 –Brake Horsepower (bhp) at Wheels Required for Various Level, Highway Speeds. (Flat, perfect road, no wind, no curves and Steady Pace) = Total Drag x mph/375 x 1.25
10 mph = 2.46 bhp
20 mph = 5.49 bhp
30 mph = 11.40 bhp
40 mph = 19.86 bhp
50 mph = 32.32 bhp
60 mph = 49.80 bhp
70 mph = 67.66 bhp
80 mph = 103.68 bhp
So what’s the point? You say - - Well, for those trying to get a bit better fuel economy while navigating you 4600 pound, brick shaped, full time four wheel drive rig down the expressway at 80 mph; consider this:
At 80 mph it takes more than twice as much horsepower than it does at 60 mph!
At 70 mph it takes more than twice as much horsepower than it does at 50 mph!
OR – At 50 mph it only takes about 1/3 as much horsepower as it does at 80 mph.
Let’s assume that if you can get about 19 miles per gallon driving all day at 60 mph on a flat highway; what do you think it’s going to get when you put your right foot twice as far into the throttle to make it go 80 mph?
Now then; driving in hills, traffic or off-road; some different conditions do apply. But using only as much throttle as you really need really helps.
opcorn:- - - - - - - -
Some Calculations of Horsepower Required for a Medium Sized Full Time 4wd Vehicle at Various Highway Speeds.
All dimensions, measurements and calculations are for a 2004 Land Rover Discovery S with the following characteristics and modifications:
No external add-on devices such as roof -racks, lights, a-bars or winches.
Truck lifted 5 inches (3 inch spring lift and fitted with 265/75-16 tires).
Assumed a Cd (coefficient of drag) of .42 (based on typical pick-up with top shell).
Calculated fA (frontal area) of 42 ft² (6 ft wide x 7 ft tall).
Assumed a weight of 4600 pounds.
Assumed that the ratio between rear wheel horsepower and flywheel horsepower for a full time four wheel drive vehicle would be about 5:4 (multiply bhp at flywheel by 1.25 for bhp at wheels).
Calculations:
*1 - Rolling Resistance = 4600 lb x .015 = 69 lb. I actually measured this and it came out very close to 65 lb using the bathroom scale to push the truck in neutral, brake off on a flat surface. Then with the added resistance of transmission, transfer box and differential gears; 69 lb should be pretty close!
*2 – Air Resistance = fA x Cd x .00256 x mph² = 42 x .42 x .00256 x 4900 (70 mph).
10 mph = 5 lb
20 mph = 20 lb
30 mph = 45 lb
40 mph = 80 lb
50 mph = 125 lb
60 mph = 180 lb
70 mph = 221 lb
80 mph = 320 lb
*3 – Total Drag = Air Resistance + Rolling Resistance (69 lb).
10 mph = 5 lb + 69 lb = 74 lb
20 mph = 20 lb + 69 lb = 89 lb
30 mph = 45 lb + 69 lb = 114 lb
40 mph = 80 lb + 69 lb = 149 lb
50 mph = 125 lb + 69 lb = 194 lb
60 mph = 180 lb + 69 lb = 249 lb
70 mph = 221 lb + 69 lb = 290 lb
80 mph = 320 lb + 69 lb = 389
*4 –Brake Horsepower (bhp) at Wheels Required for Various Level, Highway Speeds. (Flat, perfect road, no wind, no curves and Steady Pace) = Total Drag x mph/375 x 1.25
10 mph = 2.46 bhp
20 mph = 5.49 bhp
30 mph = 11.40 bhp
40 mph = 19.86 bhp
50 mph = 32.32 bhp
60 mph = 49.80 bhp
70 mph = 67.66 bhp
80 mph = 103.68 bhp
So what’s the point? You say - - Well, for those trying to get a bit better fuel economy while navigating you 4600 pound, brick shaped, full time four wheel drive rig down the expressway at 80 mph; consider this:
At 80 mph it takes more than twice as much horsepower than it does at 60 mph!
At 70 mph it takes more than twice as much horsepower than it does at 50 mph!
OR – At 50 mph it only takes about 1/3 as much horsepower as it does at 80 mph.
Let’s assume that if you can get about 19 miles per gallon driving all day at 60 mph on a flat highway; what do you think it’s going to get when you put your right foot twice as far into the throttle to make it go 80 mph?
Now then; driving in hills, traffic or off-road; some different conditions do apply. But using only as much throttle as you really need really helps.
