Aero & rolling resistance, power & MPG calculator
 

Does anyone know anything about the MPG calculator in the Garage & Tools section? I have been playing with it and I am finding there are at least three variables that are hard to know or pin down. They are rolling resistance, engine efficiency, and parasitic load.

I have been making informed guesses and I can generate some pretty interesting numbers by doing so but as to are they are true or not I can not say.
I would invite more of you to try the calculator out and let me know what you think. I will give you an example of the values I tried on my truck to plug in so you can see for yourself. The results matched observed mpg numbers in many instances.

Ford F-150 XLT 4x4 with a EcoBoost Engine and a 6.5 ft bed.

Weight 6070
Rolling Resistance 0.014
Cd 0.34
Frontal Area 36 sq ft
Engine Efficiency 32%
Drive train .893
Parasitic load 500 watts
Air Density set to 77 F
Display Range 5 to 200 in 5 mph increments.

I produces a calculator set for the Phil Knox's truck but I do not have good specs for it so I won't share that for now. Let me know what you think and if you have better values for the three most questionable parameters i would like to hear of them.

I have tried to dial in the calculator tool too. Like you, had to guess on some stuff.

But, one I didn't, was engine thermal efficiency. Happen to have a full speed load map for my engine. At my road load (near 100 HP) and RPM, it's 31%. I know your engine is totally different, and should be more efficient, but you're probably at a lighter load, so maybe it's close to a wash? In other words, I'd say your 32% guess is pretty close.

Where did you get .893 for your driveline efficiency? Seems like it's a pretty precise number? Or just one that made your data fit? Speaking of that, when I put your numbers in I get 17.08 MPG at 80MPH, is that right? That's your baseline, right?

Phil Knox generated the .893 driveline efficiency figure so I used it rather the using .90. Also I had a sq ft figure of 35.763 sq ft for frontal area. I guessed at 500 watts parasitic load but later changed that to 400 watts to tweak the results. I am generating good numbers for my speed ranges in 5 mph increments from 45 mph to 85 mph which I already have confirmation data for. I find the tool to be very interesting and fun to use as long as you can input some reasonably good assumptions. It is possible to make mistakes and still have good results if other inputs are wrong the other way.

Thank you for your information on engine efficiency. I had assumed as much. Any good numbers on rolling resistance and parasitic loads would be welcome.