Just come back to this and have managed to get it working nicely on a graph plot (with the if statement to set var's on first frame and not get NaN's or div by 0)
Right now I have added the following variable inputs:
vehicle curb weight (sans full fuel load as per spec for curb weight, basically the car ready to drive except just reserve fuel)
vehicle fuel tank size
vehicle fuel tank % full
vehicle fuel density (probably make this a toggle for diesel/petrol true/false thingy)
That should get you an easy to input weight setup, ie, you weigh the car, know your own weight, maybe have luggage/kit onboard, and you can get quite accurate just moving the % fuel tank value to, so if you have half a tank add 0.5 variable...
So thats a nice way to try be consistent and accurate with the weight input.
Then I've added:
tyre width, profile, tyre diameter OEM
tyre width, profile, tyre diameter corrected (for what you have fitted, because the speed sensor is reading for OEM tyre size)
speedo correction factor (another correction factor, you might set this using GPS, to get the actual speed the OBD is sending the script accurate to real life)
Then I've added:
cross sectional area
coeff of drag
tyre rolling coeff
then I've calc'd the power losses through aero and tyre drag for the given speed (once corrected, and using the total mass)
I've calc'd net bhp, then added these losses to get a rough wheels figure.
My next step, after a bit of testing, is to tidy it all up so it's neat and tidy with the variable names all clearly defined/commented at the top, then add a new section.
Using the corrected speed, and the wheel correct rolling circumferance given, I will calculate the speed of the diff (have a final drive input too), and so for at least a 2wd vehicle, i can calculate the average drivetrain speed and use a columb friction to add 'losses' here too, using a friction input value.
IF I were really tempted I could record engine rpm (slows it down a bit too much on my 1999 OBD setup for super smooth graphs), then I could calculate the drivetrain speeds in two different areas.
You could then do some of your own maths, doing coastdown's and recording the power loss vs rpm of clutch up and down, of the drivetrain components vs speed, and simply add the correct friction (torque) to the system to simulate that too.
It's already giving pretty accurate figures for my car... you just have to be on a flat road