The MPH changing at all with no hp change shows why the calculation its an estimate, a tool and not an exact science, doesn't it?
I even posted where it states this on the web site.
It measures the power you used to move the weight, the distance, to the speed, not the actual power the engine is capable of making.
UNLESS 100% of the engines power can be applied, the full distance.(parasitic drive line loss is different for different trans, and rear combos, and tire spin)
And that isn't likely, on bias ply tires, by mag test guys, in the 60's and 70's
If I am wrong, then I am wrong, I have been wrong before and will be wrong again I am sure, just ask my wife.
Just used that calculator on a stock 2016 Charger R/T, 4800 with driver 103MPH claims 409HP
Dodge says 370HP how can 40HP difference be accurate?