Really bad MPG since winter hit??

[racingbmwm3]

Originally Posted by JCL

Interesting calculation. I'd use 80% for the alternator, less 10% on the belt drive.

But if it means 1.5 hp, that is about 5% of the 30 or so hp required to maintain steady state cruising speeds. That's measurable.

OK, for the 4.6is at 70mph and -5C (winter), I get 41.2HP including air drag and rolling resistance. I guessed .015 for winter tire rolling resistance and an optomistic 80% drivetrain efficiency. Have no idea what the engine efficiency is, so stuck with the default of .22, possibly higher on the 4.6?
Used this calculator: Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
And used 4.6is data from here: BMW X5 4.6is E53 (2001) - technical specifications - maximum power, maximum torque, fuel consumption - urban/extra urban/combined, maximum speed, acceleration, weight, dimensions, engine, suspension, rims/wheels, tyres, brakes, transmission/gear box,

Still, that's 3.5% going to just seat heating. I plugged in a parasitic load of 800W and it lowered the guesstimated gas mileage from 13.2 to 12.8mpg.

Interesting to see that the difference between -5C and +25C is 8HP because of the lesser air density and lower rolling resistance (summer vs winter tires).