Really bad MPG since winter hit??

[racingbmwm3]

would need to know the efficiency of the alternator, but let's say you have front and rear heated seats, each pulling 15amps at the high setting? ~200W/seat (@14V). ~800W total. If the alternator was 100% efficient, that is about 1.1HP. even assuming a horrible efficiency of the alternator of maybe 50%, that's 2HP. Same logic goes for the heater, but the heater has even less impact since the fan is the only (large) electrical load. The heater does have an impact on the length of time it takes to heat your engine up to operating temp though.
Which is the main reason for lower mileage in the winter. Longer warmup cycle, meaning the engine is running rich for a longer period of time. Combined with longer warmup times for the oil in the engine, transmission, transfer case, and front and rear differentials. That's alot of sources for increased drivetrain losses due to thicker fluids. Additionally, the cold air isn't ideal for a complete burn, but once warmed up, your O2 sensors account for this and correct it as much as possible.

[JCL]

Originally Posted by racingbmwm3

would need to know the efficiency of the alternator, but let's say you have front and rear heated seats, each pulling 15amps at the high setting? ~200W/seat (@14V). ~800W total. If the alternator was 100% efficient, that is about 1.1HP. even assuming a horrible efficiency of the alternator of maybe 50%, that's 2HP.

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.

[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).