Really bad MPG since winter hit??

[JCL]

I used a SWAG (OK, some judgement involved) for my 30 hp figure. But I wasn't thinking 70 mph; the average on my vehicle over several years is closer to 36 mph, at which speed 30 hp may actually be pretty close. But then I don't have rear seat heaters. The only point was that the 5 or so % is another hit against mileage, added to increased idling time, less engine efficiency when cold, and all the other things that bring mileage down in winter.

What is surprising to me is that we have all these posters who didn't know that their mileage would drop when it got cold out. It has been doing so for the last 100 years or so.

BCR: You have to allow engineers to be engineers every now and then

[JCL]

racing: thanks for that calculator, it is interesting to play with.

I used the 4.6 vehicle figures (weight, cd, frontal area) at 5 c (my temp today) and pushed my average speed up to 40 mph, slightly higher than my average on my vehicle. The variables were:

Vehicle weight: 2180.9 kg / 4808 lbs
Crr: .012
Cd: .35
A: 2.8 m2 / 30 ft2
Fuel energy density (Wh/US gal.): 32970
Engine efficiency: .20
Drivetrain efficiency: .95
Parasitic overhead (Watts): 800
rho: 1.269 kg/m3

I got 11 hp total, lower than I had estimated. At 75 mph (hard to maintain that over a drive cycle) it was 43 hp.

I don't know if 800 watts is a reasonable figure, but taking it on faith I got 7.63 l/100 km and 8.38 l/100 km with an 800 kw parasitic load, at 40 mph, representing a 9.8% fuel consumption penalty for seat heaters on high. That is about double what I had estimated.

[bcredliner]

Originally Posted by JCL

I used a SWAG (OK, some judgement involved) for my 30 hp figure. But I wasn't thinking 70 mph; the average on my vehicle over several years is closer to 36 mph, at which speed 30 hp may actually be pretty close. But then I don't have rear seat heaters. The only point was that the 5 or so % is another hit against mileage, added to increased idling time, less engine efficiency when cold, and all the other things that bring mileage down in winter.

What is surprising to me is that we have all these posters who didn't know that their mileage would drop when it got cold out. It has been doing so for the last 100 years or so.

BCR: You have to allow engineers to be engineers every now and then

I know, but it is sooooo painful.

How do you account for head or tailwinds, air density differences, summer/winter fuel additives, average speed or terrain. Seems like the only way to be close to accurate is for that particular X5 that the formula is based.
What would you say the +- range is for accuracy?