I think EDJACK and FUDMAN have it right regards engine performance. Engine sensors, particularly the MAF (
mass air flow) and O2 sensors, compensate for changes in atmospheric parameters. FWIW, carbureted engines were/are an entirely different matter - 5,000 of altitude would definitely require re-jetting to maintain best performance. And depending on engine and carb, best at altitude may be a far cry from sea level.
Temperature ranges are primarily an oil issue, i.e. is the oil in the engine suited to the ambient? If you use BMW LL-01 certified oil you will be fine. The owners manual says approved oils in 5W-40 or 5W-30 viscosities "may be used for driving in all ambient temperatures."
If you are in below freezing temps for a prolonged period beware of potential CCV problems. Check Best Links for the story. Bottom line: avoid a steady diet of short trips that can build up sludge in the CCV or install the cold weather version of the CCV.
My 2003 530 takes ~10 minutes driving (not idling) to reach normal coolant temp in -25C, -13F. I've no indication of engine problems using LL-01 synthetic.
Maximum power depends on the mass of air that can be drawn into the cylinders. At 5,000 ft air density is ~83% of sea level. So maximum power would be about 83% of sea level. This corresponds pretty well with the rule of thumb that max power is reduced 3% for every 1,000 of altitude on normally asperated engines, which ours are. So, WOT throttle acceleration will be a little less than at sea level.
Highway gas mileage should improve. Aerodynamic drag accounts for ~80% of the power requirement at highway speeds. Drag is proportional to density. So a 15% reduction in density should result in a noticeable improvement in MPG. City driving MPG is primarily a function of tire rolling resistance and energy wasted on stops. Neither of these vary with altitude; city MPG should be the same as sea level.
Note though, as discussed in other threads, MPG depends on so many factors that any change compared to your current driving patterns and routes may be difficult to detect.
Your car, like mine, has an open (not limited slip) differential. This is good while moving; loss of traction on one driven tire does not transfer all torque to the other tire and jerk the car sideways - especially if accelerating at the instant. Not so good if stopped and one tire is spinning on slippery stuff, in which case you get none or almost no torque on the other tire. This is called being stuck. :rofl:
But your chains will solve that problem, if anything can.
Regards tires.
There is so much difference in wet, snow and ice performace between tires, even within the same performace group, that your experience will depend entirely on what you are now running. Search for tests, surveys and reviews at Tire Rack
http://www.tirerack.com/index.jsp
for an estimate of how your tires will perform.
My personal opinion is that driver experience and skill with ice and snow are as important as the tires. Some drivers can't stay in their lane with snow tires in 2 inches of the white stuff. Others manage just fine with all season or even summer tires in 10 inches. Of course if you want maximum performance and safety in snowy conditions, dedicated snow tires are the way to go.
Best of luck in Colorado. Your biggest challenge may be keeping your eyes on the road - the scenery is stunning.