[s-cars] Cold weather and lousy fuel mileage

[Stott Hare]

Weather here in the Northeast has had the temps hovering around 0*F, and my 
fuel mileage has gone straight to the toilet. 

Nice weather (40*F>) and 95% highway I can pull 21mpg and around 400 per 
tank.  Cold weather and 85% highway and I watch my mileage plummet to 16mpg 
and maybe 250-300 miles a tank.  Watching the TDS1 when the outside air temp 
is an indicated 0 - 15*F  shows an intake air temp ~20-25* warmer. 

Patient is a 95 S6A with 252k on the clock.  Still stock.  Receives routine 
maintenance.  She plugs, oxy sensor and fuel filter were done around 204k so 
they could be considered due, but I would expect that to cause decreases 
across the board, not temp specific.  Coolant temp is rock solid at the 
first fat bar (~10 o'clock).  Multi-function temp sender was done at 204k.  
The HVAC temp sensor in the coolant line between block and heater core 
replaced about 230k.  No CEL or codes. 

My prior experience with the CIS cars didn't yield such temp sensitive 
performance, if you had the car tuned right and all sensors working.  Is 
this something particular to the Motronic / hot-wire MAF systems?  Is the 
incoming air so cold that the hot wire isn't?  I don't have my mixture gauge 
in at the moment, but I suppose I will hook it up this weekend and watch 
mixture over the next week.  When I had it hooked up in normal temps 
previously (within 5k), mixture behaved as expected at idle and cruise, on 
throttle and off. 

This just the way things are? or is this an indication of a (temp) sender or 
something that is begging for replacement. 

Thanks!
 -Stott