Fuel economy getting worse

[jambo101]

Started the big trip to Fla and after the first days driving my Scanguage read average fuel economy for the day as 42.5mpg..great
Second day of driving and that 42.5mpg reading slowly dropped to 35.4mpg

Only thing that changed that i could see was the outside air temperature which went from 25 degrees on day 1 to 80 degrees at the end of day 2.Only possible theory for the poor fuel economy i could think of was poorer rolling resistance of the winter tires as the weather got warmer.
Any one have any other ideas?
Guess i'll have to break out the old pencil and paper system on the way back just to make sure this electronic gizmo is working correctly.

My other electronic problem was with the Garmin Nuvi 255 GPS unit which inexplicably wont turn on any more
I've had it for about a year and a half and its worked well on the half dozen times i've used it.It worked for 15 hours on day 1 of the trip but wont turn on on day
2 of the trip
Any ideas to get it working again?
Thanks..