Well gang... I made a trip down to Mountain Home ID in poor weather and the outside temp was ranging between 50-70*F and the IAT was range 85*F to 110*F. Now what I found out with cold wet weather is that it tends to kill off the MPG rather quickly. Now with flooded highways and cold moist air just pulled my MPG's down to about 18 MPG which happens to be about -2 off from my typical normal. Now the Next day making a routine trip to Riggins, ID I still came back to my typical 20 MPG average but the weather was dry for the moment, warmer, and IAT was staying more so in the 100-110*F bracket.

Just speculation but I guess wet, or rainy humid are has less oxygen per unit of air and I would imagine that wet roads may have a slightly higher rolling resistance.

I relize the rolling resistance and hydro-plane problem... But I think the moist air tend to retard burn rate of the fuel... (theory...) What I'm finding out is that IAT is a big controlling factor in MPG's... Colder the intake air the more fuel must be delievered because of cold air is more dense so keeping the balance the fuel rate must be increased. But bring on a hot summers day and 100*F dry weather and you most likely find a increase in MPGs... Warm air is less dense and requires less fuel... Not to mention warm air creates less wind drag...

I seriously think it is IAT. I'm just back from a weekend jaunt. I had a 300 mile run out the Mass Pike & NY Thruway. The overhead is dead on with my stock 01 CTD... 4.10 ratio. Eventually it got it up to 18.9 I naturally enjoy working the motor to keep ideal speed with just enough throttle to do so. With a cold start & a couple of short local trips dropped the mileage average by 1 mpg to 17.8-9.The return trip again had the average back at 18.9. I'm thinking of blocking off the stock fender air intake for faster warm up. It's really rare to ever get 100 here, even more rare to get a few degrees hotter. More common to get highs of 80's. I think faster warm up will get better mpg in my environment.Russ