Winter gas and cold is the main culpret as previously mentioned I think but there is something most overlook. Check your tire pressures. The cold weather will drop the psi way below where it should be. My truck tires were 6 pounds low and the car tires were down to around 27-28 psi instead of 32 where I usually run them. That was last week. I need to double check them again. It has been very cold here.
Was the "farm tank" off road fuel? Assuming it is off road gas, all bets are off because one would assume it's not the same as road fuel. An example is that off road diesel has higher sulfur (I think) than regular pump diesel. That may not be accurate anymore, but I think it used to be that way. I would assume gas is the same. Off road fuel must be delivered and is typically died a different color so the authorities can tell you are running off road fuel. It's mixture may produce similar results to winter fuel as it is intended to be stored for long periods that may stretch into winter.
I second the PSI check.
Even in a 32df space (or higher), it takes longer to warm the truck up at that temp than at 70df. If you let it sit and warm up even for a couple of minutes it can reduce your MPG.
If you are running the wrong viscosity lubricants you can be hurting FE too.
If you drive through snow/weather, that reduces FE.
If you changed to snow tires they reduce FE.
If you put sandbags/extra cargo in the back you can reduce FE. Most pickup owners do this for added traction.
If your commute/daily drives are now in the dark, the added altenator load will reduce FE.
Other possibilities might be your routine has changed. Maybe you are hauling more weight, driving at different times of the day (necessitating headlights, heater), taking different routes, etc.
Ok thank you for all the ideals so far so keep them coming. Now I keep the tire presure at 40#, the gas from the farm tank is the same as you buy at town, i allways drive with my lights on summer or winter for safety. I have a scan gauge that is very close to what the pump calulates out at and i can fill the tank with everything warm and drive 50 miles and it still very low.
I have keep track off every fill up for the last 3 years here is a sample.
10/6-20.4 mpg, 10/14-19.25, 10/26-18.37, 11/9-18.1, 11/11-19.6,
11/21-17.4, 11/26-18.4, 12/9-17.1, 12/15-16.6, I live in northeast indiana and as it gets colder the mpg goes down. I use this truck to go for parts so the better mpg are from longer trips (60 miles) the lower mpg are from shorter trips (20miles)........Dave