Where did the fuel go?!?!
 

So... I typically am able to find answers or a direction to follow by searching the forums but I have not found this addressed. I have a 97 250 4x4 with 310,000. It has been well maintained and I am pleased with its overall performance. I pull a 6x12 covered trailer everyday for work, loaded I'd say under 4000lbs. I just installed a TS 6 position chip with canned tunes mainly for the 50 hp tow with the delayed TC lockup. I have been very happy with the hp gains and the shift points. I ran approximately 30 miles on the 75 hp and was thrilled with the throttle response and the power. Now to the problem. I filled up both tanks towed my trailer as usual. On the front tank I was seeing about average mpg, got down to a 1/4 tank and swapped to the back... Less than half a tank!?! Refueled to eliminate the possibility of a stuck float or faulty gauge. It took 9 gallons. Hoping to hear some wisdom. Could this be related to the chip?

Any fuel in oil or coolant?

1st, welcome to the forum. Head over and post an introduction in the 'Diesel Bomber Introduction' sub forum.

As far as your missing fuel goes, i've read that the tanks can rust at the seams and cause the fuel to leak, the filler neck clamp. You really need to just pull the skid plate and try to find where its leaking from. Also, I know this may sound dumb but look at your receipt and make sure you filled both tanks. On more than one occasion while i've filled up the pump stopped before the tank was full due to ventilation issues with the filler neck or the pump nozzles here in Cali (not sure which) and I didn't notice.