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I recently purchased a 1997 Ford F-250 Heavy Duty with the 7.3L Powerstroke diesel and 212k miles on it. Two fuel tanks (front and rear) with a two-way selector on the dash. When I drove it off the lot, both tanks showed near E so my first stop was the nearest gas station.

I put 18.3 gallons in the front tank before the pump clicked off, which seemed about right because the owner's manual specs the front tank at 19 gallons. Then 16.7 into the rear tank before I ran out of fuel discount at 35 total gallons.

Started her up, and the reading on the front tank was about 1/4 full. Okay, maybe something wrong there (sending unit, electrical fault, etc.) but nothing to be too worried about. The family truck growing up did the same thing. Switched over to the rear tank and the needle slowly went up to F, which seemed reasonable as the stock rear tank is specced at 18.2 gallons. Drove home and was able to switch between tanks without a problem.

Today I decided to take a quick look underneath just to see what I might be dealing with and to my surprise the rear tank is plated for a 45-gallon capacity: Tank

Clearly an undocumented option or an aftermarket swap that was completed when the truck was new. Explains why there's no room for a spare tire where it should usually go.

Bottom line: I filled a 19-gallon tank completely and the gauge reads 1/4 full, and filled a 45-gallon tank less than 40% and the gauge reads full. What might be going on here?

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The fact that the gauges read empty when the tanks were empty and changed after the tanks were filled is encouraging, it means the gauges at least partly work.

The simplest explanation is that they do work, and that they are accurate, just that they are connected to the wrong gauges. You filled one tank to 40%-ish and one tank shows just over a 1/4 full, the other is full and the gauge shows full. Those readings are consistent with the fuel levels. If you fill the large tank and the gauge goes up accordingly that's what has happened. At 212k miles it's bound to have a few quirks.

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  • I had this thought, too. Would it mean that I'm also drawing fuel from the "wrong" tank?
    – Peter Schilling
    Commented Oct 8, 2020 at 15:33
  • Sure, it's completely possible. You'll just have to work it out from experience, or get under the truck and trace things.
    – GdD
    Commented Oct 8, 2020 at 15:45
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Worth following up on this with what ended up happening.

Turns out the total system capacity was 45 gallons across both tanks. It also turns out the tanks were rusting away from the inside, which conveniently clogged my fuel line while I was cruising down the freeway and forced a tow.

I had both tanks and all the fuel lines replaced, as well as the fuel filter and both sending units. Ended up paying a ridiculous dealer markup to get all the right parts, but now the gauges read accurately and there is room to mount the spare tire where it should be, once I find or build a mounting mechanism.

The downside was a return to the stock fuel capacity and known issues with fast fueling on these trucks. Another project for later.

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