I thought I'd ask you guys, maybe you've had this problem. When totally full gauge reads between 1/4 and 1/2. I pulled tank down put another sending unit inline same thing. Checked grounds, same. If I start off quick or brake fast the fuel gauge and oil pressure gauge will twitch up and down the scale. I'm thinking its a gauge problem, your thoughts?

Some of the 80's model trucks had problems with the gauge grounding. Seems like the most common place was on the back of the instrument cluster. If I remember correctly the ground for the gauges ends up in the firewall/fuse panel area.

I forgot to mention its a 95 6.5TD. I didn't pull the sending unit out of the tank, cause the other unit acted the same. And why would the oil pressure gauge act up with the fuel. I'll check for grounds first, then pull the dash cluster.

I have a similar problem with my '91 Ranger.

Unfortunately, I haven't had the time to investigate it thoroughly.

Ranger Symptoms:

• Gauge often maxes out at 1/4 to 1/2 tank.
  
• If Gauge shows I have gasoline, then that is ALWAYS true (never reads high).
  
• If Gauge shows no or little gasoline, that may or may not be true.
  
• Gauge works best when doing normal city driving, and may recover to some extent over time (sometimes a few months of short driving, infrequent fillups).
  
• Gauge works worst when doing long "road trips".

Normally I just push the trip meter button whenever I fill up. Then I refill every few hundred miles if the gauge is going funky. If the gauge shows gasoline in the tank, then I'll go by the gauge.

The problem is when doing roadtrips carrying a load and pulling a heavy trailer, going over mixed terrain including mountains and the continental divide. Then the mileage plummets, and is somewhat less predictable with more dire consequences if one runs out.

On the Ranger, I had been thinking it was the sending unit.

Experiments on my VW Caddy showed essentially the fuel sending unit is a variable resistor, from 0% to 100% resistance.

Full = Closed Circuit (to ground, I think) - 0% resistance
Empty = Open Circuit (to ground, I think) - 100% resistance

Assuming you can find access to the wire to the sending unit.
Then you should be able to test it.

Ground the wire, and it should be equivalent to a full tank. Assuming a slow response gauge, it would take a few minutes for the gauge to recover, but it should show Full.

Open Circuit - This would cause the gauge to show empty.

(Or, visa-versa, of course).

The sending unit of most older cars is a simple float attached to a variable resistor.

It is possible that "newer" cars are different.

For example, I was playing with my Washing Machine Recently.

It essentially calculates the weight of the water column.

This works great in an "open system" like a washing machine.

However, a similar system might have issues in a "closed system" like a fuel tank.

If this is the case, then you would have to look at your tank ventilation and vapor recovery system.

You could try leaving the gas cap off of both tanks for a little while and see if that helps.