I have a couple issues with my '87 F250

The gas gauge is stuck on full, when I turn the key on, it moves further past full. When I turn the key off, it just moves up to full. Has about 15 gallons in the rear tank.Switching tanks has no effect (front one is empty from what I was told). What is wrong,and how do I fix this?

Other issue, is the speedometer is reading 4mph fast.It has a matching set of P245/75R16 tires on it. Stock size is P235. What can I do to get it to read correctly?

 

Anyone??
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When I have my tank over half full it does the same thing. In my case, it is the sending unit on the fuel pump assembly in the gas tank. Once I hit about half a tank, the gauge more or less works with some flickering. The faster I go, the worse it is.

That would be the first thing I would look at if I were you.

On my 88 f250 the stock size is 235/85/R16, or approximately 31.7 inches high X 9.50 inches wide. If your tires are a little shorter than stock then your speedo will run fast. I'll see if I can find the numbers and post back. I've got a couple really busy days coming up, so sorry if I don't get it done today or tomorrow. It is possible to calibrate the speedo for about any tire, but I don't know the process on these Fords yet. Still learning

 

I found an online calculator, the ones I have now are 30.7 inches tall. Is there a different speedo gear I can get, to compensate fot the shorter height tires? They are new tires and I wanna wear them down before swapping them out

I noticed, someone installed an aftermarket pump on the frame rail. I might try removing the cluster bezel,and try tapping the gauge needle, and see if it is merely stuck

 

What I find very odd, is how both sending units could go bad. I did observe, previous owner had an aftermarket inline electric fuel pump installed,guess I will have to dig into this further when the weather warms up next week

 

Is there any way to test to see what the issue is, all the wiring to the sending units appears intact,and not cut. I ran out of gas this morning,and need some way to get the gas gauge working.Has an aftermarket electric pump,not sure why this was installed, as everything else appears normal.

I can't imagine both tank sending units going bad at the same time.

 

I'm not sure what stock rim size is on that year truck but mine is 15" rims... 235 75 15

 

For the fuel gauge issue, you have a bad ground, most likely in a harness before the side mounted tank (which is why it pegs full on both tanks).

As far as the frame mounted pump, that's a factory high pressure pump. The tanks have low pressure pumps in them, that feed the high pressure pump.

About the speedo, if you're going to replace the incorrect sized tires with the factory size at some point, it may be easier to just leave the speedo gear as is and use a GPS for a speedo, or an app on your smartphone if you have one.

 

The frame mounted pump on Mine is an aftermarket Mr Gasket brand universal one. Is this ground for the fuel gauge that you mentioned which is in a harness before the side tank go to the frame, or on the body?

The tires on it now are like new, so I want to run them awhile.

 

Stock tires on F-250s were LT235/85-16. P series tires are passenger-car rated, LT are light truck rated. If your current tires are P245/75R16 take a look at the load rating on the sidewall, they might not be up to what you think your truck is capable of.

245/75-16 tires are 4% shorter than 235/85-16, so that should only make your speedo about 2 mph high at 50 mph, but a speedo reading within 2 mph is pretty good in my experience, so you might just have a couple of errors adding together. I don't know for sure, but I think an '87 has the mechanical speedo cable off the trans (or t.case) tailshaft. I've never recalibrated that style, but yes, it's done by changing gears, either in the tailshaft or on the speedo cable.

 

I know that the ones I have aren't LT tires, they were on it when I got the truck,they are practically new,and I am going to run them awhile. I rarely haul anything heavy (except for My lard butt that is ). On My street they installed electronic signs that show your speed, When it says 30, My speedo is reading 33, so maybe that isn't too bad

 

That pump may just be a replacement. From the factory it uses two low pressure pumps, and one high pressure. That shouldn't cause any issues with your fuel gauge.

I'm not sure where the ground goes in that harness, I don't have a schematic for an 87.

 

Ok thanks I guess I will have to look around

 

I believe this is the factory high pressure pump, they have the aftermarket pump spliced into the blue line from the rear tank.Looks to be al there wiring wise. Not sure what to check next.

 

Anyone have an '87 schematic I could view??