So I need a new gauge or sending unit

 

Sending unit if it's reading off like that. Rarely is the gauge in dash at fault. Usually when the gauge in dash is at fault it doesn't work at all.

 

Where can I get them for a reasonable price or is there a way I can get a gauge that will work with the 81 sending units because they ar like $30 cheeper and I would not have to change tanks

 

Well first off, what configuration is your 87 sender? is it an EFI model, electric fuel pump on sender, or one of the extremely hard to find engine mounted fuel pump and two prong connector models? there is no way to make the 87 gauge work with the 86 and older resistance values. The 87 sender is calibrated to work with the new style resistance values.

Just curious, why did you swap clusters like that?

 

My dash was completely trashed and a friend had the 87 and auld not get the titel so he said I could use it for parts

 

What all is wrong with the original cluster?

 

THere was nothing wrong with the original cluster the 87 had a tack so I wanted to chang it out and then when I was having problems with the fuel gauge I took the one out of the 81 cluster to see if I could make it work and I broke the gauge

 

Honestly, I would just go to a junk yard and find a cluster with a factory tach for your year model. I just went to the JY last weekend and saw about 6 of these trucks, 2 had a tach cluster.

 

I don't have the old dash to put it back in my truck could I make that cluster fit