fuel guage problem

I was planning on fixing this in the future, but I am leaving for montana in the morning and figured I should fix it before I go. I have another mazda I can use for parts if there is something wrong with mine but here is my problem.

When I fill my tank, my gas guage reads empty, actually it reads nothing at all, but when I get to about 200 miles, it goes up to a quarter tank, then when I get to about 230-250 it goes past a quarter. now it doesnt move untill it reaches about 200. I dont know whats wrong with it.

if anyone has any idea what I could do to fix it before I leave, I would greatly appreciate it, if not I have to stop every 250 miles and fill up just to be safe.

another question, does anyone know how big(in gallons) my tank is in a 86 Mazda B2000 SE-5 Extended Cab?
thanks
-Mike

the fuel gauge sending unit in the tank, its attached to the fuel pump assembly, drop the tank ( or pull of th bed) pull out the unit and swap it with the other one

B series trucks are notorious for having bad fuel gauge sending units.They are located in the tank itself and u have to either drop the tank or take the bed off to replace them-not something I would want to do tonite before leaving for a trip.The stst sheet on Bseries says 12 US gallons for tank capacity but I have run mine as close to dry as I can and cant get but 10 US gallons in it.Hope this helps.BJ

Bump.Sorry Mike I missed ur post.BJ

so with it moving with it getting low, it would still be the sending unit? When I drove the truck 260 miles, I put 10.6 sumtin gallons into it.
thanks,
Mike

sending unit is bad..or going bad, its on top of the tank, 4 or 6 screws, unplug a wire, pull the old one out & drop the new one in, making sure not to get any trash into the tank, youll either have to drop the tank or pull the bed.....i think the regular cab tanks are close to 12 gal, & the ext cabs are close to 15 gal.....not sure i'll have to do some checking.

I am trying to get my aftermarkey guage to work correctly, has anyone done this w/o a new sending unit?

I have guages of this series.
I think I'll have to get a new sending unit for it to work i,ve probed the wires from the factory harness but I cannot get any data that makes sense to the new guage, must be a sweep of different resistance, (range of Ohms)

On factory sending units the fuel level sliding contact is what goes bad after use.Even if u put a new sending unit in it will also wear over time and give inaccurate readings as the replacement is the same part with the same weak mechanism.I would try an afetrmarket unit if u can find one.Also might try unplugging the harness at top of the tank and spraying contacts with electrical contact cleaner.That might help.BJ

my truck has the same problem and i already have my bed off so i am gonaa fix it before i put it back on