Hi Cheap,
Please make certain (using your handheld meter) your ground wire is making a good connection and then please bend upward (enough that it clearly does not touch the mounting part of the sender) the center (red wire) connection. It may simply be the image you've sent, but it appears there is a chance it is making contact with the part of the sending unit that will be grounded. That (as has been pointed out here several times...maybe several hundred) will cause your gauge to read incorrectly. The whole fuel gauge and sender works on the principle of a balanced magnetic scheme and not to bore you or anything but variable resistance to ground and two differently wound electromagnets (along with a small hourglass-shaped permanent magnet) are what makes it all work. You are getting an incorrect variable resistance amount once your sender is installed it seems. Since it is new, I'm going to assume your float is good. It ought to be. At empty your sender should be sending zero resistance (resistance to ground that is) to your gauge. At full, 30 ohms. Naturally at 3/4 of a tank around 22~23 ohms. Our chore here is simply to find out why the resistance you're getting is more like (what I'd expect) to be maybe 8 ohms with 3/4 of a tank.
