HD now make a decent fuel gauge for Sportsters, the only problem is you've got to install the fuel level sensor inside your tank (part: 61200008A).  My bike came with the combination speedo/tach gauge (part: 70900100C), which has the fuel gauge and range displays built in.  But it didn't have the fuel level sensor in the tank, so these features didn't work.  I bit the bullet and decided to install the fuel level sensor.  It was a bit of a pain getting the fuel pump assembly out and back in to the tank, but it's done.  I now have a decent digital fuel gauge with range function on the Sporty.

Getting the fuel pump assembly out of the tank.  This is difficult because of the tight clearances around the sides.  I found removing the spring clip from around the fuel filter housing made it easier to jiggle it out of the tank:

Pic with upgraded fuel level sensor installed:

Everything back together and configured.  At first I couldn't get the option to enable the fuel gauge in the speedo setup.  This setting allows you to also select your tank size.  

** I had to pull the main fuse to get the speedo to reset itself so I could configure the fuel gauge.  Once that was done I got the option to enable the fuel gauge and select tank size.  **

The last part of the install requires you to fit a jumper plug to the low fuel sensor harness connector.  On 2014 model Sportsters this plug is located under the ECU electrical cover, behind the rear cylinder.  See my post below with a picture of the jumper.

This is what my speedo looks like (from when I was taking pictures of the light install :D )

Underneath the tank:

The only way you can tell if the fuel level sensor is installed from the outside is to check the jumper connector.  I think you've got it there in your second photo on the side cover.  If there's a jumper wire in the end of the connector you most likely have the fuel level sensor installed.  

The fuel gauge needs to be enabled to show on the display.  Most likely your bike is in the same state as mine was, having the fuel gauge switched off because you don't have the fuel sensor installed.

Here's a pic I downloaded showing what the jumper connector looks like on mine now the fuel sensor is installed.  Yours is under the seat but the same connector.  When it's not installed the plug you see below is just solid plastic on the end.

what range do you get and how many liters does tank hold.

I'm still with carb Harley's never ridden an efi yet. I ride till reserve then fill up.

I then proceeded to the servo and filled the tank up.  On starting the bike after fill up it took a few seconds but the fuel gauge updated to show four bars (full tank).  The range display also updated to 232km.  This didn't seem accurate, but the instructions for the fuel gauge state that the range is updated during the first 48km's.  So I took the bike on a short run and was pleased to see the range slowly increase the farther I rode.  It got up to 245 and then started to go down slightly.  But by the end of my run which was 33.2km's the range now displays 239km's remaining, so 272km total.  This seems close to what I'd expect.  Possibly a little low, but considering it was mostly city riding and I didn't do the 48km recommended, I'm pretty happy with the numbers.

At least this seems to prove once and for all that the fuel level sensor can be installed after the speedo/tach gauge.

Range display after filling tank to full and doing a 33.2km test run:

I recall reading in my owner's manual (2014), even the factory gauge should display the range feature.  So for those without the upgraded speedo/tach they should still be able to get range if they have the level sensor installed.  You'd have to check what year the range function was introduced into the factory gauge, not sure how far back it goes.

I also found out that the low fuel light still works.  Low fuel light went on at around 85k remaining on the range display.  Once I was below 60km on the range display, the fuel gauge went down to no bars and "LOW RNG" now displays on the range display.  

The trip meter was at 210km when range remaining was at 70km.  So at 250km currently on the trip meter I'm down to my last 30km going by that.  From a visual inspection of the fuel in my tank, I reckon there's at least one or two litres remaining so I'm sure I'd get close to another 30k from the current tank.  This would give a total range of 280km.

It's a bit disappointing that the range display switches to "LOW RNG" below 60, I'd prefer if it kept going lower.  But in any case I'm very happy with the accuracy of this device.  

I've been commuting to work, and I had my range down to 14km.  I filled up yesterday and the tank took 13.1 litres (4.5g tank).  I'm a little surprised that the 4.5g tank seems to only have a usable capacity of around 14 litres.  I would have hoped for a bit more.  But it all adds up, with say 20km per litre and 14L tank that makes 280.  280 is what my max range reports on a full tank at highway economy.

I've also noticed that the max range seems to learn based on the last tank of fuel.  A few weeks back when I was doing highway riding, I was getting a max range of 280 with full tank, now with the city commuting, it reports around 250 with full tank.

All in all, it's a bloody great mod.