I'm not with Laminar Research, just a fellow simmer in the UK and whilst I do not have any actual fuel consumption figures for the default 737-800, you can still ensure that you have enough fuel onboard in a couple of different ways.
1. If you're not sure how much fuel to put onboard, simply select aircraft type, click on 'Customize' button, click 'Weight, Balance & Fuel', click on 'Restore Defaults' & then click on 'Done'. Please see 1st photo below.
2. Similar to No 1, load aircraft, click on 'Flight' in top left drop down menu, click on 'Edit Weight & Balance', click on 'Restore Defaults & then click on 'Done'. Please see 2nd photo below.
3. If you wish to be more precise with the amount of fuel loaded onboard, you will need to use an online flight planner to generate an Operation Flight Program (OFP). I use Simbrief which is free to download and can be found at this website:- https://www.simbrief.com/system/briefing.php.
A copy of an OFP can be seen in the 3rd photo below, which will provide all required data to file a flight plan, including the amount of fuel required for programming the onboard FMC.
4. To keep all AIRAC navigational data up-to-date in X-Plane 11, or Simbrief or any other flight planner, you will need to subscribe to Navigraph or similar to obtain the latest AIRAC cycle.
Hope this helps?