I think the key is how far you want to be able to go between filling stations. The higher the consumption the less the range (obviously), or to be more practical, the more reserve you have to carry, affecting fuel consumption, stability, braking, and wear and tear. I guess this is the key (and obvious) message. The following is simply to give some additional insight and an example.
On our most recent desert crossings we needed a range of around 500 miles, At our normal fuel consumption plus 50% for the extra consumption in dunes we needed to carry significant reserve fuel in portable tanks. There’s a limit to how much you can carry in addition to the fuel in the main tank(s). It’s a good plan to keep mass to a minimum.
Example - we crossed the Simpson and needed to carry enough fuel to get across. Based on our experience in deserts we planned on 25-50% higher consumption than on tarmac. We saw advice online ranging from 50-100% higher consumption. At Mount Dare the advice was to plan on 50% higher than "normal" and to plan on the possibility of taking the longest route (478 miles), since the shortest route was badly chopped up when we were crossing.
So, we calculated needing around 51.5 US gal of fuel. Filling the tank and fuel cans gave us 55 US gal. The French Line wasn't as bad as we’d expected so we ended up covering 379 miles and using 35.7 US gal of fuel.
The point is that we normally get 14.1 mi/US gal and we got 10.7 mi/US gal on the crossing. So if you normally get 13 mi/US gal you should assume at best 9.9 mi/US gal. Your standard fuel tank holds 21 US gal, so that’ll give you a range of 143 miles (if you assume 65 miles reserve). If you want to travel further you’ll need fuel cans or an extra tank. For autonomy of (say) 500 miles with an additional 65 miles of reserve fuel you'll need 565 miles-worth of fuel, which means 57 US gal. So an additional 36 US gal, which weighs 216 pounds (gasoline) or 252 pounds (diesel). Either way this is a lot more mass, affecting fuel consumption, stability, braking, and wear and tear.
So back to the first paragraph - how far do you want to be able to go autonomously? If it’s no more than 150 miles or so, no problem, although as you note, you’re "getting 13 MPG with just lift tires wheels and soft top. Nothing added yet.” If you add more mass your consumption will increase. ; if you want 500 miles range then you’d be better off, in my view, with a vehicle with better fuel consumption.
