I found I got about 55 mpg to the gallon in YMR. This is because I drove the schmitt hard, with my weight, two up remember, I expected it to cruise at 57 ish realtime. To get it into top you need to rev it in third. I do not do the plop round at 45 mph bit. To produce a realistic value comparison the cost of two stroke needs to be added. (I am still using up a clearout of oils, some 8 barrels!, I did years back so I have not bought two stroke for a long while so I do not know what the price is at the moment). On the basis that on using the car then half the time I would have no two stroke so would have to buy some from the garage at top price against home brew or ST30 at sensible price I get my estimated MPG figure. This was backed up by the fact that a tank full of fuel did about 150 miles so on tour it was stop every 100 miles or two hours. Maybe my two stroke additive calculation is wrong.
Now with the benefit of some mild tuning, a better carb and exhaust I would expect the new Schmitt to do the same speeds with better climbing for a far better return on economy, say 60 mpg, as the thing will be far more efficient than back in the day when we ran using cobbled up second hand stuff due to availability issues, being skint and a lack of knowledge. Also the two stroke can be got in bulk cheaper, as I have money now and the mix weakened a tad as it has a roller top end.
My Goggo 300 used to be more expensive to fuel than a Mini. Though true one should add into the Mini running cost the change of oil to balance the books.