I have not driven Fonteny yet, but it looks hella long. Must try someday. It looks awesome.
When you say that rears were shot, do you mean that they were on yellow or red indication color ?
I have used this video as referance for tire wear, tells that the tire was shred in four laps of Nordschleife.
I assume that is what should happen if you go maximum attack with this car. In your source text it is said that Mercedes had more problems. I guess mercedes was wearing tires more if pushed to the limits.
I have a bit reading through the book I have, and I think may have had these lines missed, or I am just confused right now. It states that Mercedes engine ran 4mpg, but later as they developed the engine it went up to 2.5mpg. I don't know whats the fuel tank size of your car, but I guess it is about 400 liters ?? , which would make it about 2mpg for 22 laps of old donington, I guess, you are right that would probably mean that it is more likely too high. It may be that I did mess up W125 consumption a bit, by making it too high, iiric.
By the way, I just learned that W125 later used extra fuel tank in front of the driver, so I suppose it must have ended with something about 500L of fuel
I would guess that they were rear tire limited indeed, because all pictures of pitstops there is in a book - the rear tires are being changed, and fuel is being filled in. Interesting fact from the book, Mercedes were capable to fill 5 gallons per second and change the rear wheels in less than half of a minute, that is very impressive.