A better way to compare the real world efficiency of the series hybrid with the parallel is to compare the Tesla with the Prius. Although the Tesla is a pure electric vehicle, it can be evaluated as if it had an on-board generator.
The Prius has a combined 55 mpg vs the Tesla’s 7.85 km per kwh of utility energy. The Tesla is slightly lighter than the Prius but the Prius was designed to maximize efficiency, whereas the Tesla was designed to maximize performance.

7.85 km / kwh from a 35% efficient Diesel Generator, assuming an equivalent conversion efficiency as the Tesla’s battery charger, yields 68 mpg for the Tesla vs. 55 mpg for the Prius. Reasonable to assume weight of Diesel Generator plus fuel vs removed battery size would be similar.

There is an additional efficiency gain with the series hybrid, however, since a portion of the generator output goes directly to the PEM (power electronics module) without charging the battery, so avoids the Tesla’s 86% charging efficiency. For instance at a typical driving speed of 60 mph, the Tesla would be using about 11 hp, so for a 15 hp generator, 73% would be sent directly to the PEM and Motor. Also charging efficiency could be improved in a series hybrid since less than 1 kwh of energy storage would be used for normal driving (i.e. acceleration & modest hill climbing), the balance would be for the (normally) very unusual climbing mountains at speed also sustained travel at greater speeds then the generator output to the PEM DC Bus. You could therefore optimize the efficiency of the 1 kwh storage to ~95% with various means, at a higher cost per kwh, but an efficiency gain that might warrant the increased cost.

Another factor is the fact that the Tesla was optimized for performance, i.e. 0-60 in 3.9 sec, and top speed of 130 mph. This feature required the use of a 2 speed (forward only) transmission, since I believe 1st gear gets you to 80 mph. There would be somewhere between 5-10% loss of energy (motor shaft hp) in the transmission, and a further loss of 10% in the rear differential. Both of these losses could be eliminated with the use of one or two rear wheel motors for propulsion.

These factors could push the Tesla series hybrid mileage up to about 83 mpg. In addition the scale economies of a standard generator module and much smaller generator size – 15 hp vs 76 hp for the Prius, would probably lead to much more efficient generators like LNG gas turbines or the StarRotor Brayton cycle which is claiming 45-60% efficiency. This could easily push the Tesla series hybrid mileage up to 107-142 mpg. Also it should be noted that this is the first generation Tesla vs the 9 year design advantage of the Prius, tweaking always improves performance significantly. The Tesla was also designed with an R&D budget that Toyota would consider Pocket Change, and is, much more than the Prius, an entirely virgin technology with virtually no scale economies.