Such a graph must be stupid, of course consumption rises
with rpm

| gas mileage usually decreases rapidly at speeds above 60 mph.
Not necessarily fr 60 mph, big factors incl
[i] exhaust*manifold : cheapo 4-1 short branch * can give
¾ of maximum torque @ low rpm <1000, but gives <½
of maximum torque @>3000 rpm, & even less @ higher
rpm
[ii] voltage & ampere from small single coil ( esp if hot )
drop fast @>3000 rpm : sparks will be too small to
ignite enough fuel before pistons reach b-d-c, esp where
only cheapo carbon core cables are used
[iii] cheapo mineral oil's coarseness are apparent only @ high
rpm when only the most slippery oil ( e.g. synthetic with
fullerene ) can produce high torque.
[iv] gear ratios & axle (tyre) height