The winner of the top X prize 100+ mpg competition 5 million dollar purse
went to a 400kg 2 seater with a supercharged 380cc bike engine.

If I could weld up the doors, chop the roof off and strengthen the chassis
of my 1000 tudor with a twin hoop bar and run a 600cc bike engine with lil
SCharger then I could have a 80mpg sub 500kg B10 that spins to 17000 rpm.

eg.
minus engine/box/radiator/fluids/exhaust/battery/bumpers 625 - 150kg = 475kg
tube chassis strengthening and twin hoop 475 + 80 = 555kg
minus roof/ windscreen/door mechanism and gutted/h145 diff/tailshaft 550kg - 80kg = 475kg
minus front leaf springs and add coilovers 460kg
600cc bike setup at the rear with volvo modded single leaves minus stock springs,
custom de dion tube, shortened driveshafts, shockers and hubs + 60kg = 510kg
further lightening using hole and stamping -10kg
disc brakes all round instead of drums -5kg
Boot area shortened and modded for fibreglass see-through fluted back lid -10kg
Tilt front setup with alloy foam filled fuel cell -5kg
Magnesium rims and lightweight seats equal weight
no back seat -2kg
Final target should be below 490kg so with 90hp NA should be nice but boosted
to 130hp with small supercharger will be lovely equal to 250hp per tonne.
Go crazy with hayabusa setup and go up another 40kg and will be lethal.