What is the maximum speed of a car per hp ?

I am trying to make electric battery powered car.
I must know what should be the max. speed it can attend for designing required gears and other parts. I know speed depends upon machine rpm and diameter of tyre and gears.
Machine RPM and diameter of tyre are fixed. So only design depends upon gears ?
When I strudied Eicher truck manual with 16 ton total wt. and 147 hp it can get 84 kmph max. speed from there literature. I do not know wether this speed is with full load. ( 10.3 ton payload) It comes to just 0.91875 hp per 100 kg gross wt.
Take Maruti 800 which claims to have ma. speed of 125 kmph with 1000 kg laden wt and 37 hp engines.
My car will have total gross wt 500 kg with 9 hp motor can it travel with max. speed of 90 kmph ? When speed is halfed power required is just 1/4. On Maruti data for 500 kg 18.5 hp for 125 kmph, If power is half 62.5 x 1.44 = 90 kmph
Is my math correct ?
Or is there any formula in Automobile eng. for max./ speed calculation for any given hp+ gross weight ?