my b18b revs at 3900rpm for at 120kmh.. i have a friend with same ls motor that says same thing.. and i seen first hand that b18a's are the same.. Not saying its THAT bad just could be better gas millage if it revved lower.. If the OP wanted low gas millage in a motor its something that should be noted if driving on hwys or in town..
jesus christ this thread is full of BS! Guys your speed is not only achieved by gears x gears, there are tire,rim sizes involved aswell as inflation etc that effect the speed however those even added alltogther should not make such a difference so someone here either has a cluster thats off,doesnt know what tranny he has(assumes he has a b18b tranny because he has a b18b motor) or is just BS ing! there are several sites which can calculate your speed with each gear set and an ls 5th should be around 34kmh per 1k rpms so the correct answer for 120km 3500rpm but please also note that there is not one cluster out there that is bang on. what this means is for the ones who have the reading to off from 3.5 at 120km is that there clusters need adjusting(check honda-tech eg faqs section). a person who gets a reading of 3k at 120 is actually only doing 102 so his cluster off by 18km whereas a person reading 4k at 120 is really doing 136km so his cluster is off by 16km(this is highly unlikely because barely ever read less they will usually do the opposite making me think you have a gsr gearbox specially since the numbers fit perfectly for that gearbox)
now back to the op's question a b16 eg will get very similiar mpg an ls eg IMO!
they get the same mpg reading stock (ls dc vs si ek) online but also note thats with conservative driving a b16 will require more throttle both on highway and city too meet a person who likes to speed a little.
just an input but how much gas u get on ur motor is more based on ur throttle positioning then anything else the fact a b16 revs high doesnt mean it uses more gas, the reason being its throttle position
another way to explain is this:
a b16 red lines at 8200 rpm
an ls red lines at 6800 rpm
so a b16 it being at 4500rpm at 120km has the same throttle position as an ls being at 3400 rpm at 120 km