

Sorry, it’s rare people are not asking for trouble here :)
I mean that any (reasonable) distance travelled at 90km/h will have consumed less fuel than that travelled at 30km/h; given the same car is used, reasonable gear is selected, etc, of course. If there is less fuel consumed, then, naturally, the emissions are lower as well, since that’s just a byproduct of combustion.
While travelling at 90km/h will consume more fuel per unit of time than at 30km/h, the speed increase makes up for that.
Thanks for a good chuckle!