Well, actually it's exactly 80 km/h and for mph exactly 80/1.61 which is, as NetDemon correctly pointed out 49.71 mph.
You have to consider though, that LFS uses the
exact speed you have at the moment, not just what the speedo shows.
That means, you are allowed to drive 80 km/h, but as soon as you drive 80.00001 km/h you get a penalty (so when 80 shows on the speedo, don't get your hopes up it was exactly 80 - you will get a penalty
). On the mph scale you've got a buffer though. There you can drive 49 mph all you like, as long as you stay under 49.71. That may result in you driving through the pit with 49mph and suddenly you get a drive-through because you reached 49.8 for a split second... bad luck then, I guess.
Basically, just go with 78 / 48 or even a bit slower, the extra speed of going faster doesn't make up for the risk of a drive-through.