When displaying your km/h data, suffix it with one or two ASCII spaces ($20 Decimal 32). This will overwrite any characters remaining from the end of the prvious display.

It's neater to RIGHT JUSTIFY numeric displays, this saves the km/h waving about along the right-hand side of the numeric value which becomes a distraction... this probably will look a lot more professional...

Code:
LCDOUT $FE,$80
If kmh<100 then LCDOUT " "
If kmh<10 then LCDOUT " "
LCDOUT #kmh," km/h" ' note no trailing spaces required
rather than this, which despite being simpler code, ends up looking tatty...
Code:
LCDOUT $FE,$80,#kmh," km/h  " ' note the couple of trailing spaces here