Your diversion will be planned Vmo/MMO as that is the worst fuel situation but there is never any requirement to actually fly that, you save gas if you fly normal speeds.
”It should be noted that the speed/FL used for the determination of estimated time of use in either method is for flight preparation purposes only. The use of a speed/FL during flight preparation does not imply that the same speed/FL must be used in the event of a diversion. In other words, it is perfectly acceptable for the flight crew to select a more appropriate speed/FL for an actual diversion.”
from
http://www.ifalpa.org/store/doc9976.pdf