One thing to know about traveling in Africa is you can't depend on traveling the same miles or kilometers per hour that you would at home, no matter the speed limit.  It could be pot holes or some other somewhat regular obstical slowing you down.  Or perhaps it will be the elephant in the road that you can't just drive around or the mama rhino and her baby that should you try to go around may just try to "rearrange" your car that's going to slow you down!  Why are you in a hurry anyway - there is so very much to see.