When using the latest nightly build I am finding than when viewing vehicles to buy in the depot that weights for most vehicles are shown as 6645045t and that the speeds of those vehicles are shown as being slower than they should be. When you then buy a vehicle it shows the correct speed underneath the just-bought convoy and it behaves normally in the game. Most, but not all, powered vehicles are affected though it doesn't seem to affect unpowered vehicles (trailers, rail carriages etc.). I have had this happen with both pak64 and the recent nightly of pak128.britain.
What nightly build do you use? did you compile yourself? or did you take the one from nightly download webpage.
I cannot reproduce your bug report :/
I am using the 9th August nightly (r3636) downloaded from the nightly download page. Here is what I am seeing (if it helps):
(http://img193.imageshack.us/img193/1118/simscr08.png)
This is an experimental version translation string. You somehove overwrote the stranslations with the one from simutrans-experimetnal. Please reinstalle simutrans for a zip file (the 102.2.2 version). The you can put a nightly on it.
Further to what Prissi wrote, I'd advocate keeping a separate directory for your Simutrans-Experimental install to your Simutrans-Standard install to prevent this sort of thing from happening.
Oops, sorry :-[
I was pretty sure I was doing as james suggests, but clearly got confused somewhere!