There's another consideration for why the tank speeds are slower. Two considerations, in fact. 1.) It could be due to server performance. If they and everything else moved at absolute real-world speeds, the servers might have too much strain put on them. 2.) The maps in stock RO are friggin' tiny, especially the tank maps. They may SEEM large, but that's because you're impatient and want to go shoot something.
Orel is about what I'd want to see, only maybe larger and with longer viewdistances. You'd need that kind of space to maneuver and take at proper speeds.
There's a lot of other factors that aren't modeled as far as the terrain goes. Any terrain modification would be a gross oversimplification, and so would still need to be "averaged." Then you have to consider the effect of "road treads" vs "cross-country treads". As I recall, some tanks did use different types of treads (though I don't remember which armies did this, or if all of them did).
Regardless, it's an estimation. People need to let go of the notion that RO is a sim or some hard-core realism game. It isn't. It never was designed to be. It was designed to be more realistic than other stuff out there, and it is. But it was never supposed to be a 1:1 representation of the real world.