I read the rules for acceleration and deceleration one way and a guy I play with has read them another.
In the Ultimate Combat book under Vehicle combat the text for acceleration goes like this:
"With a successful driving check, the vehicle’s current speed increases up to its acceleration (in 5-foot increments; minimum 5 feet), but no higher than its maximum speed. The vehicle can move forward or forward diagonally. In other words, each time a vehicle enters a new 5-foot square, it can choose any of its forward-facing squares—the ones directly in front or either of the squares directly forward and diagonal. This allows the vehicle to swerve. A driver who fails her driving check can only move into squares directly in front of the vehicle’s forward facing."
Deceleration is fairly similar with a few changes to account for the vehicle slowing down.
the way I read these two actions is that, once a vehicle is at it's max acceleration, It can't go any faster. It has to double move as that is all the vehicle can do. So by decelerating you would be reducing the max speed by ten each turn.
So, if you're using a carriage pull by two horses. It's Acceleration is 50, while it's max speed is 100. If it is already at its 50 acceleration, and the driver decides to slow down. It would only move 90 ft that turn instead of 100 essentially reducing its acceleration to 45.
The my friend read it is that on the drivers initiative he gets a standard and a move action, he can do any of the regular actions as well as any of the driver actions. Which I completely agree with. The difference comes into that if the driver were to take a standard action, the vehicle would only move it's acceleration and not its max speed which makes no sense to me.
As an example, if on the drivers first turn he then kept it going twice(two move actions) , the total movement would be 100. On the next turn He pulled out a weapon(move action) then decelerated, he would only move 45 as he doesn't have that second move action. On the driver third turn, he would decelerate and keep it going as his action, bringing his movement that turn to 40.
Are we both wrong? Are we both kind of right? Are we both so far in left field we aren't even in the stadium? I plan to use these rules in my next DM session so not having to to make a "I'm the DM, this is it how it is" call would be nice. But I will make it if I have to.