add keyframes at your start and end point of your speeder animation. The first position keyframe will be the speeder offscreen . your last one will be at its destination. then create keyframes at the same points in the scale section, again first will be maximum size last will be minimum. Next highlight the keyframes in your "scale" section and click on the graph icon next to your timeline. now you can adjust the velocity of your keyframes animation. You want to slow the scale down as it hits the vanishing point so lower the velocity at the end. Most of this is trial and error to get it to look right so play about with things a bit. You could also right click on the last scale keyframe and select "keyframe assistant/ easy ease". this will do all this for you but its better to tweak the settings yourself until you get the desired result. once you have the animation settings you want then right click on the speeder layer in your timeline and select "switches/ motion blur". when adding this motion blur drag your last keyframes (which should be at the last frame of your animation sequence) for position and scale 1 fame after the last frame of the animation or the last frame ends up with no motion blur.
without playing about with the animation myself i can't be any more help with this but this should give you the basics