First of all check if the speedo and motor can take seven cells and whether or not your willing to risk it if so then read on:

Do you want to make a six cell pack into seven? If so then your goin to have to take one of the tabs that have been soldered to one end and solder it to your new cell (try and get a cell the same as the oher six otherwise make sure they are the same ampage and voltage). If you have taken the tab from the posotive end of the battery then solder it to the negative of the cell and then have the wire comming off the posotive of your new cell, Thats it really.

As for charging times it is going to be a bit longer, take your origonal charging time and divide by 6 then times by 7 to give your new charging time.

The final result will be a much faster motor, i would guess about the same length run time, also your steering servo will be slightly more powerful.