Different Tamiya tanks use different gearboxes and hence different ratio's and speed. The King Tiger is extremely fast out of the box and the sherman is a little slow out of the box.
You usually have to use gear reductions, different motors, or different motor pinions to dial in your speed and still have torque for manuverling on that last hit. Here's a speed chart using a 25 foot course I believe. Throw some tape on yourfloor or driveway and time it at full throttle from tape to tape. (25ft) Make sure to use a fully charged battery and the floor or driveway is level too.

http://www.bayareatankers.org/Intelligence/Article_00/Apndx_b_Rev_b_Ratified03_.htm<br type="_moz" />