The pot controls the PWM duty-cycle which has the effect of varying the output voltage.
If you connect the pot to a foot pedal, you can have an accelerator.

That's easier said than done. The peddle is a simple foot switch, if I were to try to make it operate a pot I'd need all sorts of stuff to modify it. Besides if I could modify it I wouldn't really need the PWM anyway as the power could be fed it gradually.
If I wire it in after the on/off (peddle) and set the pot to max does will gradually increase to max voltage or deliver an instant 12v ?

 

That's easier said than done. The peddle is a simple foot switch, if I were to try to make it operate a pot I'd need all sorts of stuff to modify it. Besides if I could modify it I wouldn't really need the PWM anyway as the power could be fed it gradually.
If I wire it in after the on/off (peddle) and set the pot to max does will gradually increase to max voltage or deliver an instant 12v ?

Think I've got what I need. found this video to visually showing me a PWM.

So back to ebay..
http://www.ebay.co.uk/itm/DC-12V-24...918370?hash=item211aa4a462:g:VbIAAOSw5cNYLDMH
This covers me for 30amps and as I understand I can limit the cars maximum speed. Dial it to the max should slowly give 12V, dial it halfway and it'll slowly deliver 6v.
Right, wrong ?

 

If I wire it in after the on/off (peddle) and set the pot to max does will gradually increase to max voltage or deliver an instant 12v ?

My understanding would be, upon power on, it would slowly ramp to the voltage set by the pot.

 

@crutschow has it right. In the real word, it could be a modified version of PID, where dp/dt or the rate the output can change has a limit.
e.g. %/cycle.

Sewing machines may use SCR phase control per discussions on ETO.

 

Thanks for such a detailed reply.
This looks excactly what I need, not sure I have the skills to build it though. Do you think the
SoftStart Reversible Motor Speed Control PWM Controller PWM DC 20A 12V I found one Ebay is the same kind of thing. I'll have to get a meter of the motor and find out how many amps the motors draw. Might have to have 1 for each motor.

Many thanks again for your time and advice and everyone else that's offered me suggestions. I really appreciate it

We really need to know the stalled current of the motor. You can either measure it's resistance (pretty low) or it's current (maybe to high for your meter). Maybe the number of watts is printed on it?