No they are electronic i think, check the plug on the G/B around the back by the starter, im pretty sure thats the speedo one..i might be wrong though lol
the speedo is electric on top of the diff, 3wire plug real bitch to unplug.
go to p-a-p get 1 out of any toyota thats the same (e.g AE101 or coronas, anything of the same sort of years) fit it and it should be fixed.
i can get 1 if u can't find 1.
hahahahahaha mine to does this but only on a hot day
there are indeed run off a speed sensor that is pooked in to the back of the g/box
i wouldnt reqomend just going to a wrecker and slaping any sensor in as you may find that they might have diff input/out puts
just match up the numbers on the back of your existing sensor to the second hand one.
or just go buy new
but hay who needs a speedo 5th gear 3tho rpm is a 100km
forget what 60km is