speedo + overdrive

[Steve P]

Testing my o/d gearbox i just fitted to my Herald,its a single rail with J type,speedo connected via the angle drive,when the o/d is engaged.i.e wires shorted the speedo goes up not down,disengaged makes the speedo indicate lower speeds,am i being dumb or is this arse about face?

There's no prop fitted just the flange spinning on the box,oh and first gears a bit noisy too :-/
Steve

[Davemate]

Yes its correct
When you don't have a prop connected or the wheels off the ground when you engage the overdrive the
the speedo will rise

Just think,
        1st gear at a set rev -----10mph
        2nd gear at same rev -----20mph
        3rd gear at same rev -----30 mph
       4 th gear at same rev -----40mph
  Overdrive at same rev -----50 mph
With the wheels off the ground or with the prop not connected there is no drag or weight to slow
the revs down ---so the speedo will shoot up

I hope I have explained that correct !!!

You lucky sod you only got a noise in first !

[timraven]

Isn't that what should happen if you don't have the wheels on the ground? The revs will stay the same, and the lower ratio will increase the speed. When driving the speed can't increase suddenly so the revs decrease? Makes sense in my head anyway!