would it be possible to make a kph speedo into an mph speedo by changing the internal cog to one that is 1.6 times the diameter, so that it goes up to 140 mph ?

I used GS instruments in my Lomax and IIRC a firm called Speedy Cables recalibrated both tacho and speedo for me. Pretty sure they made up a new speedo cable as well. I've just googled them at http://www.speedycables.com. This was 20 odd years ago which is why memories are a bit vague sorry.

Yes you can buy an adapter that fits between the end of the cable and the speedo. I opened on once, just out of curiosity, clever little thing, got a load of tiny cogs inside.

I've no idea if the threads would be correct to fit your speedo though.

http://www.ebay.co.uk/itm/Mechanical-sp ... 500wt_1170

A converter which converted kph to mph was at one time availalable which screwed into the speedo drive [usually on the front wheel] of imported motorcycles.
It fitted in between the speedo drive [on the front wheel] and the speedo cable.
I dunno if one would fit behind the dash of a 2CV [I doubt it would fit on the "gearbox end" of a 2CV speedo cable]
I never got 'round to buying one for my [imported] 'bike ......I could'nt see any sense in having a speedo which went to 200 and only using the first 60% of the dial.

btw .....its a 0.6* reducer you need .....not a 1.6 multiplier [do the sums ]
* 0.62 ....if you wanna be really accurate.

T.

Thats exactly wot I meant .....[I must learn to type faster] ...

T.

As far as I know you don't need an adapter on a standard 2cv speedo.

I replaced Hetty's ropey KPH speedo with a D*lly one using the same cable and it works fine. Speed is accurate to GPS speed as is milage.

I think all the clever goings on are done in the dial itself because the cable is going at the speed of the gearbox, the mph/kph is calculated by the instrument using the speed of that cable.

Sam, it depends on the final drive in the gearbox. Louis wants to make a kph speedo accurate to mph, eg when the needle points to 100kph the car is doing 100mph. I think.

Sam, the only difference between a 2cv kph speedo, and a 2cv mph speedo are the numbers on a dial. I don't know if the rpm of the cable is constant across different a-series variants, ie the drive to the cable is matched to the ratio on the diff. But it would seem easier for the factory to do that than make different speedo internals.

I'm assuming Louis's got a particular reason for wanting to keep the kph speedo, either he thinks that his 2cv is going to do 140mph, or he talking about a speedo that he can't get an mph version of.

Bugster,
I've been using a GS speedo in my Dyane for the last few years, mostly because I couldn't be bothered to go through the rigmarole that's needed to change the cable on a Dyane and it reads correctly without any modification.

If anything, it's more accurate than the original speedo, with the trip counter corresponding to the distance given by Google maps on any long trips I've undertaken.

@ Joolz,
as you've suggested, there are different sized speedo take-off pinions in the gearboxes, dependent on both the crownwheel and pinion set and the pinions used for 3rd gear.
If a 2CV gearbox is fitted in a Dyane the speedo will read just a shade low, vice versa for a Dyane or Ami box in a 2CV...

@ Louis,
GS speedos top out at 100 mph.

ken

Sorry, I didn't read Louis' initial post correctly. Now that I have, it would be nice to keep the original speedo and use an adapter, I see what he means