C4 Speedo Drive Gear Size

[dirtbike]

Good day gentlemen

I have recently installed a 'MPH' speedo (was originally KPH) and was wondering if anybody knew the correct size speedo drive gear for a C4 Auto.

I am still to drive the truck with new speedo but it hits 60MPH very quickly on jack stands

[bruceandersson]

The speedo drive gear depends on your rear axle ratio and tire size as they are all driven from the transmission tailshaft. I'd either use a GPS or have someone tail you to get a reading of how fast you are really going at a given reading on your speedo, then pull your gear and either go with more or fewer teeth depending on how far your are off.

[dirtbike]

Thanks Bruce. I was hoping to get it relatively close as the vehicle is currently unregistered so I can't drive it on the road (well not legally).

[Toyz]

Post the data Bruce mentioned, and I should be able to steer you to the correct color-coded driven gear. I was of the opinion that the conversion from kph to mph was all in the head calibration, though.
Paul

[Leeroy]

When you say you changed back to mph, did you change the whole odometer and gauge? I know there is a kph speedo available (DC still lists them last time I looked) so if it was a kph speedo then the gearing should be the same on the gearbox, it'll be internals of the mph gauge that are different.

My last slick had the gauge converted to display kph even though it was still the original mph gauge, so it moved very quickly also. Made 110kph a PIA too because it was off the dial!
I think I do still have a spare gauge (mph) if you need to troubleshoot.

Cheers,
Leith

[Toyz]

Wow, that's good info! I would have expected it was like modern day dual reading setups, where 100 kmph would be around mid-sixty mph, and about 3/4 gauge.
Paul

[Leeroy]

I think you are correct Toyz. When they were listed by D.C, they still displayed the mph picture so it wasn't possible to tell where the 100 was on the dial.

It would need different internals though so the odometer counted km rather than miles. Annoyingly the one I had that was converted still counted the odometer in miles, and now at a much faster rate! So totally inaccurate.

[dirtbike]

The speedo was originally KPH, I bought a complete MPH one that I have installed. The KPH one was quite accurate if I remember correctly. As mentioned I have not driven it on the road, just had it in gear on stands and the speedo seemed to hit 60 MPH very quickly. I may need to go out on a midnight run in a couple of weeks...

[Leeroy]

Maybe check the full length of the cable, I have seen 'converter boxes' spliced in that slow down or speed up the cable when converting between k/mph.
There's only a small range of driven teeth available for the gearbox end, (17-26tooth gears?? Can't remember) but these aren't enough to make the conversion alone.

I had a 351c with an fmx auto and a 3.5 lsd rear end (AFTER the wheel came off) and it had an 18toorh gear to make the speedo correct.

Edit: forgot to add, that was with a 29" rear tyre. (275/55 17")

[Toyz]

I suspect one would need to convert the drive gear on the tailshaft. I hadn't thought of the odometer; looks like it would require quite different gearing also!
Paul