Speedometer gear swap SM240

[baldilocks]

I have a 49 3/4 ton with a granny gear 4 speed tranny(SM420). The speedometer gear set is GM# 3689178 which matched the original 4.57 differential. While back I found a 4.10 differential that I was able to swap into my original rear axle. So now my speedometer is indicating low and I have to rely on a GPS to see how fast I'm going. I'd like to swap the speedo gears in the tranny to GM# 3609223 which matched 4.11 differentials in the 1/2 ton trucks. I figure that would get me close enough that I could ditch the GPS. Both of the gear sets have 4 teeth on the drive gear. I would be swapping out a 13 tooth driven gear for a 12 tooth gear. Can I just plug in the 12 tooth driven gear with the original drive gear or will I need replace the drive gear too?

[baldilocks]

I'll answer my own post here. The speedo gears must be changed as a set. Anybody know where I can find a GM# 3609223 gear set?

[Deve]

If I had anything to add, you can bet I would. Its REALLY hard to know everything. LOL! You are doing great! The admirable thing is you are not just texting stupid crap, but delving into the more intricate part of the craft. Research is everything. If you find a good supplier of this stuff, please share, its why I started this forum. It doesnt get enough traffic, but whats here is real information everyone can count on.