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1926 Chrysler 50 diff ratio and speedos


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Pondering diff ratios started when I was investigating a replacement Smith's period speedo.

American Speedos of the 20s were plagued with poor diecast metals - most samples are just rotten inside, where as the Smith's chronometric and magnetic instruments usually are just running fine still. Proably because of quality brass construction.

Luckily here down under Smith's Speedos are easily found and with a Smith's repair specialist located nearby I learned he can create a new drive cable, and fit a speed ratio adaptor inline.

All he needed was some numbers:

Radius of rear wheel, normal tyre pressure, actual ground to centre. Check 14.75"

Diff ratio, or wheel to prop shaft ratio. Check 47:1.

Speedo drive ratio, gearbox/cable. To be determined.

So how did I find my diff ratio? Believe it or not take the rear cover off and it is stamped on the ring gear. On my Model 50 it's 10x47.

A bit of research showed the the 6 cylinder may have a 10x46 gears which would drop the revs a bit on open road to help it get 70mph.

PS, I just picked up a nice working period Smith's for $50 on eBay. I would have preferred the AC Speedo, but inside is just disappointment.





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Hi Tonz,

 The Chrysler 50 and 52 both had 4,7:1 diff ratios, however the speedo drive gear and pinions in the gearboxes have different part numbers and probably different ratios, because they have different speedos.

The model 58 4 cyl had 3 diff ratios available 4.3:1 , 4,6:1 and 4,7:1 and there were 3 different part numbers and ratios for the speedo drive gears in the gearbox. Whether the model 58 rear axle will interchange as a unit onto your model 50 I do not know, you will have to research changes in hubs, axle shafts etc.

 As it is possible that your gearbox may have been changed over the years, the only way to find out your speedo drive gear and pinion ratio, will entail stripping the back off the gearbox and counting the teeth.

 Now, regarding changing to a 6 cyl differential !!!  I have had many years looking for a G70 diff pumpkin for my rear axle and eventually found one, after buying 3 wrong ones where nothing interchanged. I can assure you almost 100% that the 6 cyl pumpkin will not fit your axle housing. The gears themselves will not interchange, nor the bearings, shafts etc. 

 If you decide to fit a complete 6 cyl axle to a mechanical brake car , you may have to modify the brake backing plates, the hubs or shafts and change the propshaft. On a model 50 with hydraulic brakes, you will find the original brakes are 12in diameter with smaller cylinders, backing plates and drums than a 6 cyl axle. The 6 cyl brakes are 14in diameter with bigger bands and wheel cylinders.

 Pre 1930 Chrysler's are a nightmare for parts, there are so many subtle differences in parts, and it might look right but isn't. Check very carefully before buying .

 Looking at the rust in the diff pictures you uploaded, I hope you are going to strip and clean it completely, or you won't be driving long.

 Good luck with your car



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Thanks for the info Viv. That photo is from a spare rear end I picked up, it is identical to the good unit in my car. Yep! you should have seen it before I cleaned it! Inside housing was black semi solid sludge. I was hoping it would be useful, but it may end up as garden art.

As for year/models and parts I reckon there were numerous rolling production changes and options that don't coincide with "26" or 27 year model. Who knows🤔

Just read up the posts on different brakes .. mechanical, fluid, fronts, rears only.

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