Speedo / Tach / Gauges: Speedometer adapters

[c locke]

Hi gang,
With tens of thousands of threads all over, i'm surprised I can't find what i'm looking for, so with fear for getting beat up, i'm starting a new thread...

I'm changing the rear end in my '51 2R5 from the optional 4.89 to a 3.73 twin traction. 4 speed standard transmission. looking in my parts book, i need to replace the speedometer pinion gear and add an adapter. Looking around on fleabay and other stude parts places, there are stewart warner adapters all over, and all with many different numbers and what not. I have one that will not work, i'm sure. Here's what i'm figuring. going from a 4.89 to a 3.73 means i need to twist my speedometer cable 1.310 times faster than normal. The parts book calls for a 13 tooth pinion rather than a 14, so if I change pinions, i'll now need a 1.217 adapter: neither of which match the one i have, which is 1.0400. SO, I know the 666 SW adapter is an offset adapter, and the 777 SW adapter is straight through, but there are tons of other numbers, V6, T3, T12, R12, no idea what they all mean. Does anybody have a cross reference chart for stude part number 675982 to a Stewart Warner number?

Though the chart in my book doesn't show a 3.73 rear end for the 2R5, the E series does, with the same 4 speed transmission, so the speedometer adapter parts should work just fine. Also, i'll be happy to listen to anybody's advice on my project...

thank you in advance!
c

[Warren Webb]

Go to this site:

http://speedometershop.com/rep-pag.htm

Under ratio test pick "method 2". There you will find a test you can do on your own. With the results you can order an adapter if nessisary.

[PackardV8]

I've got two or three S-W speedo gearboxes. When you figure what you need, let us know and I'll check mine.

jack vines

[WCP]

If you are using radials, totally disregard the Stude chart. You have to base your pinion selection on "revs/mile" rating for your specific tires. Radial tires of "comparable" size, will spin the speedo faster by up to 10%, so the pinion count needs to go up to compensate. As an example, with a 3.07 RA with 6.70-15 tires the Stude chart says 18 pinion count, but with P205/75R15 tires requires a 20 count pinion to correct speedo reading to GPS measured mile reading. Chartwise, that's a shift from a 3.07 to 3.31!

[c locke]

Thanks wcp, but alas, I have the wonderful bias ply tires that were spec'd with the truck, 7.10x15's, so everything from the chart works out. when i first got it i threw some radials under it just to be able to move it around and take it for a test drive, and boy was the speedo off! Once the right tires were on, and I removed an adapter that wasn't supposed to be there, the odometer is right smack dab nuts on the money. the speedo was off about 5 mph, but i found the adjustment on the hairspring and futzed with it till it was right on, at least right on to a couple of GPS units. so, after all that work, i'd like to keep it working right by getting the right adapter and pinion. thanks! Jack, i'll let you know what I find out, and thank you for your offer, we may have a transaction coming up if you have the right stuff! thanks
chris

[1961lark]

I'm confused, what difference would radial vs. bias ply tires make? if they're the same size, the speedo should read the same, right?

[WCP]

Radial tires have a lower "rolling diameter" than bias-ply tires, due to the sidewall flex or squat. The rolling diameter is twice the length from the axle center to the ground at curb weight and proper inflation. This is why the tire manufacturer or vendor will provide a "revs/mile" rating for each tire brand, style and size.