Differential ratios

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Model T Ford Forum: Forum 2015: Differential ratios
Top of pagePrevious messageNext messageBottom of page Link to this message  By Bruce Compton on Monday, November 16, 2015 - 06:35 pm:

I'm getting ready to replace the thrust washers in my '25 Coupe and was wondering about changing the ratio at the same time. I have an excellent 11 tooth pinion an an equally nice 39 tooth crown (recessed) which when put together gives a ratio of 3.54. My coupe has a Z head and an OF Stromberg and runs really strong but I'd like to reduce the RPMs if possible although going from a 3,63 to a 3.54 isn't a big change. Is it worth the effort, and would I have to shim the crown gear? Thanks : Bruce


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould, Folsom, CA on Monday, November 16, 2015 - 07:20 pm:

I didn't think the 11 tooth pinion mated with a 39 tooth ring gear.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Allan Bennett - Australia on Monday, November 16, 2015 - 07:40 pm:

Bruce, I am not familiar with a recessed 39 tooth crown wheel, so I don't know if this will work for you. I had a 39tooth ring gear and 13 tooth pinion gear set in my speedster. This 3:1 ratio was too tall, making braking a real problem.

I have changed the diff into my new roadster, but have mated a 12 tooth pinion gear to give a 3.25:1 ratio. This will give me an overdrive over the standard Ford ratio, without going too far. I am not yet ready to road test it, but am confident it will be OK in a light roadster.

Hope this helps.
Allan from down under.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bruce Spainhower - Portland, Oregon on Monday, November 16, 2015 - 08:17 pm:

Glen or Dave at Chaffin's are the ones to ask about this. My experience includes 4.00:1 gears in my '26 Fordor, 3.25:1 in my '15 Touring, and stock gears in other Model T's I've driven. My '15 has a Ruckstell along with the 3.25:1 gears, which in my opinion is the best combination for a Model T on today's roads and today's traffic. At 35, the car is pleasantly quiet, and it'll do 50 in a pinch. If you're going to open up the rear end, consider installing the 39/12 gears. The only gotcha is, the 12-tooth pinion won't fit through the hole in an assembled differential, so assembly becomes a timed event, with sealant, gaskets, etc.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Lonnie Smith Baxter MN on Monday, November 16, 2015 - 08:22 pm:

3.63 to 3.54 would be 2.5% change so 25 rpm change at 1000 rpm's. Doubt you would notice it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan McEachern on Monday, November 16, 2015 - 08:59 pm:

The cone angles of the 11 tooth pinion and the 39 tooth ring gear are different so you will never attain a workable contact pattern.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bruce Compton on Monday, November 16, 2015 - 09:33 pm:

Thanks for all the informative feedback guys. Now I'm looking for a 12 tooth pinion. What a great forum. Bruce


Top of pagePrevious messageNext messageBottom of page Link to this message  By Allan Bennett - Australia on Monday, November 16, 2015 - 11:05 pm:

Bruce, Chaffins have the 12 tooth pinion in their catalogue.

Allan from down under.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Harper - Keene, NH on Monday, November 16, 2015 - 11:38 pm:

I have read this thread with more than passing interest as I have a '24 coupe and have been considering a similar gearing change. The engine is going to be rebuilt (joined the two piece crank club) and I was running a Stromberg OF. I have acquired a Ruckstell and already have a 39 tooth recessed ring gear.
I am not trying to turn the coupe into a hot rod or a coupe/speedster, but as Bruce spoke of in his original post, dropping the RPMs at cruise would be nice or maybe gaining three or five more MPH for the same RPM. Here in hilly New Hampshire the Ruckstell will be a welcomed addition.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Erik Barrett in Auburn Ca. on Tuesday, November 17, 2015 - 10:46 pm:

With a strong running engine you could run a higher gear ratio successfully in your coupe but you will want a Ruckstell or other auxilliary transmission. Without one, bumping the final drive ratio up in a heavy car will really stress your crankshaft when you shift into high. I have taken a lot of 3:1 gears out of T's at the same time as replacing the crank. The latest in a 26 coupe.


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