rear gearing

[RichardG]

still learning ,i heard long ago if you lived in hilly country, the dealer would put lower gearing in the rear end, just one of those thing ive wonder'd about over the years, how many gear sets available for the t, i know my 21 likes to scamper down the road at 30 mph, and it takes a good hill to have to shift.

[ModelTWoods]

It probably has 4 to 1 gearing, which, as far as I know, was the only other Ford authorized gear ratio. It just substituted a 10 tooth pinion for an 11 tooth pinion with the same ring gear. Many other aftermarket gear sets were available, though.Some under 3 to 1; some over 4 to 1. $ to 1's weren't limited to hill or mountain use. They were useful in muddy roads, too.

[TonyB]

Some time ago I computed the various ratios that are possible in a Model T rear axle as follows:

Model t gear ratios
Rin pin Ratio. Per 1000@30” wheel
40. 10. 4.000 22.31 mph
40. 11. 3.630 24.6 mph std.
40. 12. 3.333. 26.8 mph
40. 13. 3.070 29.7 mph allowed??
39. 11. 3.545 25.2 mph
39. 12. 3.250 27.5 mph. Unreliable too many shims
39. 13. 3.000 29.75 mph

I have never come across the 40/13 and I don’t know if it is allowed.
I have done several 39/12 axles but I have found it unreliable as it seems to require too many shim which eventually leads to the failure of the ring gear bolts. Two out of three caused problems so I will nit do it again.

[RichardG]

TONY, WHEN YOU SAY TO MANY SHIMS, HARD ON RING BOTS, ARE YOU SHIMMING THE GEAR OR PINION, I KNOW YEARS AGO (50S,) I LOOKED HARD TO FIND 4-11 GEARS FOR MY 40 FORD, THANK YOU FOR TAKING THE TIME WITH THE INFORMATION, IVE FOOLED WITH ONE OR TWO REARS, USING PERSION BLUE, ONE WAS A BENTLEY, MAN IT WAS TRUCK SIZE 1925 6.5-L.IT HAD RACING GEARS, I HAVE A RUSTY --T---REAR IM GOING TO TAKE APART TO SAVE THE PARTS IN OILY RAGS,JUST TO BE WITH THE CAR IF EVER NEEDED.

[Allan]

My wide body 25 roadster has 39 tooth ring gear and 12 tooth pinion, giving the 3.25 : 1 ratio. There are no shims in the diff. Shimming the crown wheel on the diff carrier is needed if the carrier is in the wrong place within the pumpkin. It is better to use a thinner thrust washer on the left side so that the carrier is moved over. Then the right hand thrust is machined to suit. Sure, the carrier will not be in the centre of the pumpkin. It will be just the thickness of the shim that would have been used off centre, a negligable amount in the scheme of things. This often makes it easier to get the correct rolling fit between the crown and pinion gears.

In my opinion, the only place a shim might be used is when the thrust washer/disc combination leaves the diff centre able to move side to side in the housing. Then a stationary shim can be used behind one of the steel discs.

Others do it differently.
Allan from down under

[Wayne Sheldon]

Tony, I know for certain that Ruckstell themselves made and sold 13/40 ring and pinions way back when. I had a set myself early on in this hobby, and have seen probably a dozen others. Many years ago, I was trading favors with a friend, helping him work on his T's rear end. He was installing a "3 to 1" gear set. He was tightening the ring gear bolts and wanted to hold the differential in his big vice and complained that there had to be a better way to hold it. I just said to use a standard ring gear nested in the teeth to get a gentle but firm hold. He exclaimed it couldn't work because the 3 to 1 had a 39 tooth gear and it wouldn't match up. I said "you sure? Try it." I had my set, and had noticed his was the same, so I pretty much did know. He was quite dumbfounded when it fit in perfectly, and told me he had never seen one before either.
I know that both types have been made from time to time.