Opinion Re Differential Gear Ratio

[Bob Bidonde]

My piggy bank state rules-out getting an overdrive unit, so I am considering the installation of a 3.54 gear set. The engine in my Victoria is a Model B with a 6:1 compression head, chopped flywheel & V8 clutch. In your opinion, will my engine have enough power for good cruising with the 3.54 gear set?

[denis4x4]

Absolutely! Been there, done that and pleased with the results.

[Seth Swoboda]

Yes. I would like to do this to my '31 Victoria. I have many 3.54 gear sets in V8 Fords and even a 3.25 gear.

[GPierce]

I was happy with the 3.54 in my early 31 CCPU with a stock engine. I am even happier with it since I added a 6.1 Snyder head. Bear in mind though the CCPU is lighter than any other body style.

[JayJay]

Originally Posted by Bob Bidonde

My piggy bank state rules-out getting an overdrive unit, so I am considering the installation of a 3.54 gear set. The engine in my Victoria is a Model B with a 6:1 compression head, chopped flywheel & V8 clutch. In your opinion, will my engine have enough power for good cruising with the 3.54 gear set?

Bob - What you gain at the top you lose at the low end. Also, with an O/D you gain an intermediate gear in that wide gap between second and third, useful both at lower speeds and climbing/descending hills. As you make up your mind you might get in the queue at Mitchell (I hear rumors that it's now close to a year), they don't require commitment or deposit until about a month before production. No foul if you end up cancelling out when you get the call to commit to the order and send a deposit.

I just installed my Mitchell O/D in my '31 (stock engine with 5.2 head, stock diff). I entered the queue last May 2023, got "the call" in late December, and took delivery mid-March. I took a long tour on back roads with it last weekend and am very pleased.

[PC/SR]

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I run a 3.54 with an HC head and it is great. The engine does not work as hard.

[BRENT in 10-uh-C]

Bob, I think with the right camshaft and engine package, a 3.27 will be just fine. I have installed both 3.54 and 3.27 gear sets in customer's vehicles. The biggest thing I think that dictates which is better is more about whether it is city or country driving, -and who you are traveling with. If you are in a group of 3 or 4 Model-As that have stock ratios, you generally will find yourself not like a 3.27 simply because you are not in sync with their shift points. If you are by yourself on a rural road, then a 3.27 is a great combination.

Something I want you to think about, ...three of my Model-Ts have a 3.00 rear gear ratio with 30x3.5 rear tires, and my Depot Hack has a 3.25 ratio with 21" tires. All of my engines are basically the same with 6.0 compression ratio, a 280" lift camshaft, an oversized intake manifold and a stock carburetor. Realistic horsepower with my combination is probably in the mid-thirty range. While all of these cars have a Ruckstell (two-speed) rear axle which gives me 4 forward gears vs. a Model-A's 3 gears, I can cruise 48-50 mph all day long in my cars. Those that know me know that I have toured extensively with my cars, -been up to Mt. Rushmore, been up & down the N. England state mountains, all over the Blue Ridge Parkway, Kentucky and W Virginia mountains, and many places in-between. I have never had a mountain that I could not pull.

My point is my engine and gearing combination is not unique as there are many others with similar engines and rear ends. If a Model-T with an inch or so larger diameter rear tire can pull a 3.0 Ring & Pinion, then a Model-A with an engine making twice the horsepower and at least 30% more torque should be able to capably pull a 3.27 gear ratio, ...even if it is a Sedan weighing 1,000 pound more. Now before someone suggests that 50mph is different than 60mph, its not that my cars can't do 60, but more about that I run out of brakes & steering in my Ts before I run out of power. I would strongly consider a 3.27 if you are going to touring by yourself, -or with others who have similar gear sets.

[alexiskai]

I don’t have Brent’s extensive experience but my research aligns with his advice here. An engine with the three core upgrades (head, carb, cam) will run 3.25:1 no problem.

[denis4x4]

Sold this 1800 pound hot rod with 1931 Model A running gear a year ago. Engine put out 60HP at the rear wheels on the dyno during the Speedster Reunion meet. The 3:25 gearing with a '39 transmission left a lot to be desired, so went back to 3:54 gears. I live at 6700' and often times have lunch at a resort that is almost 9,000'. Gear choice has more to do based on where you live/drive and tire diameter.

[jeepguy1948]

The top mechanic in our club installed a 3.54 in one of his cars because he likes messing with things (as do I LoL). I do not know anything about the motor but I do know that the car already had an overdrive. He liked the 3.54 but it would not work in 3-high so he removed the OD (he has multiple cars and he liked this one to be “a bit different”). I understand the pro’s and con’s involved here and being as I only have the one Model A I would not do it, I would keep the OD (although at the current prices and lead times from Mitchell I might need it elsewhere (if I had another car). From talking to many people, I find that 4.10 and OD make a good pairing with a stock motor. Most people would not know if they liked it or not because have not had an opportunity to try it.

[jeepguy1948]

I think that 28-29 with 21” wheels vs 30-31 with 19” wheels might make the difference too.

[GPierce]

I have a Mitchell overdrive with my 3.54. At 45 mph I shift into 3rd over. Everything quiets down and smooths out. I love the combination.

[WerbyFord]

Originally Posted by alexiskai

I don’t have Brent’s extensive experience but my research aligns with his advice here. An engine with the three core upgrades (head, carb, cam) will run 3.25:1 no problem.

Any examples of what head, carb, cam to run?

Obviously too much cam will take away low end power, but sometimes just more lift on a modern grind will help.

Our 28 A still has the original engine but I have a spare engine I might play with someday!

[Keith True]

I have a late 31 coupe with a Mitchell OD and 3.54 gears.Seems to work fine for me.I forget to take it out of OD a lot when I stop,I just drive off again.Very seldom do I get into a situation where I HAVE to take it out OD to drive it.Sometimes I'll have it in OD when I've stopped on a hill,and I do have to yank the lever back to take off.I have no clue what the engine is,I bought the car a few years ago and discovered the 3.54 gears when I went to put the OD in it.I also have a 30 open pickup with 3.54's in it,BF head,B cam,.060 over,zippy little truck.It used to have 3.78 gears and a BW overdrive,that was just a little too tall in that truck.I have never once seen a need to split the gears with the Mitchell in mine.I have another new Mitchell,I wanted to use it in my 28 Phaeton,but it has the early design ring and pinion,not compatible with the Mitchell.

[alexiskai]

Originally Posted by WerbyFord

Any examples of what head, carb, cam to run?

Obviously too much cam will take away low end power, but sometimes just more lift on a modern grind will help.

Our 28 A still has the original engine but I have a spare engine I might play with someday!

I think the Burtz head and cam would be solid recommendations. Carbs are much more about your personal taste. If you want to keep the original profile, converting to a Model B carb and intake is a common choice. After that it's downdraft carbs.

[Seth Swoboda]

I like what Brent said. My 1937 Ford Tudor has a 85 hp V8, 16 in wheels and a 3.25 rear gear. It will drive down the highway very well. I'm in flat land country, no mountains.

[Bob Bidonde]

Thanks all for your comments. Unfortunately, I do not see my piggy bank getting enough to buy a Mitchel OD this year or in the near future. So I will likely opt for the differential gear change.

Considering the 3.27:1 and the 3.54:1 gear sets, at 55 MPH the differences in RPM are 14.5% less for the 3.27 and 6.5% less for the 3.54 gears. At 55 MPH, the motor turns approximately 2357 RPM with the 3.78 stock gears. Thus with a 3.27 gear set at 55 MPH, the RPM should drop 341 RPM to 2016 RPM. If I choose the 3.54 gears, the RPM at 55 MPH will be 2357-153 = 2204 RPM.

I will opt for the 3.27:1 gear set, so the search for the gears begins. I live on Long Island which is mostly flat. My annual long distance touring is to the New England Meets which can be a hilly ride. As I mentioned earlier, the engine in my Victoria is a Model B with a 6:1 cylinder head, a chopped flywheel and a V8 clutch. My guess is the this motor has between 50 & 60 HP.

[Keith True]

I didn't realize where you lived.From what I remember of Long Island a high gear ratio would be of zero concern.I hauled seed potatoes from there about 40 years ago.I could zip right along with 220 HP pulling 50,000 pounds of seed spuds.Got pretty agonizing once I got off the island and headed for N.H.though.

[jeepguy1948]

If you need to save $$ just put synthetic oil in it and drive it the rpm might bug you but it won’t hurt the motor (to be clear, I’m talking 55, not 85)

[jetrod]

Bob, I installed the 3;54 in my RPU and it is an improve ment on level ground. The RPU is the lightest of model A's---but in the hills in WV, my best budddy in his stock 3;78 coupe takes great pleasure in riding my bumper --up hill --and oogahing his damn horn. So I guess it is a mixed blessing. Toodles, JR