The Ford Barn

Synchro909 · 12-21-2022, 05:01 PM

Quote:

Originally Posted by Bruce of MN

The correct math is 3.78 / 1.26 = 3.00. No subtraction with gear ratios.

You beat me to it. As I read this thread this morning (It's 9.00am on 22/12 now), I was going to post exactly what you have said.

nkaminar · 12-21-2022, 06:11 PM

Bruce of MN has the right math.

Keith True · 12-21-2022, 06:43 PM

I have been running a 31 coupe with 3.54 and a Mitchell 26% over for a couple of years now.I get along with it just fine.If it starts to get a little chuggy around town it's because I forgot and left it in OD.I have a BF head,a B dist cam,and an Aries muffler.

Model A Ron · 12-22-2022, 11:35 PM

My math tells me the Mitchell takes you to 2.8.....regardless look at the chart. Seems to me all you would have to do with a 3.25 is run 2nd out a bit. I see no reason why any good running A would have a problem with it unless your in the mountains. I can drive my car in Mitchell high and do just fine......even with funny math that 3.00 but we know its really 2.8

Synchro909 · 12-23-2022, 05:22 PM

Quote:

Originally Posted by Model A Ron

My math tells me the Mitchell takes you to 2.8.....regardless look at the chart. Seems to me all you would have to do with a 3.25 is run 2nd out a bit. I see no reason why any good running A would have a problem with it unless your in the mountains. I can drive my car in Mitchell high and do just fine......even with funny math that 3.00 but we know its really 2.8

You are welcome to your incorrect maths.

29vintage · 12-23-2022, 08:23 PM

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Why there appears a difference in opinions, is due to the different opinions in what the 26% reduction means. It's a reduction in the engine rpms, and not the gear ratio. You'd think they'd both work out to the same answer, but unfortunately they don't.

Whey the O/D is in low, the input shaft (engine side) rotates at the same speed as the O/D output shaft. When the O/D is in high, the output shaft rotates 1.26 times faster than the input. For any given road speed, the tyres, axle shafts, and pinion, rotate at the same speed, regardless of whether the O/D is in low or high. With the O/D in low, the engine side of the O/D is the same rotational speed as the pinion side. When the O/D is in high, the engine side of the O/D is 26% lower (1.26/1) than the pinion side, due to the gearing of the OD. Hence the notation of 26% reduction.

If you assessed it as a 26% gear reduction. For a 1:1 drive, 26% of 1 is 0.26. Therefore final drive is 1-0.26 = 0.74, which is unfortunately incorrect.

The actual gear reduction of a 26% O/D is 1/1.26 = 0.794. Which makes the final drive ratio if 3.78 drive of 3.78*0.794 = 3.0, which is the same as 3.78/1.26.

Synchro909 · 12-23-2022, 09:20 PM

Quote:

Originally Posted by 29vintage

Why there appears a difference in opinions, is due to the different opinions in what the 26% reduction means. It's a reduction in the engine rpms, and not the gear ratio. incorrectYou'd think they'd both work out to the same answer, but unfortunately they don't.

Whey the O/D is in low, the input shaft (engine side) rotates at the same speed as the O/D output shaft. When the O/D is in high, the output shaft rotates 1.26 times faster than the input. correctFor any given road speed, the tyres, axle shafts, and pinion, rotate at the same speed, regardless of whether the O/D is in low or high. With the O/D in low, the engine side of the O/D is the same rotational speed as the pinion side. When the O/D is in high, the engine side of the O/D is 26% lower (1.26/1) than the pinion side, due to the gearing of the OD. incorrectHence the notation of 26% reduction.

If you assessed it as a 26% gear reduction. For a 1:1 drive, 26% of 1 is 0.26. Therefore final drive is 1-0.26 = 0.74, which is unfortunately incorrect.

The actual gear reduction of a 26% O/D is 1/1.26 = 0.794. Which makes the final drive ratio if 3.78 drive of 3.78*0.794 = 3.0, which is the same as 3.78/1.26.

A 26% O/D is, as the name says, an OVER drive. That is, the speed of the output is OVER the speed of the input. If it were less, we'd have an UNDER drive. Your logic is unfortunately, flawed. A 26% overdrive means at the car will be travelling 26% faster at the same engine revs once the O/D is engaged (ie, travelling at 26% over the speed without it.)
If the input shaft to the overdrive unit is rotating at say, 1,000 rpm. the output is also rotating at the rate when the O/D is not engaged. When it is engaged, the output shaft is rotating at 1260rpm. You can't work the other way at 26%. In this example, the output shaft is turning at 260 rpm faster than the input shaft. If you try going the opposite way, 26% of the output rpm is 1260 X 26%= 328 rpm. 1260 - 328 is 932 rpm and that is clearly wrong.
26% applies to the speed of the input shaft which is directly related to the speed of the car. If the revs do not change, a car travelling at 50 mph without the O/D will be travelling at 63 mph when it is. (50x 1.26= 63)

daveymc29 · 12-23-2022, 11:39 PM

Running a 33% Borg Warner ahead of a 4:11 rear in my coupe, and love it. Good engine, 92 psi compression is all I know about engine, runs swell and use the gears in the hills. Sierra's.
3:78 behind a 26% Mitchell. Love it also, had a strong inserted engine that blew a cld. wall after about 1,000 miles, now have a refreshed stock A engine with about 64 PSI in the roadster. use 2nd over a lot in the hills. Not so much on the level, just 3rd then 3rd over. Mitchell advised the 4:11 with his 33%, but I already had the B/W 33% on hand. He will build a 33% if you like, or said that a couple of years back.

Dave Mc Arthur

Hitman · 12-24-2022, 12:08 AM

Ladies, ladies……. Does the math really matter? What’s the final drive ratio in your wife’s front wheel drive car? You don’t know, and it doesn’t matter. Does that same car downshift often? Probably.

Who cares if you have to downshift in the switchbacks at 8000’? Your modern car shifts gears too! That’s why you have choices, to keep the engine where it makes power. I don’t brag how long I can pull third gear up a hill.

I’d personally install a 3.54 gear set behind my Mitchell. It’s a coupe that has a 3.78 with the Mitchell. I live at 2800’, can drive to 9000’ within 20 miles of my house, and regularly cruise interstates all over the west at elevations up to 11,000.

Have fun, run the gear combination you want. If you hate it, change it to the direction that makes you happy.

old31 · 12-24-2022, 05:31 AM

My head is starting to hurt with all this math stuff.

Model A Ron · 12-25-2022, 02:38 PM

Quote:

Originally Posted by 29vintage

Why there appears a difference in opinions,

If you assessed it as a 26% gear reduction. For a 1:1 drive, 26% of 1 is 0.26. Therefore final drive is 1-0.26 = 0.74, which is unfortunately incorrect.

The actual gear reduction of a 26% O/D is 1/1.26 = 0.794. Which makes the final drive ratio if 3.78 drive of 3.78*0.794 = 3.0, which is the same as 3.78/1.26.

Thank you for the explaining things and now I do agree that the Mitchell OD is 3.0. That said I can drive my Mitchell in high and it seams to do fine as long as I wind out 2nd a bit more. Any reason not to go with a 3.25 and no Mitchell as long as you do not live in the hills?

Thanks
Ron

tinkirk · 12-25-2022, 04:00 PM

Quote:

Originally Posted by Model A Ron

Thank you for the explaining things and now I do agree that the Mitchell OD is 3.0. That said I can drive my Mitchell in high and it seams to do fine as long as I wind out 2nd a bit more. Any reason not to go with a 3.25 and no Mitchell as long as you do not live in the hills?

Thanks
Ron

None what so ever your choice go for it

Sent from my iPhone using Tapatalk

Synchro909 · 12-25-2022, 04:20 PM

Quote:

Originally Posted by Model A Ron

Thank you for the explaining things and now I do agree that the Mitchell OD is 3.0. That said I can drive my Mitchell in high and it seams to do fine as long as I wind out 2nd a bit more. Any reason not to go with a 3.25 and no Mitchell as long as you do not live in the hills?

Thanks
Ron

If the motor has the torque, I also say "go for it".
There have been times when I am towing a trailer and I've come to a stop without taking the O/D back to direct drive. When I move off again, I'm pulling about double the weight of the car alone with an effective ratio less favourable than 3.27, yet the car does it (with some riding of the clutch).

Richard Knight · 12-25-2022, 04:53 PM

Reading thru this I see very little mention of tire size. So I would guess most all the responders do have the stock size for there cars. I ran a Murray sedan with B motor, mild cam, stromberg 97, brumfield head, with 3.78 gears, 16 inch wheels and borg Warner overdrive for years and thousands of miles. This combination worked well for me. Now I have a 30 coupe currently with a B with the goodies and I have built a burtz I hope to install. This car happens to have 17 inch wheels. I have been considering 3.54 gears and had not considered 3.25s. Anyone have any thoughts on this?

ModelA29 · 12-29-2022, 08:59 PM

Easy math - one step if you just multiply by .74 (100% - 26% = 74%)

Synchro909 · 12-30-2022, 01:49 AM

Quote:

Originally Posted by ModelA29

Easy math - one step if you just multiply by .74 (100% - 26% = 74%)

That gives 2.8. As has been said, there is no addition or subtraction in calculating gear ratios I divide by 1.26. 3.78 ÷ 1.26 = 3.0. Easy!

jeepguy1948 · 12-30-2022, 11:49 AM

Not an expert but here is what I know; It is really all about an overdrive, I may have missed it but I don’t see that you mentioned that. If we have or plan to have an overdrive you will want to stick with the 3.78, if no overdrive, 3.54. We can argue all day long but unless you never go up hills you will not want to go higher than these ratios.

12-21-2022, 06:11 PM	#22
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 3,897	Re: Ring and Pinion 3.78, 3.54 or 3.25 Bruce of MN has the right math. __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past.

12-21-2022, 06:43 PM	#23
Keith True Senior Member Join Date: May 2010 Location: Epping N.H. Posts: 2,989	Re: Ring and Pinion 3.78, 3.54 or 3.25 I have been running a 31 coupe with 3.54 and a Mitchell 26% over for a couple of years now.I get along with it just fine.If it starts to get a little chuggy around town it's because I forgot and left it in OD.I have a BF head,a B dist cam,and an Aries muffler.

12-23-2022, 08:23 PM	#26
29vintage Junior Member Join Date: Jan 2014 Posts: 12	Re: Ring and Pinion 3.78, 3.54 or 3.25 Sponsored Links (Register now to hide all advertisements) Why there appears a difference in opinions, is due to the different opinions in what the 26% reduction means. It's a reduction in the engine rpms, and not the gear ratio. You'd think they'd both work out to the same answer, but unfortunately they don't. Whey the O/D is in low, the input shaft (engine side) rotates at the same speed as the O/D output shaft. When the O/D is in high, the output shaft rotates 1.26 times faster than the input. For any given road speed, the tyres, axle shafts, and pinion, rotate at the same speed, regardless of whether the O/D is in low or high. With the O/D in low, the engine side of the O/D is the same rotational speed as the pinion side. When the O/D is in high, the engine side of the O/D is 26% lower (1.26/1) than the pinion side, due to the gearing of the OD. Hence the notation of 26% reduction. If you assessed it as a 26% gear reduction. For a 1:1 drive, 26% of 1 is 0.26. Therefore final drive is 1-0.26 = 0.74, which is unfortunately incorrect. The actual gear reduction of a 26% O/D is 1/1.26 = 0.794. Which makes the final drive ratio if 3.78 drive of 3.78*0.794 = 3.0, which is the same as 3.78/1.26.

12-23-2022, 11:39 PM	#28
daveymc29 Senior Member Join Date: Sep 2010 Location: Danville, CA Posts: 1,554	Re: Ring and Pinion 3.78, 3.54 or 3.25 Running a 33% Borg Warner ahead of a 4:11 rear in my coupe, and love it. Good engine, 92 psi compression is all I know about engine, runs swell and use the gears in the hills. Sierra's. 3:78 behind a 26% Mitchell. Love it also, had a strong inserted engine that blew a cld. wall after about 1,000 miles, now have a refreshed stock A engine with about 64 PSI in the roadster. use 2nd over a lot in the hills. Not so much on the level, just 3rd then 3rd over. Mitchell advised the 4:11 with his 33%, but I already had the B/W 33% on hand. He will build a 33% if you like, or said that a couple of years back. Dave Mc Arthur

12-24-2022, 12:08 AM	#29
Hitman Senior Member Join Date: Sep 2014 Posts: 521	Re: Ring and Pinion 3.78, 3.54 or 3.25 Ladies, ladies……. Does the math really matter? What’s the final drive ratio in your wife’s front wheel drive car? You don’t know, and it doesn’t matter. Does that same car downshift often? Probably. Who cares if you have to downshift in the switchbacks at 8000’? Your modern car shifts gears too! That’s why you have choices, to keep the engine where it makes power. I don’t brag how long I can pull third gear up a hill. I’d personally install a 3.54 gear set behind my Mitchell. It’s a coupe that has a 3.78 with the Mitchell. I live at 2800’, can drive to 9000’ within 20 miles of my house, and regularly cruise interstates all over the west at elevations up to 11,000. Have fun, run the gear combination you want. If you hate it, change it to the direction that makes you happy.

12-24-2022, 05:31 AM	#30
old31 Senior Member Join Date: Jan 2016 Posts: 2,101	Re: Ring and Pinion 3.78, 3.54 or 3.25 My head is starting to hurt with all this math stuff. Last edited by old31; 12-25-2022 at 08:51 AM.

12-25-2022, 04:53 PM	#34
Richard Knight Senior Member Join Date: Mar 2021 Posts: 130	Re: Ring and Pinion 3.78, 3.54 or 3.25 Reading thru this I see very little mention of tire size. So I would guess most all the responders do have the stock size for there cars. I ran a Murray sedan with B motor, mild cam, stromberg 97, brumfield head, with 3.78 gears, 16 inch wheels and borg Warner overdrive for years and thousands of miles. This combination worked well for me. Now I have a 30 coupe currently with a B with the goodies and I have built a burtz I hope to install. This car happens to have 17 inch wheels. I have been considering 3.54 gears and had not considered 3.25s. Anyone have any thoughts on this? Last edited by Richard Knight; 12-25-2022 at 04:58 PM.

12-29-2022, 08:59 PM	#35
ModelA29 Senior Member Join Date: Sep 2022 Location: SoCal Posts: 591	Re: Ring and Pinion 3.78, 3.54 or 3.25 Easy math - one step if you just multiply by .74 (100% - 26% = 74%)

12-30-2022, 11:49 AM	#37
jeepguy1948 Senior Member Join Date: Jan 2022 Posts: 721	Re: Ring and Pinion 3.78, 3.54 or 3.25 Not an expert but here is what I know; It is really all about an overdrive, I may have missed it but I don’t see that you mentioned that. If we have or plan to have an overdrive you will want to stick with the 3.78, if no overdrive, 3.54. We can argue all day long but unless you never go up hills you will not want to go higher than these ratios.