The Ford Barn

B0B · 03-14-2011, 04:34 PM

I know I saw on the forum sometime back the engine RPM's at various speeds 30,40,50 Mph in third gear for a 28-29 model A with standard 21 inch wheels BUT I can not find that info now when I need it does anyone have this info ? THANKS B0B in MICHIGAN

Ian in Mississauga · 03-14-2011, 09:12 PM

Bob, here's the formula I have:
RPM=MPH x axle ratio x 336/tire OD
Most 21 inch tires have an OD of about 30 inches, and the standard axle ratio is 3.78.
eg RPM at 50 MPH=50 x3.78 x 11.2 = 2116.8

Logan · 03-14-2011, 10:57 PM

Quote:

Originally Posted by Ian in Mississauga

Bob, here's the formula I have:
RPM=MPH x axle ratio x 336/tire OD
Most 21 inch tires have an OD of about 30 inches, and the standard axle ratio is 3.78.
eg RPM at 50 MPH=50 x3.78 x 11.2 = 2116.8

What about 19 inch wheels?

Tom Foster · 03-14-2011, 11:09 PM

Quote:

Originally Posted by Logan

What about 19 inch wheels?

What matters is not the diameter of the wheels but the rolling diameter of the tires. You'll find that most 19" tires also have a rolling diameter of about 30" so the same figures would apply.

Fearless

sturgis 39 · 03-15-2011, 05:50 AM

I always thought the rear end ratio was different for the 19 and 21 wheels. I guess you can learn something everyday.

Tom Foster · 03-15-2011, 06:59 AM

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Quote:

Originally Posted by sturgis 39

I always thought the rear end ratio was different for the 19 and 21 wheels. I guess you can learn something everyday.

Well, in a way it was. Early '29 roadsters came supplied with a 3.54 gear, because they were about 200# lighter than sedans and coupes. Makes a diff in acceleration, though.
Fearless

Tom Wesenberg · 03-15-2011, 07:29 AM

Quote:

Originally Posted by Tom Foster

Well, in a way it was. Early '29 roadsters came supplied with a 3.54 gear, because they were about 200# lighter than sedans and coupes. Makes a diff in acceleration, though.
Fearless

And, many 1928's came with a 3.70 ratio, which is just a hair faster than a 3.78 ratio. 10:37 vs. 9:34 for the ring and pinion

kenparker · 03-15-2011, 07:49 AM

I used to keep a business card stuck to the dash rail that I called my "po mans tach". The numbers are approx and were calculated for 19" wheels on my 31 coupe.
400 rpm - 10 mph
600rpm 15
1000 30
1600 45
2000 55
2200 60
2400 to fast

Tom Wesenberg · 03-15-2011, 09:05 AM

Quote:

Originally Posted by kenparker

I used to keep a business card stuck to the dash rail that I called my "po mans tach". The numbers are approx and were calculated for 19" wheels on my 31 coupe.
400 rpm - 10 mph
600rpm 15
1000 30
1600 45
2000 55
2200 60
2400 to fast

That sounds really low, unless you have an overdrive.

sturgis 39 · 03-16-2011, 01:36 AM

The best little book I bought was FORD MODEL A PASSENGER CAR AND MODEL AA TRUCK ENGINE CHASSIS MANUAL. I use the thing for everything including a coffee cup coaster.
It is about 75% of an 8.5 x11 piece of paper.

10 mph=429 rpm
15 mph=643 rpm
55 mph=2357rpm
60 mph=2571rpm
65 mph=2785rpm

It says there were two rearend ratio-3.77 and a 4.111 for mountain driving.

That is what my favorite book says. It has wiring diagrams and all kinds of good stuff. It is getting in rough shape. I have had it for about ten years.

Jim Brierley · 03-16-2011, 10:22 AM

That sounds closer to being right. If I can remember, I'll check it on my dream wheel.

John Stone · 03-16-2011, 04:59 PM

Here is a little table that I made up.

Speed -mphTire DiameterRear End RatioEngine RPMTire DiameterRear End RatioEngine RPM0293.780303.7805293.78219303.7821210293.78438303.7842415293.78657303.7863520293.78876303.7884725293.781095303.78105930293.781314303.78127135293.781533303.78148240293.781753303.78169445293.781972303.78190650293.782191303.78211855293.782410303.78232960293.782629303.782541Speed -mphTire DiameterRear End RatioEngine RPMTire DiameterRear End RatioEngine RPM0293.540303.5405293.54205303.5419810293.54410303.5439715293.54615303.5459520293.54821303.5479325293.541026303.5499230293.541231303.54119035293.541436303.54138840293.541641303.54158745293.541846303.54178550293.542052303.54198355293.542257303.54218260293.542462303.542380

John Stone · 03-16-2011, 04:59 PM

Whoops, that didn't work. Looked good for a while.

John Stone · 03-16-2011, 05:14 PM

Can't paste an Excel file. Bummer
I give up

sphanna · 03-17-2011, 02:28 PM

Bob
Some of the info on your request is not very accurate. I have mathematicly correct info for the 19 in wheels.
( 19" Wheels with 4.75/5.00/19 tires with 1/2 wear left Rolling distance was 90.75")
If for example (and I do not know the distance) the rolling distance for your wheels was 93.5. You would add 3% to the MPH figures assuming you have the 3.78 gear ratio.

65 MPH 2860 RPM 60 =2640 55 =2420 50=2200 45=1980 40=1760 35=1540
30=1320 25=1100 20=880
For RPM TO MPH THEN: 3000=68 2900=66 2800=63.5 2700=61.5 2600=59 2500=57 2400=54.5 2300=52.5 2200=50 2100=47.5 2000=52.5 1900=43 1800=41 1700=38.5 1600=36.5 1500=34 1400=32 1300=27.5 1100=25 1000=22.5

03-14-2011, 04:34 PM	#1
B0B Member Join Date: Jun 2010 Posts: 82	What is the engine RPM's in a 1929 Ford with 21 inch wheels ? I know I saw on the forum sometime back the engine RPM's at various speeds 30,40,50 Mph in third gear for a 28-29 model A with standard 21 inch wheels BUT I can not find that info now when I need it does anyone have this info ? THANKS B0B in MICHIGAN

03-14-2011, 09:12 PM	#2
Ian in Mississauga Senior Member Join Date: May 2010 Posts: 136	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? Bob, here's the formula I have: RPM=MPH x axle ratio x 336/tire OD Most 21 inch tires have an OD of about 30 inches, and the standard axle ratio is 3.78. eg RPM at 50 MPH=50 x3.78 x 11.2 = 2116.8

03-15-2011, 05:50 AM	#5
sturgis 39 Senior Member Join Date: Aug 2010 Location: Lead. South Dakota Posts: 963	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? I always thought the rear end ratio was different for the 19 and 21 wheels. I guess you can learn something everyday. __________________ IF IT CAN NOT BE FIXED WITH BLASTING WIRE, JB WELD OR DUCT TAPE ---IT CAN NOT BE FIXED Do not get me started on the stupidity of ethanol. I think one of the monitors is from Iowa and he will delete the thread.

03-15-2011, 07:49 AM	#8
kenparker Senior Member Join Date: May 2010 Location: Van, Texas Posts: 1,122	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? I used to keep a business card stuck to the dash rail that I called my "po mans tach". The numbers are approx and were calculated for 19" wheels on my 31 coupe. 400 rpm - 10 mph 600rpm 15 1000 30 1600 45 2000 55 2200 60 2400 to fast

03-16-2011, 01:36 AM	#10
sturgis 39 Senior Member Join Date: Aug 2010 Location: Lead. South Dakota Posts: 963	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? The best little book I bought was FORD MODEL A PASSENGER CAR AND MODEL AA TRUCK ENGINE CHASSIS MANUAL. I use the thing for everything including a coffee cup coaster. It is about 75% of an 8.5 x11 piece of paper. 10 mph=429 rpm 15 mph=643 rpm 55 mph=2357rpm 60 mph=2571rpm 65 mph=2785rpm It says there were two rearend ratio-3.77 and a 4.111 for mountain driving. That is what my favorite book says. It has wiring diagrams and all kinds of good stuff. It is getting in rough shape. I have had it for about ten years. __________________ IF IT CAN NOT BE FIXED WITH BLASTING WIRE, JB WELD OR DUCT TAPE ---IT CAN NOT BE FIXED Do not get me started on the stupidity of ethanol. I think one of the monitors is from Iowa and he will delete the thread. Last edited by sturgis 39; 03-16-2011 at 01:46 AM.

03-16-2011, 10:22 AM	#11
Jim Brierley Senior Member Join Date: Jun 2010 Location: Temecula, CA Posts: 4,085	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? That sounds closer to being right. If I can remember, I'll check it on my dream wheel.

03-16-2011, 04:59 PM	#12
John Stone Senior Member Join Date: May 2010 Location: Wichita, Kansas Posts: 710	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? Here is a little table that I made up. Speed -mphTire DiameterRear End RatioEngine RPMTire DiameterRear End RatioEngine RPM0293.780303.7805293.78219303.7821210293.78438303.7842415293.78657303.7863520293.78876303.7884725293.781095303.78105930293.781314303.78127135293.781533303.78148240293.781753303.78169445293.781972303.78190650293.782191303.78211855293.782410303.78232960293.782629303.782541Speed -mphTire DiameterRear End RatioEngine RPMTire DiameterRear End RatioEngine RPM0293.540303.5405293.54205303.5419810293.54410303.5439715293.54615303.5459520293.54821303.5479325293.541026303.5499230293.541231303.54119035293.541436303.54138840293.541641303.54158745293.541846303.54178550293.542052303.54198355293.542257303.54218260293.542462303.542380

03-16-2011, 05:14 PM	#14
John Stone Senior Member Join Date: May 2010 Location: Wichita, Kansas Posts: 710	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? Can't paste an Excel file. Bummer I give up

03-17-2011, 02:28 PM	#15
sphanna Senior Member Join Date: Jun 2010 Location: Polk City, Iowa Posts: 526	Re: What is the engine RPM's in a 1929 Ford with 21 inch wheels ? Bob Some of the info on your request is not very accurate. I have mathematicly correct info for the 19 in wheels. ( 19" Wheels with 4.75/5.00/19 tires with 1/2 wear left Rolling distance was 90.75") If for example (and I do not know the distance) the rolling distance for your wheels was 93.5. You would add 3% to the MPH figures assuming you have the 3.78 gear ratio. 65 MPH 2860 RPM 60 =2640 55 =2420 50=2200 45=1980 40=1760 35=1540 30=1320 25=1100 20=880 For RPM TO MPH THEN: 3000=68 2900=66 2800=63.5 2700=61.5 2600=59 2500=57 2400=54.5 2300=52.5 2200=50 2100=47.5 2000=52.5 1900=43 1800=41 1700=38.5 1600=36.5 1500=34 1400=32 1300=27.5 1100=25 1000=22.5 __________________ Steve Hanna, Polk City, IA