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09-25-2018, 09:03 AM | #1 |
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30 PU max safe driving speed
Have been looking at what max speeds are safe for the motor.
Found info on the Barn and other places on the Internet. Everyone seems to agree 2200rpm is max, due to lubrication issues above that rpm. I have the 4.1 rear end in my truck, 16 inch wheels but the tire circumference seems to be the same as the 19s The speedometer is spot on/correct at 40mph. Measured at one of the speed indicators sometimes put at the side of the road. Some say 50 is max, some say 45 is max, some say 40 is max. Which is correct? If I want to convert to 3.78, is it a simple diff gear swap, or do axels have to also be changed? Would I experience a noticeable loss of power and acceleration? Am considering this, most in clubs prefer to cruise at 45-50. May change if changing is not a major expense like when putting in an overdrive. |
09-25-2018, 09:26 AM | #2 |
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Re: 30 PU max safe driving speed
I have a 30 pick up which is completely stock with the 3.78 rear end . It cruises very happily at 45 MPH . At 50 it is not so happy I can tell, this is for extended periods . With modern higher octane gasolene your truck will easily pull a 3.54 or even a 3.27 if you live in a flat area no long big hills. It might take some years but you will get your money back with way better gas mileage. Only the ring gear and pinion need to be changed nothing else.
John in lovely sunny afternoon Suffolk County England. |
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09-25-2018, 09:48 AM | #3 | |
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Re: 30 PU max safe driving speed
Quote:
Thanks for the info you provided. I assume I have the 4.1, since I struggle to keep up with others. It is not happy/smooth at 40 and above, vibration, sounds busy. 55 mph (not KM) is max on the flat. Wisconsin has hills, minor, not even foothill type. But some hills on secondary road will slow me down to 35-40 in 3rd. The truck gets less than 500 miles a year - not too worried about cost, I also run ethanol free which is another 40 cents/a gallon. More of a concern where you are. If I remember correctly your gas is 3- 4 times as much as here in the States. Priced similarly as here except ours is per gallon, yours is per liter, a liter is about 1/4 gallon. I am hoping others can definitively confirm where my motor is at 2200 RPM. The motor had a recent head gasket and valve job, the lower end condition is unknown. Babbitt's/rods/etc. work is expensive to have done. |
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09-25-2018, 09:52 AM | #4 |
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Re: 30 PU max safe driving speed
I would skip 3.78 and go to 3.54 which is what have in my '31 RDPU. I do have a counterbalanced crank but 55 is comfortable. Above that speed the fenders make the front end light and uncomfortable to drive.
Charlie Stephens |
09-25-2018, 10:10 AM | #5 |
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Re: 30 PU max safe driving speed
What Charlie said.
I have a 3:54 rear end, stock engine in my 31 CCPU. No problem running in the Alabama hills at 45 and 50. Will do fine at 55 if I need to keep up with the pack. |
09-25-2018, 10:18 AM | #6 |
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Re: 30 PU max safe driving speed
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Wayne @ Barb Malvern '19 T Speedster '26 T Touring '29 A Roadster '30 AA Stakebed '30 A Pickup '30 A Town Sedan '31 A Station Wagon '38 Columbia Girls bike (WWII) '40 Elgin Boys bike (WWII) '42 Super Deluxe Tudor '42 Willys MB Jeep '43 Willys MBT Trailer '43 M3A4 Hand Cart '43 Harley Davidson 42WLA with sidecar |
09-25-2018, 10:19 AM | #7 |
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Re: 30 PU max safe driving speed
You probably should determine your current axle ratio for certain before doing any swaps.
Jack up one rear wheel. Put a chalk mark on the jacked up tire and a board with a finish nail in it pointing exactly at the chalk mark. Take out your spark plugs. Put the trans in high. Take any slack out of the drive train with the crank (make sure you didn't move the elevated rear tire) Mark exactly where the crank is. Turn the crank until the tire has turned exactly 2 revolutions. The number of times you had to turn the crank to get those 2 tire revolutions is your gear ratio (4 times=4.11/1...3 3/4 times=3.78...3 1/2 times=3.54)
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09-25-2018, 10:22 AM | #8 | |
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Re: 30 PU max safe driving speed
Quote:
If your rig is not happy nor smooth above 40, you may need to do some routine maintenance, or repair. Wheel balance, tires out of round, worn bearings in diff or trans, fan out of balance, or even a poor quality engine rebuild by a prior owner. |
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09-25-2018, 12:02 PM | #9 |
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Re: 30 PU max safe driving speed
I appreciate the advice - Thanks
I've had the oil pan dropped because the drain plug spun, someone before me had pressed in a repair insert, but did not solder/braze/weld it in place. I looked at things and know what they are. but am not a mechanic and would not know what is good/bad. Need a mechanic to tell me good vs. bad. Only have basic tools, not a good place at home for major work. After the recent day tour noticed it has an engine knock, most notable at very slow idle. Going into storage soon, next year have it looked at. Right now using it minimally, no more long rides, and concentrating on small things that I can actually do. Things like adding the tire balancing beads, tires were not balanced when new installed, adjusting the generator - was at 15 amps, etc. Willing to learn, but I am overwhelmed, too many things are needed. The purpose of this is string is to see if I want/need to change the diff. When I take it somewhere next year will have a prioritized list for quoting, and get it all done at once. Still looking for the rpm/mph answers. Was wondering if I ran too high of a rpm on the day trip. I've seen the article in this link that running above 2200 rpm over time can cause Babbitt and Rod issues due to lubrication issues at more than 2200 rpm. http://www.hangtownas.org/TopSpeed.pdf |
09-25-2018, 12:15 PM | #10 |
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Location: Fountain Valley, Calif.
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Re: 30 PU max safe driving speed
An unladen Model A Truck normally does not lower gears, unless you are hauling loads or driving up steep hills most of the time. Many Model A’s have been re-built over the years, and not very well balanced, and do not like to be driven more than 45 mph. Like the other post said, you really need to determine your rear axle ratio first. If you do have the 4.1 ratio, then your choices are either changing it to a 3.54 or adding an overdrive.....you will be much happier driving it around. With either of those you can drive 50 at a comfortable rpm range for that engine.
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09-25-2018, 12:23 PM | #11 |
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Re: 30 PU max safe driving speed
I'd have the engine looked at now, so any repairs can be made over the winter.
I agree with the others that 3.54 or 3.27 would be the gears to use. I plan to buy 3.27 gears for my 28 Phaeton. |
09-25-2018, 01:19 PM | #12 |
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Re: 30 PU max safe driving speed
I have 3.78 gears fed by a 27% OD transmission. In the hills without OD, I can maintain 50-55 with no problem and still stay under 2200 RPM. On the flat, the RPMs drop to around 1700-0800 maintaining the same speed.
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09-25-2018, 03:17 PM | #13 | |
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Re: 30 PU max safe driving speed
Quote:
Thanks Tom. Will determine what I have for diff and make a decision. My only concern is if my A has enough power for the 3.27. Have yet to do a compression test since the top end job. Will research some more and mull over what I want to do. I am known for taking a long time in decision making, my thought process can be slow. I am going to wait till next year, closest A repair places are a 2 hour drive in the A, or have it towed. My storage place only charges $140 for 6 months, have paid and is not refundable. But the space is assigned, am 6 cars deep, has to go in when they say so and can not take it out. Am more comfortable waiting and have time to think things over. |
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09-25-2018, 04:01 PM | #14 |
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Re: 30 PU max safe driving speed
My suggestion if you plan on changing the ratio would be 3:54. A 2 hour drive is nothing in a model A. Just set the throttle on the column and sit back and watch the scenery go by.
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09-25-2018, 04:22 PM | #15 | |
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Re: 30 PU max safe driving speed
Quote:
Thanks, that was what I was considering (the 3.54). Would love to drive it. But am concerned about the engine knock, not knowing the cause of the knock, and making things worse. m |
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09-25-2018, 04:58 PM | #16 |
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Re: 30 PU max safe driving speed
If I change the ratio, will the speedometer be correct?
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09-25-2018, 05:09 PM | #17 | |
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Re: 30 PU max safe driving speed
Quote:
Charlie Stephens |
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09-25-2018, 05:11 PM | #18 |
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Re: 30 PU max safe driving speed
You have to match your speedometer driven gear with the rear axle ratio for the speedometer to be correct. I live in the mountains area of North West Washington state and I have 3.27 gears in the rear axle of our 1931 pickup. It works well around here for our pick up. With you living in much less mountains than us, you should get along fine with 3.27 rear axle gears. Our 1930 Phaeton and our 1929 Tudor both have 3.54 gears and they do well with them. They haul up to 6 people and I felt that the 3.54 would work better with a full load being carried.
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09-25-2018, 05:18 PM | #19 |
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Re: 30 PU max safe driving speed
I don't know A's as well as a T, but if the speedometer drive is anywhere other than the front wheel...it will be off.
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09-25-2018, 05:19 PM | #20 |
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Re: 30 PU max safe driving speed
No you will need to change the little gear in the turtle, it's attached to the drive shaft tube on the end of the speedometer cable.
My Tudor came to me with 16in wheels and 3.78 gears,it didn't like anything above 40mph. Went with 19in wheels with 31in tires and and 50-55mph doable. I type really slow :-(
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