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how to speed up a 37 dodge D5 coupe now with 4.11 rear


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Ya something does not seem right, my 28 with 4.45 gears will cruse 55 all day with sprints to 65. Even at 65 I’m only using about half throttle on flat ground but limit my rpm to 3500. At 55 mph it’s turning 2800 and has great power and seems very happy. With your 4.11 gears 55-60 should be easy for a 37. 

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I agree you need to look at other things first.  I have a 1937 Dodge D7.  When I 1st got it in June it would barely do 40mph on level ground 45 down hill, with a tail wind!  Now that I have cleaned out a lot of oil sludge out of the engine, put gas tank/cleaner (many times) cleaned the carburetor etc., it will now do 55mph all day long.  It will do 60+mph but I feel that is pushing the engine harder than it needs to.  Also the car get a bit twitchy to steer at high speeds!  Also flushed the rad and measured engine temperatures in many different places, with and infra-red sensor to make sure no hotspots.

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I saw your post and it reminded me of a similar question I had with our 32 DeSoto. How fast can it go.

If you measure the circumference of your tire in inches.

Knowing there are 5280 feet or 63360 inches per mile.

Also you need to know your final drive ratio.

With this information our DeSoto with 4.62 final drive and will red line at 63 miles per hour.

I usually drive between 45 - 50.

As an added note the speed limit in Ohio in 1932 was 45 MPH.

 

This should give you an idea of what your car is capable of speed wise.

I generated a spreadsheet for our car to see rpm per miles per hour.

It aggravates me that I cant keep up with a Model A Ford because of their 3.73 rear end.

 

Bob

 

 

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Remember that tire size is not always what it seems.  To calculate accurately you have to know the distance the wheel travels when loaded.  I found almost 4 inches difference per revolution in different makes when I was looking at 550x19 tires.  The tires measured the same when unmounted.

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I tried to copy and paste my spreadsheet from excel but it did not want to work for me.

 

So here goes with the math.

 

Redline on our engine is 3400  revolutions/minute. (75 horsepower)

4.62 final drive ratio will give me 735.93 tire revolutions per minute wide open.

92 inch tire circumference takes me 67,705.56 inches in that minute. (18 x 5.50 tires)

That is 5,642.13 feet (a shade over a mile)  1.0685 miles.

60 miles per hour is a mile a minute.

I would be going 64.12 miles an hour to cover that mile and use up all of my tire revolutions.

I will try to get the spreadsheet posted.

 

Bob

 

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A while back I wrote up some thoughts on how fast my '33 Plymouth could/should go. Some of the musing is also appropriate for other cars. It is at https://www.ply33.com/Misc/speed

 

There is a engine RPM to road speed calculator on that page.

 

14 minutes ago, Touringcuda said:

. . .

Redline on our engine is 3400  revolutions/minute. (75 horsepower)

4.62 final drive ratio will give me 735.93 tire revolutions per minute wide open.

92 inch tire circumference takes me 67,705.56 inches in that minute. (18 x 5.50 tires)

That is 5,642.13 feet (a shade over a mile)  1.0685 miles.

60 miles per hour is a mile a minute.

I would be going 64.12 miles an hour to cover that mile and use up all of my tire revolutions.

I will try to get the spreadsheet posted.

 

Bob

 

 

An 18-5.50 tire having a mounted tire diameter of 29.28 (92 inch diameter) seems reasonable. The Coker website shows tires of that size having a diameter between 29 and 30 inches. My calculator assumes 29.11 based on the generic cross section of tires of that era.

 

My calculator comes up with 64 MPH for that tire size with a 4.62 final drive at 3400 RPM.

 

But I assume your “tire revolutions per minute wide open” means with the engine at the RPM of maximum brake horsepower (BHP). The engine can probably go faster than that, it will just be putting out less power. On the other hand, that would be “over speeding” the engine. You really should keep the RPM lower than that for longer life. Knock a couple few hundred off that 3400 RPM and for 3100 RPM which works out to 58 MPH.

 

I am a little surprised that a '32 DeSoto had such a low maximum cruising speed. The '32 Plymouth also had its maximum BHP at 3400 but it had a 4.33 rear end and 18-5.25 tires. That works out to 67 MPH at max BHP and 61 MPH at 3100, slightly faster than the DeSoto. I’d have expected the DeSoto to be faster than the Plymouth not the reverse.

 

Getting back to the original poster's '37 Dodge with a 4.11 rear end. That engine has its maximum BHP at 3600 and is likely on 16-600 tires. Plugging in those numbers you get 73 MPH at max BHP and 65 MPH for long distance cruising once you back off to 3200 RPM for longer engine life.

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1 hour ago, dwollam said:

BTW, '37 Coupes had a 3.91 rear gear. That would help get ya more speed.

 

3200 RPM, 3.91 rear end and generic 600-16 tires works out to long distance cruising at 68 MPH (77 MPH at 3600 RPM at max BHP). Seems fast enough to me when driving a car with a 83 year old design for suspension and brakes. But then I live life in the slow lane.

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