Auburn Engine RPMs In High Range

  • sds1861
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12 Jun 2009 14:14 #13741 by sds1861
sds1861 created the topic: Auburn Engine RPMs In High Range
Many of you are probably already aware of this information, but I thought I would pass it along for those who are not.

My 1932 Auburn does not have a factory tachometer so I recently purchased an N.O.S. electronic Sun Brand unit for temporary use in tune ups and setting idle speed etc. I mounted the sending unit and the tach head on a board that will sit in the passenger floorboard and can be read from the driver's seat.

I had always wondered what the engine RPMs of my car would be on the highway in high range, so after a tune up last week I left the tachometer hooked up and hit the Interstate. Here are the results with 7.00 x 17" tires on the car. I am impressed.
55 MPH = 2,000 RPMs
60 MPH = 2,200 RPMs (Rounded Up)
65 MPH = 2,400 RPMs (Rounded Up)
70 MPH = 2,600 RPMs (Rounded Up)

These results indicate a "high range" gear ratio just about the same as most modern cars, in overdrive. The engine is not laboring at all at highway speeds. What an innovation for 1932. "Dual Ratio" the best of both worlds.

Kindest regards,
Steve Stevens
Evansville, IN

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  • mikespeed35
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13 Jun 2009 00:48 #13744 by mikespeed35
mikespeed35 replied the topic:
This is all predicated on the rear end ratio. There are several for dual ratio rears. So other cars with dual ratio might not have the same results. I don't want any one over reving their engines at 60 mph thinking they are running at 2,200 RPM's. Also I have found that both my L-29 and my 35 Auburn say 60MPH on the speedometer when the Garmin says 55MPH.
CORDially Mike

Mike Huffman

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  • sds1861
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15 Jun 2009 14:06 #13760 by sds1861
sds1861 replied the topic:
Mike:
I am only a student of the 1932 Auburn models because that is the car that I own. The factory liturature for 1932 shows high range ratios of 3.4 to 1 for the Eight (this is the ratio that my car has by mathmatical confirmation using wheel circumfrance) and 3.0 to 1 for the Twelve. What were the "high range" ratios that Auburn offered in their two speeds for 1933 -1936? Were any lower than 3.4 to 1?

Your speedometers both being off five miles per hour is odd. The factory would not have produced something that far out of calibration. They would, however, have needed to have different speedometer calibrations available for each different rear end ratio. People using replacement rear ends or speedometers out of parts cars during a resoration could easily cause a mismatch to happen in this area resulting in an inaccurate speedometer. As we all know, tire size can affect speedometer accuracy also. I plan to check my speedometer on the Interstate by following my wife's 2009 Lexus at 60 MPH. I'll report the results when I have them.

Thanks for your comments. This is an interesting area of discussion. It is good advice for anyone who drives his or her antique or classic car at highway speeds to check the accuracy of the speedometer as well as hooking up a temporary tachometer to check engine RPMs. Just check it at any given speed in high gear / high range and then the RPMs can be calculated mathmatically at all other speeds.

Kindest regards,
Steve Stevens

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  • Josh Malks
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15 Jun 2009 14:16 #13761 by Josh Malks
Josh Malks replied the topic:
I don't believe that a speedometer that's 5mph off at 60 is unusual. I have no data on the 1930s, but in the 1950s this was a normal variance on American car speedometers. ('Course, how well could you read some of those bizarre dials of the 50s anyway?)

The only good answer is a GPS check. Bill Hummel explained how to do it in ACD Newsletter 2005-1, page 10.

Josh B. Malks
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  • mikespeed35
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16 Jun 2009 03:59 #13768 by mikespeed35
mikespeed35 replied the topic:
Hi Steve, You are right that tire size does effect speed. I went from 6.50x16 to 7.00x16 and according to calculations I gained about 3 MPH. This of course would not be recorded by the speedometer. You will find that the speedometer is run of the axle shaft thus rear end gear changes will not effect speedometer readings. As far as the ratios are concerned every differential carrier has the ratio stamped on it. These ratios are given in low range. The ratio that is stamped on my car is 43+10. This translates to a low ratio of 4.30. But this does not tell the high ratio. This is determined by the planetary gear set. I have been told by Stan that there were different planetary gear sets, thus effecting the high ratio.
If you are coming to the Spring Warm Up Meet in Auburn this week end you can use my Garmin to test your cars speedometer accuracy.
CORDially Mike

Mike Huffman

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  • sds1861
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16 Jun 2009 13:21 #13769 by sds1861
sds1861 replied the topic:
Mike:
I must correct you on one of your previous statements. 1932 Auburn speedometers are not geared off the axle shaft. They are geared off the transmission output shaft. Every other rear wheel drive car I have ever worked on was also. Therefor, the rearend ratio does affect the speedometer. This is why the Auburns with dual ratio have a transmission attached to the back of the speedometer that is actuated by the dual ratio dash lever and changes the speedometer gearing to keep it accurate when the ratio is switched back and forth.

The "high range" ratios that I mentioned in my previous post came from the 1932 Auburn dealer specifications liturature. If someone has this liturature for the 1933 - 1936 models, please post the "high range" ratios for us to review.

I have no plans to come to Auburn this year. I enjoy driving and tinkering with my car, but haven't attended the ACD club meets. Other interests have always taken higher priority. I will check my speedometer with our Lexus (I'm sure the Japanese calibrated it with GPS) in a couple of weeks, when I get back from a trip to Montana, and report the results. I'll also get a second reading using mile markers and a stop watch (old school, but very accurate).

regards,
Steve

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