Odd speedo questions

[builtfercomfort]

Does anyone know the number of turns a speedometer makes to go one mile? (Specifically for a 39 Chrysler C-22, but it's probably the same for the D-24 as well) I think they standardized on 1000 turns/mile at some point but I don't know if this is true, or when it happened.

Second, is the nut on the back of the speedo a standard item? Would it match, say, a 60's Mustang cable? (pictures at link below)

Reason for these questions: I've got an AOD transmission and I'm trying to figure out the number of teeth needed in an AOD transmission to drive my stock speedo - with a 4.10 rear end and 16" wheels. They make 16 to 23 tooth gears. (I'm looking at the #7300 cable from PATC's Mustang to AOD page so that I would have a 'speed sense' signal for a later cruise control.)

(Odd enough questions? :-) )

[Normspeed]

Don't know the answers but I'd sure like to know what they are. All the cable assemblies always looked pretty similar to me except for length but I don't recall swapping any.

[greg g]

You can chuck up your cable in your drill, run it a mile and time it then divide the time by the drill rpm. to figure turns per mile. YOu might need to run the drill in reverse as it might take a mile off instead of putting one on. The drill rpm should be on the drills data plate.

[Don Coatney]

You can chuck up your cable in your drill, run it a mile and time it then divide the time by the drill rpm. to figure turns per mile. YOu might need to run the drill in reverse as it might take a mile off instead of putting one on. The drill rpm should be on the drills data plate.

I started that test last year but after 2 hours of running I ran out of extension cord and I was not yet to the end of my driveway:rolleyes: Also my drill motor is variable speed. How can I do the math with a variable speed drill?

[martybose]

If I remember right, the standard was 1000 revs/mile. When they calibrated my car, they used a gearbox to get the count down to 1000/mile, then used a 1000 RPM bench source to calibrate my speedo head to 60 MPH.

Marty

[Norm's Coupe]

I started that test last year but after 2 hours of running I ran out of extension cord and I was not yet to the end of my driveway:rolleyes: Also my drill motor is variable speed. How can I do the math with a variable speed drill?

:D Good question Don as I would think a 3/8th inch drill would make the speedo spin too fast over 120 MPH. However, if they were to use a 1/2 inch drill, they only run about 400 to 600 RPM's and lock it on full blast. Myself, I would just ask a speedo shop, or take it to one.

[TodFitch]

Back in 1978 I had the speedometer in my 1933 repaired and I ask when I got it back if the fellow could calibrate it. He said he already had. I asked how he knew how many turns per mile. He said all American cars used 1000 turns per mile.

I later sat down and figured out the turns based on the manual's speedometer gear selection table, my rear end ratio, and mounted tire diameter and found that, at least for my 1933 Plymouth, he was correct.

[builtfercomfort]

Great, thanks, so I'm going to assume if it was 1000/mile in 1933, they didn't change it by 1939. That just leaves the back-of-the-speedo nut question, and that I can hopefully figure out before spending money on the wrong cable.