speedometer

JFromm

Are the speedometers for a hydramatic calibrated differently from the speedometers on a stick with overdrive? I put the speedometer from a stick 1950 into a53 hornet with hydramatic and it doesn't even come close to recording the correct speed.

Thanks,

John

Jon B

John:  I'm curious:

1. did the previous speedometer on the '53 Hornet come close?
2. Have the Hydramatic and the rearend been in the car since it came from the factory?

Correct speedometer calibration depends on the rearend radio of the car.

If the rearend and Hydro transmission in your car came from the factory, all should be well. (Obviously it isn't, though!)

If the car was originally 3-speed / overdrive, for example, the rearend would have been mated to that ratio.  The speedo gears in the 3 speed / OD transmission would have matched that rearend and the speedometer would have given a correct reading from those gears.. 

However, if a previous owner yanked the 3-speed / OD and replaced if with a Hydro, he should have changed the speedo gears in the hydro to match the (3 speed / OD) ratio of your car's rearend.  Unfortunately, the Hydro's speedo gears are probably mated to the rearend that originally came paired with the Hydro (in a long-dead Hudson)..  Thus, your speedometer is giving you a false reading because  the number of RPMs of a Hydro rearend are different from the RPMs of a 3-speed / OD rearend.

Have I confused you sufficiently?

Park_W

Speedometers in Hudsons were calibrated differently starting in '51.  Earlier ones read 60 mph at 2,000rpm of the cable.  '51 and later read 60 mph with just 1,000 rpm of the cable.  So, if you put a '50 speedo in a '51, it'll read about half what it should.

Lee ODell

John

You didn't mention how great the speedometer error was. Did you have it checked at a speedometer shop? The error is probably as Park's post mentioned at half speed.

Here is the speedometer results from North Hollywood Speedometer on my 52 Hornet with hydro and correct rear gears.

Your odometer registers 100% on 100 actual miles.
Speedo reads - Actual M.P.H.
30 - 28
35 - 32
40 - 36
45 - 40
50 - 45
55 - 49
60 - 53
65 - 57
70 - 62
75 - 67

With the correct speedometer and If your speedo reads close to the above speed differences it is in the ballpark of most stepdown speedometer error.

As a child I remember on one of dad's hi speed trips in his new 51 Hornet 3 speed with O/D, I saw the speedometer was stuck at 120 MPH when he stopped. But in reality the top speed was probably around 105 MPH.

Lee O'Dell

RichardD

Best way to check is to use a GPS; it will give the actual MPH. My '53 was dead on when I checked it but I am using larger tires.

Geoff

 John, the speedos are not calibrated differently, but the drive gears in  the transmission must match up with the rear end ratio used. 

Geoff 

Park_W

Geoff, what you said is true for the particular era ('50 and earlier vs. '51 and later), but if you put a '50 speedometer in a '51 or later car, you have two choices . . . have the speedo recalibrated or tear into the transmission to change out the speedo drive gear as well as the appropriate pinion.  I'd go for recalibration.  And there may not be a '50 and earlier drive gear for a Hydramatic, since the latter didn't show up 'til '51..

Geoff

You are right, as always of course Park!   I was referring to each specific year.   However re-calibrating a speedo to bring the speed correct will  mean the odometer will still be recording wrongly.