65 MPH Speedo - interchangeable with Mustang?

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kevinshi
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65 MPH Speedo - interchangeable with Mustang?

Post by kevinshi »

Can I use a 65 Mustang Speedometer in my 65 Comet (or do I need to change out the gearing at the tranny to make it accurate)?

The Comet has part number C5GY-17255 - 120MPH faceplate.

The Mustang had (at least) two...
120 MPH: C5GF-17265
140 MPH: C5ZF-17265

Does anyone have experience using these in a 65 Comet?

Thanks,
kevin
Thanks,
kevin
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poboyjo65
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Re: 65 MPH Speedo - interchangeable with Mustang?

Post by poboyjo65 »

it'll work. I've heard of guys using them .also heard of using the complete gauge cluster from a stang with just a tad of cutting or grinding to make it fit the comet bezel.
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Johno
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kevinshi
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Re: 65 MPH Speedo - interchangeable with Mustang?

Post by kevinshi »

My question was somewhat poorly formed.

What I meant to ask is: Can I expect these to function accurately (at least as accur6 as the original)?

I agree with you - seems like the answer must be "yes."

The folks at Bel-Kirk Mustang (just down the road from me in Bellevue, WA) seem to concur. They report that of you swap a 120 <> 140 unit in a 65 Stang you do not have to change anything else for it to function properly. I was worried the number of teeth in the tranny's speedo gearing might be different (such that the cable RPMs changes at a given speed).

This must mean that the speedos all expect the same cable RPM for a given speed...

To wit: 70mph on a 140 will position the needle at noon, whereas 70mph on a 120 will position the needle at 1PM. The only difference is the internals of the speedo itself (which must be calibrated slightly differently).
Thanks,
kevin
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Jims65cyclone
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Re: 65 MPH Speedo - interchangeable with Mustang?

Post by Jims65cyclone »

I swapped a 120 for a 140 in my first Cyclone many years ago. I made no changes to the gearing, and the new speedo still read accurately. I just assumed that the spring in the 140 speedo head was calibrated a little stiffer than the 120 so that a given rpm of the cable didn't rotate the drum as far around as it did in the 120. Based on my experience, I think you'll be fine.

Jim
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kevinshi
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Re: 65 MPH Speedo - interchangeable with Mustang?

Post by kevinshi »

Oh geesh, of course you have already done this!!!

:D

I will proceed with confidence!
Thanks,
kevin
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Re: 65 MPH Speedo - interchangeable with Mustang?

Post by comethead »

I think the speedo numbers font is different between a Mustang and Comet. The face can be swapped though.

Joe
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kevinshi
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Re: 65 MPH Speedo - interchangeable with Mustang?

Post by kevinshi »

Yea but that is kind of my point. If you simply swap the faceplate then the pointer will be inaccurate. (Not sure that is actually what you are implying... if not then apologies).

Imagine your car has a 120MPH max faceplate. Now you are in your car driving a constant 20 miles an hour. While maintaining that speed you swap the faceplate to 140MPH. Suddenly your speedo will report you are driving ~25 miles an hour (about +5mph) - even though your actual car speed has not changed at all.

The speedo needle is "fixed" according to the speed of the vehicle. Thus if you swap the faceplate you are simply "labeling" that "fixed" speed with a different value.

Hence my question. I am trying to confirm whether the speedo itself is calibrated to "adjust" so that the "fixed" speed will report correctly.

To put is another way: Somewhere there must be a different mechanical element that maps to the use of a 140 (vs. 120) speedometer. I cannot be the case that you simply swap the faceplate (that will not work, by definition). So either (a) the speedo itself is calibrated differently or (b) the speedo gearing in the tranny is diffident and must be swapped when the speedometer gauge is swapped.

Based on the above replies and resources like this Mustang Speedometer Gear-Calculator I think the answer is most likely (a) - the internals of the speedometer are different. I just spoke to the folks who produced that video and they are saying this is correctly - the internals of these speedometers are different (so swapping the faceplate will not generate an accurate result).
Thanks,
kevin
1965 Caliente Convertible
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