JOHNWSMITH

Hey C3-ers... I have an 81 with an 85mph speedometer,

THE QUESTION...
Do any of you HAVE info or KNOW OF info for calibration after I swap the face to a 140mph...
(140 Faces are $30 at ZipCorvette)

I do not want to ship mine off and have somebody do a conversion.
I can handle it, its only gears and stuff.... anyway, info is what I'm after at this point...

THANKS!!

Eliredandblack

Why not download the free app that gives you the exact speed you are moving while driving. Then you can go to a vendor website (they sell the tranny gears) that helps you choose the closest transmission speedometer gear. By the way, my 73 SB with manual transmission is off around 5 or 6 mph, and I had the speedo / tach professionally rebuilt. Even though everything on my car is stock, you have to remember that my car (and it’s innards) are 50 years old. If you need the exact mph, a new Tesla or BMW would be exact.

calwldlife

the little I know about speedometers is a gear and a magnetized system to calibrate
the readings.

an electric, and gauges shop can fix you up.
if they exist anymore?

Usual swap from 85 to 140 is the whole speedometer.

stingr69

Maybe a used '79 speedo and a '79 transmission gear? Seems too easy???

sunflower 1972

Check with this company: https://deluxespeedometer.com/services.html

JOHNWSMITH

$500

JOHNWSMITH

I have noticed that even GPS speedometers are not entirely accurate, depends on the accuracy of the software...

JOHNWSMITH

whole speedo is a $500 swap now days!

calwldlife

not the point I tried to make.
your 85 speedo needs to be calibrated.
only a shop with proper equipment can do it.
it is more than a gear change.
been to one to calibrate a 68 speedo that wasn't accurate.
good luck

JOHNWSMITH

actually my 85 speedo is fine now... if I swap the face, it will need to be recalibrated... that is what my entire question is about...

7t9l82

Art carr at California performance transmission sells a device that they build to correct your speed.
Rogers made me a 160 speedometer.

MelWff

It has to be sent to a shop that calibrates instruments, it's not a DIY.

7t9l82

I unstood that . With an absence of speedometer shops art carr can make it a simple task.

pgtr

I agree - this is how I did it on a non-Corvette years ago - I swapped in a higher mph speedo from an earlier (or was it later?) model year and it worked fine. (FWIW by definition you are giving up some display accuracy but I suspect that's not an issue for most folks )

Two things I'll mention that are somewhat related - this is regarding an early C3 tach - which is essentially the same as an early C3 speedo exc for the odo - and I suspect similar to an '81 speedo...?

A) I would expect a mechanical speedometer should have some sort of (delicate) calibrated spring counter to the cable rotation to which it is coupled magnetically. Theoretically that spring could be tweaked to move the needle up or down a little (calibration). I partially cracked open my '71 mech tach once upon a time. As best I recall inside the unit towards the back there was what looked like a plate that rotated and 'might' have been a spring adjustment...? But I could be totally wrong about that. Anyway I did not touch it!

B) I did confirm the function of my mechanical tach, after it had been out for an extended period, before reinstalling. I did this by hooking it up to a drill with a spare cable and spinning the drill at a known RPM. I verified the drill RPM with one of those optical RPM meters and a reflective sticker on the chuck. The tach appeared to be indicating roughly double (or was it half?) the RPMs that were showing on the RPM meter so I figured it to still be as accurate as one might expect from such a unit and left it at that. I suppose one could do the same with a mech speedo which displays MPH based on rotations (you'd have to do the calcs of what MPH should be displayed based on your differential ratio and tire diameter).

I'm not sure one can recalibrate a speedo all the way down from 85 to 140. I probably wouldn't attempt this without more definitive how to info or a plan B.


OK I'm curious - how do you verify if the GPS speedometer on a phone app is not entirely accurate?

JOHNWSMITH

[QUOTE=pgtr;1606917532]I agree - this is how I did it on a non-Corvette years ago - I swapped in a higher mph speedo from an earlier (or was it later?) model year and it worked fine. (FWIW by definition you are giving up some display accuracy but I suspect that's not an issue for most folks )

Interesting... I had two phones with two different apps, that did not match each other. I also had a dedicated GPS speedo that didnt match either. I also checked it with a Garmin... they all are within 3-4mph but definitely not "bang-on" like you would think they would be!