Speedometer KPH to MPH

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Apr 19, 2018 | 09:02 PM

Jaye71

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Speedometer KPH to MPH

Hi everyone,

I'm getting ready to purchase an 81 C3. It's a Canadian car and has a KPH speedometer. What's the easiest way to convert it to MPH? Would the vinyl decal overlays work for this situation? Thanks!

Apr 19, 2018 | 09:03 PM

Jaye71

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Speedometer KPH to MPH

Apr 19, 2018 | 10:02 PM

7T1vette

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From: Crossville TN

KPH x 1.6 = MPH

Or, you can just tell the Trooper the car is new to you and you thought the speedo was in KPH...

Apr 19, 2018 | 10:09 PM

Jaye71

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Thanks. Lol. Hoping for something a little more permanent.

Apr 19, 2018 | 10:30 PM

Toms 80

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From: London Ontario

The speedo output on the side of the transmission has interchangeable gears inside to allow for different gear ratios. There are also aftermarket inline speedo cable gear reducers that accomplish the same thing. A combination of the two could get your metric dash to read MPH. IF you have a 3.07 rear gear ratio (my 80 did originally), then then 3.07 x 1.609 (kilometers to miles conversion) would require the equivalent speedo gears as a car with a 4.94 rear gear. An old school transmission mechanic should be able to help you out.

Apr 19, 2018 | 10:40 PM

riverracer au

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From: "ɹǝpunuʍop", Adelaide, South Australia

if distances traveled are correct, correct tyres etc, try one of these

https://transmissioncenter.net/shop/...ratio-adapter/

i used the same style unit, but went from to MPH to KPH..

Apr 19, 2018 | 10:59 PM

Grumpy 427

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From: Regina Sk Canada

Redline gauges made me a new overlay and calibration. Mine is is both MPH and KPH I also have a 150MPH speedo.

Apr 20, 2018 | 12:16 AM

Toms 80

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From: London Ontario

Quote:

Originally Posted by riverracer au

if distances traveled are correct, correct tyres etc, try one of these

https://transmissioncenter.net/shop/...ratio-adapter/

i used the same style unit, but went from to MPH to KPH..

That's precisely the type of reducer I was speaking of.

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Apr 20, 2018 | 12:18 AM

CanadaGrant

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From: BC

Quote:

Originally Posted by Jaye71

Nothing is calibrated or made differently except the markings on the Canadian kph speedo head but the odometer is. Find a US mph speedo head and that will do it. The gears, cable, and everything else are the same. I have done this a number of times on C3's in reverse (see my name) and you don't have to worry about gearing or anything else as there is no compensation for that in the head. Just get a US head. There isn't a big market for C3 speedometers so the cost won't be much and the work involved will be less than trying to remove one, take it apart and use a sticker. The reason to change the unit instead of the face plate or using a sticker is that the odometer will also be changed from KPH to MPH. Might as well do it right for no extra cost.

Last edited by CanadaGrant; Apr 20, 2018 at 12:26 AM.

Apr 20, 2018 | 06:46 AM

#10

c3_dk

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From: Aarhus, Denmark

But km/h is the right way to go :- )

Apr 20, 2018 | 11:51 AM

#11

vince vette 2

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From: PA

Quote:

Originally Posted by Jaye71

One way is a speedometer gear reducer from Stewart Warner. You'll find lots of them on ebay for $75 to $90. You might go elsewhere to find them for less. Ideally the reduction would be 38%. Thus 100 mph would appear as 62 mph which is 100 kph. Thus the kph scale would be reading in mph. Note, this also reduces your indicated mileage (or kilometerage, I guess) by 38%.

If you go this route you should use the offset style reducer (see pic). There is very little space between the speedo gear housing in the the tranny and the driver side "bucket" of the floorboard. The reducer mounts between the speedo gear housing and the speedo cable. If you try to put the straight through reducer style in that small space I guarantee that the bend it will force in the speedo cable will snap it in about 1000 miles. I know of what I speak.

The offset style reducer will hopefully drop the speedo cable attach point low enough to either get past the bottom of the floorboard or reduce the necessary angle to something tolerable. But I never tested that.

Last edited by vince vette 2; Apr 20, 2018 at 11:53 AM.

Apr 20, 2018 | 12:23 PM

#12

drwet

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From: Thunder Bay

I suspect you could just replace the speedometer. Not sure what's available. If you contact Corvette Clocks by Roger, they can probably help you out with a replacement speedo, or rebuild yours as an MPH unit.

Apr 20, 2018 | 12:35 PM

#13

drwet

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From: Thunder Bay

Quote:

Originally Posted by c3_dk

But km/h is the right way to go :- )

The US tried to go metric once. I think it was Jimmy Carter's idea. Lasted about a week as I recall. Just too hard.

Apr 20, 2018 | 12:41 PM

#14

vince vette 2

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From: PA

Quote:

Originally Posted by Jaye71

Why are there two threads with this question?

Apr 20, 2018 | 12:43 PM

#15

Jaye71

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It was my fault. I created the thread on my phone and accidentally duplicated it. Not sure how to delete or close the thread. If there are any mods reading please close this thread.

Apr 20, 2018 | 12:46 PM

#16

vettebuyer6369

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From: About 1100 miles from where I call home.

Multiple threads have been merged.

Apr 20, 2018 | 12:52 PM

#17

Jaye71

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Thank you

Apr 20, 2018 | 12:57 PM

#18

c3_dk

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From: Aarhus, Denmark

Quote:

Originally Posted by drwet

The US tried to go metric once. I think it was Jimmy Carter's idea. Lasted about a week as I recall. Just too hard.

But some new US car are metric today, right?

Apr 20, 2018 | 12:59 PM

#19

vince vette 2

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From: PA

Quote:

Originally Posted by drwet

The US tried to go metric once. I think it was Jimmy Carter's idea. Lasted about a week as I recall. Just too hard.

They managed to go partially metric - just work on an 80 vette and you'll find out. For example, last nigh I was using a 10 mm socket to turn the bolt which holds the tab that holds the speedo bullet in the transmission. On the rear suspension I find some parts with metric and others that are imperial. The ultimate was the power antenna. The mounting bolt has an imperial head while the nut or metric - or vice versa - I don't remember. But I do remember I grabbed an imperial socket to put on one end and found I needed metric. Got that and then grabbed a metric for the other end and found I needed imperial. I didn't bother to figure out what the thread was.

I'll say this, the conversion in the US was never avoided because it was too hard. The military did it. And businesses overwhelmingly support it. The only people it is too hard for is the spineless politicians who have abdicated their intended role elected LEADERS in favor of continuous reelection through the role of elected FOLLOWERS. And thus, as has been said, people get the government they deserve.

Last edited by vince vette 2; Apr 20, 2018 at 01:02 PM.

Apr 20, 2018 | 01:37 PM

#20

Bikespace

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From: Virginia

I'll second the 80 as being a weird mix of metric and SAE.

What is the top speed of the kph speedo? 135 or 140 kph? That should be a direct swap for the 85 mph speedo. Just buy a speedo, or complete dash, off ebay. Easy! Wilcox has instructions for the swap.

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