Speedometer Gear for Km?

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69cudaownr

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Don't scream my American friends, or cover your eyes if you can't bear a discussion on the metric system and don't read further.
Has anyone calculated what the gear tooth would be to get the speedometer needle to read at km rather than miles? As an example my 150 speedometer is reading higher presently because I changed my 8 3/4 gear ratio. I'm expecting my old gear will be a 30 tooth and I need a 32 tooth for the new rearend. But if I were to go less, say 28 tooth then the needle would be closer to kilometres. I'm guessing there is a mathematician out there seeing that its a known difference between the two measurements - 1 km is .62 mi.
I know this would change my odometer calculations but I'm not planning on selling my car based on "low miles" any time in the future. No one would ever know when the car is parked either. This may seem sacrilicious to some but it does make it easier for our streets.
Anyone up for the math test?
Thanks
 
Yes, I have done that in the past, what is your gear ratio and tire diameter? There is a calculator on Big Block Dart in the tech section. Just use a known speed in kilometres for example 62 mph=100 kph.
 
Just use the chart for tire diameter and gear ratio to pick your speedometer pinion gear.

The speedometer pinion gear does not care if the read result is in MPH or KPH and the selection charts to not go into MPH at all anyway.
 
You want the speedo to read in KPH? Take whatever the correct number of teeth was for MPH, and just divide by 1.609 to get the right gear for KPH. Example: 32 tooth now, change it to a 20 tooth. (Which would be great, but I believe the smallest gear they ever made had 21 teeth. Plus, has anyone ever seen a speedometer gear housing marked for smaller than 24 teeth?)
I think you'd be better off either getting a metric speedometer, or get a speedometer calibration gear box that attaches to the cable.
 
The speedometer heads are already calibrated to use the same gears.

The ONLY difference is the face of the speedometer.
 
The speedometer heads are already calibrated to use the same gears.

The ONLY difference is the face of the speedometer.
That's why I said it'd be easier to get a metric speedometer. Assuming the face is swappable, that accomplishes the same thing.
 
think you'd be better off either getting a metric speedometer, or get a speedometer calibration gear box that attaches to the cable

I agree the calibration box would be the best bet.

OR

You just memorize the key mph that are kph like 62 is 100 etc.

By the way the most accurate way to get the correct gear is what the speedo says and what a GPS says and the number of teeth on your current gear. Speedo GPS g numbers gets you a ratio, that's the ratio to apply to current teeth to determine the new teeth. Fewer teeth makes speedo read faster
 
That's why I said it'd be easier to get a metric speedometer. Assuming the face is swappable, that accomplishes the same thing.

The speedometer heads are the same only the face plate is different.

Some places sell new decals for convert to KPH, that is all it takes.
 
Thanks for your input guys. I did not want to change the faceplate on the speedometer. I was looking at the gear change only and already knew my 32 tooth gear to keep it at a MPH speedometer. I believe 69_340_GTS is correct in that I need to either divide my existing gear (32 tooth) by 1.609 or multiply by .62 . The answer is at 21 tooth which is non-existent. The lowest is 24 tooth and that won't work unless I install some whopping big tires on the rear!! Thanks again, I'll stick to the MPH and tell my son that 60 kph is NOT the same as 60 mph!

On an interesting note. While searching this stuff I found my 215/70R14 tires are equivalent to a 7.75 x 14 tire (vintage tire size 1965-72) My 69 Service Manual lists a 7.75 x 14 tire with 3:23 diff as having a 32 tooth speedometer pinion for that combination. However all of the charts on the web, and this site, suggest a 215/70R14 tire (26") with a 3:23 diff requires a 33 tooth speedometer pinion. I'm going with the Service Manual recommendation.
 
Tire sizes have never been totally standardized, so there's no way to know exactly what inch size equals which metric size tire. All you can do is get close. The speedometers are usually off too. That's why you should probably check the accuracy via GPS and adjust the speedo gear size if needed. I have always found that the perfect speedo gear is usually somewhere between two whole numbers. So you learn to live with it.

Oh and here is something I found interesting: if you are running a normal size tire, 25"-26", the speedo gear is usually going to be the same numerically as the rear end, i.e. 32 tooth for 3.23, 36 for 3.55, 41 for 4.10, etc...
 
Ha, I had noticed that pattern on gearing to tooth count. It was more obvious to me when reading the Service Manual and comparing their chart.
 
Plymouth 1969 Service Manual Page 98 in Section 21.
Speedometer Gear Chart.jpg
 
That's a great chart. Just have to translate tire size to p-metric


What I like about the tooth count and GPS / speedo method is it removes the tire / gear ratio variables from the equation.
 
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isn't there a box that you install between the speedo cable and the speedometer that has its own little gears to correct what it reads?

A friend has had a km speedometer for a rallye dash if you are interested send me a pm and I'll make the connection
 
isn't there a box that you install between the speedo cable and the speedometer that has its own little gears to correct what it reads?
Yes, it is called a speedometer calibration gear box. Do a Google search. There are a bunch of people selling/building them. No idea if one vendor/unit is better than another.
 
Any calibration gear box I’ve seen is for an electronic speedometer not mechanical.
The ones I easily found on-line mount in-line between the (mechanical) speedometer cable and the (mechanical) speedometer. You tell the vendor what ratio you need and they build it.
SpeedoGearBox-Lg.jpg
 
Those gear boxes have been around forever, ever since the first speeding ticket was issued. And owner claims he was driving speed limit.
If i dig around, there may be one or two of them in my archives..
 
That's a great chart. Just have to translate tire size to p-metric


What I like about the tooth count and GPS / speedo method is it removes the tire / gear ratio variables from the equation.
Go to the Coker Tire web site. They have all the tire dimensions listed there.
 
What vehicle?
Rally dash?
Dart dash?

I took a 67 barracuda rally dash and put a kph decal on top of the mph.

Premium Dash Decals

Solved all math and have had comments about the dash going to 200 mph... then they realize kph ... then think it's a car that came from Europe.
 
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