[TR] HVDA 5-speed in TR6--speedo gear

[Tim Gaines]

I have found several possible solutions in my quest to calibrate my TR6 speedometer after making the HVDA Toyota tranny conversion.  Thanks go out to several members of this list and several on the Triumph Experience forum.  Here is a summary of those solutions.

1.  Keep using a GPS or a phone with a GPS-speedometer app and forget about the actual gauge on the dash.

2.  Use a GPS to calibrate the speedo and mark new lines on the gauge face or glass with a Sharpie.

3.   Classic Automotive Innovations has an ingenious solution.  A gps controls a motor that spins the speedo cable (disconnected from the tranny) at just the right (adjustable) speed.  The current price is $275.

4.  As Herman van den Akker recommends, send your speedometer to a shop that will replace gears for the speed and odometer mechanisms that will accurately recalibrate the gauge.  This requires some accurate measures on your part of the number of turns of your present cable per 1/100 mile.  I just got a quote from Palo Alto Speedos for $295 to do that calibration.

5.  Buy a speedo ratio adapter for $60 - $120 that attaches to the tranny between the speedo gear and the cable and adjusts the cable turns per mile (TPMs) to match speedometer calibration.  Different speedo gauges require a different number of TPMs.  Whereas most American gauges are standardized at 1000 TPMs, mine is 1120, and I have heard of another TR6 one being 1180.  Maybe there are even more values out there.  The advantage of this solution is that there are right angle ratio adapters that will help alleviate the problem of the tight turn in the speedo cable from the Toyota tranny to the TR gauge.

6.  An in-line ratio adapter that connects via cables to both the tranny and the gauge.  It does what the above device does.

7.  Find a replacement speedo driven gear with the proper number of teeth to change the ratio to match the speedo gauge.  The new driven gear must be matched to the number of teeth in the drive gear.  From what I have read, that number is 10 or 11 in the W58 Toyota tranny that is often used for the HVDA conversion and the value should be stamped on the shaft of the current driven gear.  The driven gear can be removed fairly easily from under the car by removing a single bolt that holds the gear assembly in place with a small metal tab.  I have found only one online site that lists driven gears with details about the drive- and driven-gear teeth.  I have emailed to ask if all of the Toyota driven gears will fit the W58 transmission, but I haven't heard back yet.  I would think that there is a standard for the speedo gear assembly across tranny models. 

http://parts.lakelandtoyota.com/showAssembly.aspx?ukey_assembly=519949&ukey_make=1060

There are some calculations that must be made to find the right gear, and there is a helpful site that guides you through it.  Keep in mind that this guide assumes (without saying so) that the speedo gauge uses the US standard 1000 TPMs.  At the stage where the guide says to divide by 1000 (or 1001), you divide by your own gauge value.  That can be found at the bottom of the gauge face, partially hidden by the chrome rim around the gauge.

http://www.celicasupra.com/forums/showthread.php?67982-The-end-to-speedometer-gear-questions

I have not been able to find the precise driven gear I need, but I did find one that should get me much closer to an accurate readout.  The price of the driven gears is about $25 - $35.

Art Liefke sent me the url for a site that explains in detail how the British speedometers work and how to calibrate them.  I have only briefly skimmed it at this point, but it looks to be a good explanation of all the things (and more) I pieced together in several days of looking and reading online.

http://www.westnet.com/~mfrank/Speedometer/Smiths.html

Tim Gaines
Clinton, SC
1974 TR6
1980 Spitfire





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