Speedometer Gear Ratio Adaptor installed. Was Speedometer Cable Nut Size?

I’ve been doing research on gear reduction units to alter the indicated speed if you change rear end ratios.

No place I’ve talked to makes a ratio adaptor unit that attaches to the back of the speedometer or attaches to the angle drive. The ratio adaptor units I’ve found all use 5/8" X 18 threads of 3/4" threads. The angle drive and speedometer are about 1/2" .

The only way to change the speedometer reading, short of replacing the speedometer itself is to send your speedometer cable to a shop like http://www.speedometercablesusa.com/cables_and_housing_assembly.html . They cut the cable where you designate and install suitable connectors to put a ratio adaptor unit in line.

Hi John, Dick Maury recently gave a link to a company that made speedo ratio boxes…I just made a mental note, and didn’t book mark it.

Thanks John. Dick gave me the same link for the shop in San Antonio. I also found a shop in Bossier City Louisiana. They all do the same thing. I’ve spoken to all three at length on the phone and feel most comfortable with the Denver Store. There is also A Bob’s speedometer in Michigan that does this kind of stuff but I didn’t bother to call them.

Looking at the product they offer I believe there is probably one manufacturer of the boxes and different gears and the different stores assemble and retail what they sell. So it’s a matter of going with who you are most comfortable with.

And I think I found the thread information, looks like it’s 1/2 - 26 hex whitworth.

can you change the speedo gear in the tranny? it may be even harder to find but might be worth looking.

I am about to go from a 3.54 rear end to a 2.88. I have the car apart (1970 Series II) so I can measure stuff. The tail of the transmission has 6 teeth driving a 16 tooth. (.375 ratio).
The speedometer cable right angle drive at the transmission has a ratio of 1.2727 ( 11 input 14 output) it speeds up the transmission … The speedometer dial face has the fine print of 1312, which is the revolutions/mile. So figuring in tire size in revolutions/mile of 795 (205/75R15) we can calculate how much a speedometer ratio box needs to increase the speed. 795 x 2.88 = 2290RPM at the driveshaft, so we take this # multiply .375 x 1.272 and come up with 1092 at the speedometer. But this is too slow. We need 1312 as listed on the face. so we need a ratio box to convert 1092 to 1312 or a ratio of 1.201. See here for all the possible ratio combinations.
http://www.clarkbrothers.net/pdf/SC795_Catalog2_Page_4.pdf
and here for speedometer repair and other useful info.
http://www.tr3a.info/PDFs/speedo.pdf

I used the speed vs. RPM numbers from the owner’s manual for a 3.07 and 3.54 and came up with a table that told me if I was doing, for example, an indicated 70 the speedometer would read about 60. I did that in 10 MPH increments. Then went for a test drive with my trusty GPS. Sure enough the table I made and the GPS were pretty close. I [physically verified the indicated vs. actual speeds from 20 to 80.

I just divided 3.54by 3.058 and came up with 1.1576 which is about as accurate as I need to be since the Speedometer guy is going to choose a gear set that closest to that value and likely won’t hit that number exactly anyway.

If I divide 3.54 by 2.88 I come up with 1.229 which is very close to your 1.201 and the difference, a little over 2%, is probably within any margin of error for a mechanical speedometer.

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John,

As you know, I put a 2.88 dif in mine. After running the same rabbit trails that you are on, I sent my speedo to Morris Mintz at:

http://westvalleyinstruments.com/index.htm

Great guy! Speedo is within 1 mph of gps readout all through the range.

HTH

Just a general comment. Its not very productive to get too anal about the required accuracy of the speedo when a change in tire circumference from oe can have a distinct effect! Just saying!!

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I considered a ratio device but instead sent the speedo off for recalibration. I should have sent it to Morris but did not. Results were a total failure. Not all the claimed ‘experts’ really know how to do this.

In the end I found an NOS speedo unit for the 3.08 rear and put that in.

Ah, thanks for that resource. I’ll check with them Monday. When they reworked it did they change the ratios for both of the odometers and speedometer? I ask because I’ve been told shops doing this just change the speedometer reading, not the odometers.

True, but I’d like it to be closer than 15% off.

I believe the right angle adapter where it connects to the trans is a 5/8” connection. You could just order a new right angle drive, you would have to account for the ratio of the stock r angle drive. It is 14/11. Or 1.2727.

Mark Tangney

If you are changing from 3.54 to 3.07 , then you could also try to find the correct speedo head used, probably from the UK. Your 3.54 speedo head is marked 1321 turns per mile on the face. A 3.07 speedo is marked 1120 turns per mile on the face. The speedos seem to be calibrated to Dunlop 185vr15 SP radials. If you are still looking to change the adapter, then the speedo drive gear on the trans output shaft is a 6 tooth, and the driven pinion gear is a a 16 tooth.

Mark Tangney

John,
I just did the math based on the 185vr15. If you replace your 1.2727 right angle adapter with a 1.5 right angle adapter, that should work.
Mark

Thanks for all the responses.

I’m holding one in my hand and it appears to be just about 3/4" with about 24tpi, with an output of 1/2" 27 TPI. . I haven’t seen an angle drive with those input and output thread sizes. I’ll call the shop Monday and see if anyone makes one.

I believe this is the right unit, if they make one in 1.5

http://www.speedograph-richfield.com/html/rgb2402.html

Mark

John,
This is the calculation I am using to determine TPM(turns per mile)

Number of tire rotations per mile x rear axle ratio x transmission drive ratio x right angle drive ratio

The Pirelli Cinturato specs say it has a diameter of 26.57 inches. In practice it would be a little less because the tire squishes when it is carrying the weight of the car. You could measure the squished diameter on your car tires, mine is not together now, I cannot. A 1/8 inch squish would be a 1/4 inch reduction in the diameter, so 26.32.

So. 26.32 x 3.14159= 82.6866 rolling circumference

There are 63360 inches per mile so

63360/82.6886= 766.25 tire rotations per mile

So for the 3.07 case:

766.25 x 43/14 x 6/16 x 14/11= 1123.25 turns per mile

Which is very close to the 3.07 speedo calibration of 1120

If you do the same calculation except use 12/8 (1.5) instead of the 14/11

You get
766.25 x 43/14 x 6/16 x 12/8 = 1323.83 Very close to your speedo calibration of 1321.

Mark

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I sent them a query and they responded. They can only offer me an in-line unit in the proper ratio, same as the guys in the states I’ve already identified.

Hi John, remember this thread? I did this to first repair and then re-calibrate my speedo when I changed my diff from 3.54 to 3.07. It’s another option and not as difficult to do as you might think.
Anthony

In-line, to my mind, majes more sense than the angle drives: I fully understand the necessity for it, though.

The Rover uses a bizarre (READ: NLA) angle drive on its speedo head: gonna see if I can summon enough electrick-al know-how to make it non-mechanical.

John,
That is unfortunate that they do not have an other ratios.
http://www.speedograph-richfield.com/html/rgb1037.html Is another unit they make. It says this can be ordered with a range of ratios and they will install the fittings of your choice. I believe this would work.

Mark

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