Speedometer Off 14mph

clr0gers

New Member
Jun 15, 2012
19
0
2
Evansville, WI
I'm using a T5 out of a 1989 Mustang GT in my hot rod (1946 Ford pickup with 351W). Since I'm using Dolphin mechanical gauges I had to have a custom speedo cable built for it but when I was finally able to get it out on the road I found out the if I'm doing 58 mph I'm actually doing 70 mph & in order to do 60 mph I have to go about 48-49 mph. Figuring all this out with gear ratio, tire diameter & all I would need a 14 tooth driven gear but nobody makes one. The drive gear is yellow (7 tooth, the 6 tooth I believe is black), I've checked it several times & included that info in with the order for the cable. Now the cable manufacturer says I need a 7 or 8 tooth drive gear or a $200 to $300 ratio adapter. Has anybody had a similar experience or know of a cure other than tearing the trans apart?
Thanks

chuck
 
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See http://www.corral.net/tech/gearcalc.html for more help

Check to see if there is a speedometer shop in your area. If there is, ask about the Stewart-Warner speedometer gearbox. I believe it is either a 777 Series Drive Joint Kits and Parts or 666 Series Drive Joint Kits and Parts. It is a small gear box that fits between the speedo pickup gear on the transmission and the speedometer. It has quick change gears that allow you to choose almost any tire size and rear end gear ratio you want. This will allow you to get the accuracy within 1%-3%. The drawback is that it isn't cheap. Cost is $95-$120 from www.gaugeguys.com

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See http://www.gaugeguys.com/ratio-adapters.html for more info.

See http://www.davidsonsales.com/docs_pdf/Stewart_Warner_HD_Catalog.pdf for more information on the Stewart-Warner speedometer gearboxes.
 
You should be able to quickly figure this out. You need to know how many teeth you now have on the driven gear. The change will be in direct proportion to the actual speed to speedometer reading. For example the ratio between actual is 70/58 = 1.20. If the existing driven gear is 20 teeth you will need a 20/1.2 = 16.6 (17) tooth driven gear
 
Thanks for all the replies. Let me fill you in on the particulars. The T5 has a yellow drive gear which should be a 7 tooth gear. The rear end had 3.50 gears (verified by turning a tire with the other on the ground & chalking a line on the drive shaft), the tires are Michelin LTX AT P235/75R15 with a dia of 28.9" & the driven gear of 19 teeth that Speedometer Service Company in Milwaukee, WI included when they made my cable. If I figured correctly with the gear calculator online the driven gear should be 17 tooth. But if you figure that each tooth is about 2.5 mph I should have a 14 tooth gear. I still have to count the teeth on the drive gear to verify it is 7 teeth but I remember seeing a yellow gear in there when I looked so unless I've missed something major I'm at a loss. I am going to try the lowest tooth gear in it, which is 16 teeth, to see just where things are & go from there. I'm hoping not to have to resort to one of those ratio adapter units do to the cost but if I have to it's probably cheaper than a speeding ticket.
Thanks guys

chuck
 
You are off by 12 mph? I did some number crunching and came up with a 16 tooth. Could be left field but here goes. Actual speed 70, shown speed 58. 58/70 comes to 0.828. Which means there is a 17.2% difference. 19 tooth gear (current) divided by the 17.2% difference (19/1.172) comes to 16.2. So, unless my thinking here is in left field, you should need a 16 tooth. That same math suggests that you have either a 3.27 or 3.31 rear gear.
 
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I did verify that the rear end gear is 3.50 by turning 1 tire 2 revolutions with the other on the ground & chalking a line on the drive shaft, it went around exactly 3.50 times.I do agree with trying the 16 tooth gear as soon as my local Ford dealer opens up Mon., I also want to get a 185 degree t-stat to replace the 195 degree I have in there now The hot rod won't be driven much in the winter & with a carb on it instead of fuel injection 185 is recommended. Here's a couple of photos, as you can see it is far from done but fun to drive (needs a posi for 9" Ford rear end, maybe next year, no funds for now).
Thanks

chuck