T-56 Magnum / Speedometer Gear Question

[PDC]

T-56 Magnum with Yellow, 7-tooth Drive Gear. Car has 26” rear tire and 3.70 Rear Gear. Every online calculator tells me I need the Red 21 tooth driven gear. Got that installed and factory speedo is reading 10 mph fast.

I’m happy to try the White 23 tooth driven gear, but keep running into forums where folks have posted that the 23 tooth gear has durability issues and gets chewed up. Apparently, in order to fit 23 teeth on the driven gear and keep it the right size to fit in the trans end cable fitting, the teeth end up being very fine.



One source online suggests that the problem is running the 23 tooth Driven Gear with the 6-tooth Drive gear that was common in the T-5 transmission. The size discrepancy between the teeth on those gears is what (that source) says causes the 23 tooth Driven Gear to take a beating. Those folks end up pulling the trans to replace the 6 tooth Drive gear with the 7 tooth Drive Gear to get them into a range where the stronger 18-21 tooth Driven Gears can be used.

I’m wondering if anyone has run the White 23 tooth Driven Gear with the Yellow 7-tooth Drive Gear in the T-56 Magnum (or TKO 600) and if the 23 tooth Driven Gear held up in that application?

Thank you!

[TheJDMan]

I bypassed all that gearing to run the speedo with my T-56 install and went with a GPS based speedo system. That way I can change gears or tire size and none of it has any effect on the accuracy of the speedo. I think the GPS is actually cheaper in the long run and I have a small postage stamp sized antenna on the dash that is connected to the AutoMeter speedo gauge.

[Vimes]

You say 10MPH fast. Is it 10MPH fast at all speeds or a specific speed? A speedo gear inaccuracy generated a percentage inaccuracy, not a set MPH inaccuracy. If it's 10MPH fast at all speeds there is some other issue. If a percentage, and the gears should be the same, I'd both verify that you actually have 3.70s.. With the rear jacked up and the car in neutral, have someone rotate the rear tire one rotation while you watch the driveshaft. Put tape on the shaft to keep track of the rotations. For 3.70s the shaft should go around about 3 and 3/4 times. I doubt the tire is the issue, because a 10 percent difference in tire diameters is over 2 inches and that would stand out. A tape measure ground to tire top would show this.

[PDC]

Love the idea of GPS based speedo - and will be going with Dakota Digital on this build at some point. But they are currently backlogged 14 weeks and all gauge kits are made to order.

Correct - it not 10 mph fast across the board, just a generality. I do know these are for sure 3.70 gears in a Ford 9” from DSE. Using an online calculator, I figured what RPM matches with 50 mph in 4th gear (1:1) with the 3.7 ring gear and the 26” tire. At that rpm, my speedo says 60. If I divide the actual 50 mph by the reported 60 mph, I come up with 83.3333 - so my speedo is reading approximately 17% high at that point. If I do them math with the tach, this set up should read 2400 rpms at actual 50 mph, and when my speedo says 50, my tach says 2000. Divide 2000 by 2400 and you get the exact same 83.333 difference which points me to the same ‘approximate’ 17% discrepancy.

I did reach out to PATC - although the ‘Ford’ speedometer reducer is out of stock and they no longer carry it, they will make a custom speed reducer that fits between the speedo cable and the trans and reduces the speed by a custom calibrated %. It’s only $136.00, so that seems like the best way for me to get the speedo ‘closer’ until next season when I can take the plunge on digital gauges.

[Vimes]

Will be interested in seeing how the solution goes.

[PDC]

Got this solved - went with the Red 21 tooth driven gear. Too many reports of reliability issues with the 23 tooth gear. At that point she was about 10% fast across the board, so I ordered one of those Speedometer Reducer gear boxes that threads in between the speedo cable and the cable input on the trans. It fit perfectly and now the speedometer is dead on with my gps phone app. And that’s with a 21 tooth gear that won’t wear prematurely.