Hello all. Happy British Car Week!!!!
Questions follow this lengthy, and somewhat understandable explanation.
There have been a few questions about TR6 speedos lately, but none seem to
have applied to my situation. Prior to rebuilding my '76 TR6, my speedo
read too high for my speed. I then replaced my transmission with a J-type
OD tranny, installed the required angle drive, and called it good. My
speedo still read too high. "It's my speedo's fault, of course." Sounds
reasonable to me. After all, it read wrong in both cases, and the
differential is the proper differential ratio for my speedo.
I've gone on that assumption for a year.
Due to an upcoming TSD ralley, I recently took the opportunity to run into a
local Speedometer shop in Denver to have them go through the speedo to
recalibrate it and clean it up while they were at it. Upon arriving, the
owner placed my late model speedo on his diagnostic machine and says it is
accurate to within one mph at 30 mph and two mph at 60 mph.
Now what???
Well, now my assumption has to be that speedometer drive gears in the tranny
must be the problem. The non-OD tranny that I replaced was not the original
tranny - it was from a much earlier TR6 based on the tranny's "commission"
number. The J-type tranny that I have since put in my car is from very
close to my '76 TR6's build date. It came from Canada.
After laboring over the catalogues, I've realized that there are two
versions of the driving gear inside the OD; one an 8 tooth gear from early
in the J-type run, and a 7 tooth driving gear that began after commission
number CF34000. I'm not in a hurry to take the OD apart to check on the
gear tooth-count. I've also realized that the OD tranny's only have one
driven gear version throughout the J-type's run - unlike the non-OD tranny
that used different driven gears to drive the speedo cable at different
speeds.
Based on my knowledge of bicycles, a larger driving gear will drive the
driven gear faster than a smaller driving gear - thus causing a higher than
actual speedo reading. I'm betting that my J-type has the 8-tooth driving
gear, and not the proper 7-tooth driving gear for my '6's speedo.
QUESTIONS: 1) Does anyone know what "higher" reading would come about from
the larger toothed driving gear? 2) Anyone know if a speedo shop that is
very knowledgeable about TR6s would be able to build an adaptor box without
seeing the car? I've head that Quantum Mechanics (Espisito's shop) is very
knowledgeable. Any other shops I should check out?
Thanks a bunch,
Eric Conrad
'76 TR6 (CF52678U +O)
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