Ok, just had the diff in my US spec car changed from 3.54 to 3.07. Money well spent! Obviously, now the speedometer reads incorrect. I am assuming the way to fix this is to get a new angle drive for a European car. Does anyone have a parts #?
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Hi Dennis…no…you need to get your speedo recalibrated…Steve
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Good discussion here:
This is interesting; I have an S1 XJ with 3.31 diff and the speedo says 960 TPM. A nearly identical unit from a 420 with 3.54 diff says 1020. I stripped the 420 unit to see how it works and there is a worm gear on the main input which drives two nylon gears to the odo and tripmeter. I assume that any change in gear ratio ( from the factory) is accounted for in those gears to make the odo read correctly. I assume, but don’t know, that the rotating magnet poles sizes are altered for speed calibration as there is no internal reduction in the speedo head. The magnet is square shaped with cutaways, and I assume the size of the cutaway affects how much deflection the needle gets. Anyone confirm or explain how they work? Calibrating a speed accurately just by tweaking the return spring sounds a bit hit and miss to me.
Yes, the odo is a mechanical change done with the gears. As I understand it the speedometer gets adjusted by either tweaking the spring or changing the strength of the magnets (more magnets to increase strength, degaussing to weaken).
The shop I used in NY was incapable of doing this (they finally gave up working on speedos altogether) but I expect Morris Mintz at West Valley Instruments in Reseda can handle it.
An adapter may be another way to go.
I took the easy way out and sourced an NOS speedometer for a 3.07.
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Dennis,
who did the conversion and what was the final cost, if you don’t mind
Sports Car Center in Phoenix, AZ. Around 4 G’s ! Seemed a little expensive to me when they quoted me the price, but when you consider a 5 speed conversion would be around 14 G’s, its a great alternative.