The car that I converted from an automatic to a manual is having some speedometer issues.

As per all instructions, opinions and plain common sense...I swapped the 4.1 (gray) calibrated gear out of the S5 N/A gearboxe I used and replaced it with the '88 (S4, orange-ish) calibrated gear so that my speedometer would read correctly. This is because I have retained the 3.909 LSD rear diff.

So, the car is an '88
3.909 LSD rear
S5 N/A gearbox (0.697 5th gear, IIRC)
3.909 orange-ish calibrated speedo gear
'vert wheels, correct size tires

Yet when I do 73 on my speedo, it takes me (slightly vague) a minute from mile marker to mile marker. This would imply that I'm really doing 60mph when my speedo says 73mph. And while doing 80mph (according to my speedo) I still get passed like a mother ****** on I-85.

Also, prior to my swapping transmissions, my speedometer seemed perfectly accurate. Odometer from my driveway to my store was dead even with my blue car (Accurate speedo, to my knowledge).

So with my setup being mechanically correct in all aspects, why the hell would my speedometer have suddenly taken a turn for the worse like this? Via searching I also learned that the speedometers themselves have absolutely no bias to automatic or manual transmissions.

 

The transmission 5th gear ratios will not affect the speedometer readings. Think about it for a minute...if it did than every gear would give a different speedometer reading. Your problem is likely due to mismatch between diameter of the speedometer gear and diameter of the output shaft. I would recommend reinstalling the gear that came with the 5-speed. The speedometer error due to rear end ratios will be 4.1/3.909 or 5% higher. (Or if you prefer lower, you could get a gear for 4.3 rear end.) For example, if car is clocked by radar at 60 mph than speedometer will be reading 63 mph, assuming speedometer is error free.

Hot_Dog
90 RX7 GXL

 

For an accurate speedometer reading, you must use the proper calibrated gear for the differential you have...This is what I have done. Putting the 4.1 (gray) calibrated gear back in the transmission would throw it off further.

 

SEARCH!

haha just messing with you collin

 

just go to walmart and buy a garmin gps and it will tell you how fast your going lol.

 

Sounds like you did what was needed to be done. I've no solution, but, I'm curious. What was the result with milage markers when using the grey gear?

 

I haven't put the gray calibrated gear in. That would be a step backwards, wouldn't it?

I'm not entirely worried about it right now. I've got the TB/UIM off the motor of that car right now because I am sending the injectors off to be repaired. Also, while I'm in there, the P.D. is going buh-bye like all the others I have encountered and some lines are getting replaced. Also righting the wrong in the sense that the one bolt the P.O.'s installed BACKWARDS ( for the transmission) is being put back in place the right way. They installed it backwards, while the UIM was off (The only way you can get at it where it sits). *******.

Ultimately, the motor/tranny/diff from my blue GXL are being swapped into the silver car.

I'm also re-working the transmission crossmember setup. I used four inch wide channel and it's TOO wide, disallowing me to use my RB header/presilencer setup on the car. I'll probably end up with two inch channel, drilled offset to allow the most room. None of this compromises rigidity, of course.


Edit: By the way, would anyone like a working but needing a rebuild automatic transmission?