I have a '88 convertible. It has a turbo trans in it now (because I have a built engine in the car) and it's now reading about 20km/h low on the speedometer. Are there different speedo gears in the turbo vs. NA transmissions that could be causing this issue?
Will the NA gears fit in the turbo transmissions to correct it?

I have an S5 Turbo trans, I do believe.

 

Did you change the rear end?
What TII trans used JDM?

 

Rear end is NA with Mazdatrix conversion driveshaft. The T2 trans is something I bought from someone in Canada, so assuming it's a Canadian market trans.

 

Reason I ask is that the only way a lower speedometer readout can be seen is if the diff is changed, or if the trans contains a speedometer gear for a lower rear end ratio. Believe the JDM or non US market Turbo cars had a 4.3 gears, or at least some of them if I remember correctly.
Assume the '88 has a 3.909 gear and a trans were installed for 4.3, it would be off (low) maybe 6mph, or 10 km/h at 60mph.

 

As far as I can tell, all NA and Turbo except auto and gtu-s are 410, maybe the 88 convertible only may have a 390...though I'm seeing references to the Speedo gear being one for coupes and one for convertibles, as well as with the cable.

 

Jdm?

 

have you thought about difference in Stock tires to what you run now?

 

There's nothing JDM in the drive train. I may have figured some of it out though. Running a t2 trans and Speedo gear, but the 88 convertible seems to have come with a 390 rear end instead of the 410 in the coupes. My plan is to swap in the convertible Speedo gear.

Tire size, I'm not running too far off stock, as the wheels are FC fitment and lower profile tires to make up the difference in wheel size.

On a side note the Speedo stopped working completely the other day, so I guess the problem took care of itself haha

 

Thought about that also, but having a 3.909 and using a 4.10 calculation @ 60 mph, would only yield a difference of <2mph (3km/h) @60, nowhere near what you see.
Now, if you were doing 150mph then the speedometer would be off 7mph or @12km/h.

 

I don't see how that could work.
Without changing their relative pivot points, you can't change gear tooth count and expect the pair to still mesh properly.

Especially now, since your speedo has apparently died, I'd say your best move would be to obtain a turbo gauge cluster and hope the gauge itself is geared properly for the new sending unit on your trans.

There are more involved solutions I can think of- for instance, after swapping drive gears, the FD's VSS bolts right onto a FC trans and allows the use of an electronic, rather than cable driven, speedo and that can be calibrated with a Dakota Digital box if necessary.
(In my case I mated the FD VSS with a Miata cluster...speedo, tach, and fuel guage all work perfectly. I swapped VDO oil pressure and water temp guts into the Miata housing.)

 

I've swapped speedo gears before and its been fine.. i converted an auto to a manual and used the drive gear out of the auto in my manual transmission to account for the 3.9 rear end.

Id pull your cluster out and check to see that the cable didnt simply come out of the back of the cluster. It happened to me once when i didnt clip it on all the way.

 

I pulled the cluster and it's plugged in there and in the trans. I might actually have to fix it now though, because it took out the speed sensitive power steering with it haha

Now my power steering is....intermittent.
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​​​​​I have a new cable around, and the convertible drive gears, should be able to rig something up.