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speedo error

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procharlie

Well-Known Member
I have TTA #162 that had been modified extensively before I got it, including a beefed-up trans and 3.42 rearend from a GN. My speedometer reads quite high, about 15% too fast as well as I have been able to measure. I thought it might be that they didn't change the speedo drive gear when the rearend was changed, so I pulled the drive gear from the trans and found it is essentially the right gear for that rearend/tire combination, maybe one tooth off but that would only be about 3% error. But I did find that this car has an electronic speed sensor, not the speedometer cable like the GN has, and when searching online I found someone who sells these electronic speed units and they have the ability to be adjusted for changes in rear axle ratio, tire sizes, etc without fooling with gear changes.
SO, my question is does anyone know if the TTA came with this electronic speed sensor standard, and can it be adjusted to make it read the correct speed? I can't figure out how else it could be reading so far off. DontKnow:
I'm new to this forum but it looks great, and I am hoping you can give me some help. When I get my car checked out better I will let you know how it performs.
Thanks,
Charlie
 
It would take quite a bit of fabrication to install a GM 10 bolt 8.5" coil spring axle into a an F-body with a torque arm axle. And you'd gain nothing. What you probably have is an aftermarket 3.45 B-W 9-bolt ring and pinion set.

I bought mine with that ratio in it and it was off 7 MPH at 60 MPH. I did nothing other than make a mental note of the error while cruising and told Eric at TT so he could compenasate for that error in chip programming.
 
Thanks for the info. I checked the diff and it has 26041414 1 on the bottom of the case, and looks original. It looks like it has been opened, so it appears you are right in that the gears were changed rather than the whole axle being replaced. But they did measure to be very close to 3.41 rather than 3.45, and I think my accuracy was pretty good in measuring it, so could they have put a set of GN gears into this diff?? I am researching to see if I can find out who the original owner was to ask what he did, because the person I got it from said that work had been done before he got the car.
I still have to find out if the speedo electronic control has an adjustment for scaling. I'll let you know what I learn.
Charlie
 
I don't recall the ring gear diameter in our BW 9 bolts, but the ratio's available in that range are 3.27,3.45. The GN and T/A axles are totally different animals.
 
Can anyone tell me if the speedometer can be adjusted electronically since it is driven by an electronic module rather than a speedo cable?
Thanks,
Charlie
 
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