Tire size and speedometer (Testarossa)

The speedometer in my US spec 88 Testarossa is inaccurate. It reads slow. At 100 mph indicated speed, the actual speed is 107 mph. if it was a little fast it wouldn’t be a big issue, but with it reading slow it greatly increases the chances of getting a ticket.

I’ve reached out to a few technicians, but no one seems to know how to calibrate the speedometer. The most likely reason the speedometer is off is the tire and wheel size. The car has 18” wheels and 295/35/18s on the rear and 235/40/18s on the front. I am in need of new tires and the easiest way to improve the accuracy may be to slightly adjust the tire sizes. Does the speed sensor get its reading from the front or rear wheels?

 

Rear wheels (really rear axle rotation). However your speedometer seems to be way of compared to the very small tire OD size difference, and retweaking the speedometer calibration would be the better approach IMO. Adrian (FChatter alhbln) is the best resource. Here's a thread showing the potentiometer on a 308-2Vi speedometer -- I'd wager there's something very similar on the TR speedometer, and you could just give it small adjustments until you get a better result:

https://www.ferrarichat.com/forum/posts/143322918/

 

It is, the problem is no one seems to know how to adjust it. I’ve spoken to two different dealerships and three independent technicians. There was a motorcycle speedometer calibration unit that one member here successfully used to adjust his TR speedometer, however that company no longer makes the unit id need.

 

Please see my edited post -- I'd wager there is a potentiometer on the TR speedometer you can adjust to change its output.

 

Thanks for that, that is very helpful. It looks complicated but doable. I’ll look into it.

According to the tire size calculator going from a 295/35 to a 295/30 would set my speedometer to just around where it needs to be, but it’s a bandaid and properly calibrating it would be preferable.

 

calibrating the speedometer myself may be a little bit beyond my skill level. I did find one company that said they could probably do it for around $1000. However, I have also found a US spec Testarossa speedometer for $250 from a recycler. I’m wondering if it might not be a bad idea for me to buy this one, pop in and hope it’s a little more accurate than mine now, or experiment with trying to calibrate it.

 

I sent my TR speedo to two very reputable speedometer repair shops. Neither could correct it, cost me. I had to revert to M/C 12 o'clock labs DRO

 

that is one reason I am hesitant to send it off. There’s a real good chance I would spend $1000 and not get a good result. I reached out to 12oclocklabs and they told me they no longer make the device that would work with the Testarossa. Would you happen to know what model unit you use? It’s possible I can find one used off eBay if I knew which one to look for.

 

Buy the used one anyway if it works.

The calibration is not complicated - search signal or frequency generator on Amazon ($12+)
and follow the instructions in the other thread.

 

Such as this one? https://a.co/d/5QvL91L

 

I used one like these - not on the TR but it should be similar.

https://www.amazon.com/dp/B0B396Q9ZD
https://www.amazon.com/dp/B0B397TLPS/

 

Thanks! I ordered

 

I decided to reach out to 12oclocklabs one more time and this time the person I spoke to was actually helpful. She hooked me up with the correct module. It arrived today.

Do you remember the color of the signal wire and the ground wire on your TR’s speedometer? The instructions say it is usually red/black. Also, how did you wire in the ignition 12v?

I am about to go pull the gauge cluster out of the car.

 

It has been a quite awhile, If you pull the cluster out. It was obvious what was going on. The lower trim piece under the steering wheel was the minor problem reinstalling. I believe I trimmed the washer, and left the nut loose to reattach it.

 

I did find the following on another thread. I assume there wouldn’t be a problem getting 12v from the speedometer’s white/green wire?

"The speedo gets ground (black wire, N), 12V (white/green, BV) and sensor signal (red/black, RN)"

 

I just want to confirm this before proceeding any further. The white/green is 12v and the black is ground, correct?

@Steve Magnusson I know you are an expert on everything TR electrical. Would you see any problem with me tapping into the 12v speedometer wire? The 12oclocklabs module just needs an ignition on 12v source. I could go to the radio but this would be much easier if it wouldn’t be a problem.
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It’s fixed. Speedometer is dead accurate now. Thanks everyone, especially @Steve Magnusson and @ago car nut

 

Pirelli have made some 265/50R16 Pirelli Cinturato P7 which were the OE 16" tyres for the pre 1986 TestaRossa

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we have a few on the shelf. give us a ring.

 

oh yeah.

contact details on here

https://www.longstonetyres.co.uk/classic-car-tyres/ferrari/testarossa.html

the tyre isn't on there yet. they only turned up today. we havent even fluffed the images yet.

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