Speedometer accuracy

My new to me 328 speedometer is way off on the speed indication.
I can't remember if there is a driven gear that can be swaped to correct it.
It is at least 10% off and maybe more.

 

High or low?

 

It read more than we actually go. We checked it again today and it can be up to 20kph faster like read 140 when doing 120.

 

It should be pretty accurate, maybe the speedo pick up is bad, I would replace it. The speedo is electronic, and the pick up comes off the transmission I believe.

G

 

Mine is off by about 5%. It reads high. Always has, and I've owned the vehicle for 36 years.

 

Same for me with my 85 QV, owned since new. Reads about 5 MPH high. Always has.

 

The one on my '89 GTB is accurate to the exact kilometer: if it says "90" it means exactly 90, not one more, not one less.
(And I have a dozen speeding tickets that I know are right...)
The one on my '89 GTS is lying by 5 km/h too much: if it says "90", then 85 it is.

Rgds

 

But 15 to 20 is a LOT MORE. I would like to fix it.

 

Is the signal read in volt or frequency ?
Can it be adjusted at the speedometer ?
I had replaced the sensor on my 308 as it was working intermitent but it was a mechanical driven sensor.
I haven't removed this one but from the pictures of the parts catalog it look like a magnetic type.

 

my '89 shows 120kph when I drive 120kph on the GPS !

 

The speed sensor is electro-magnetic:

Number 76: https://www.ferrariparts.co.uk/Diagram/Ferrari/328-GTB-GTS-(1988)/027-Gearbox-Differential-Housing-And-Oil-Pump

https://www.ferrariparts.co.uk/part/ferrari/116987

The sensor takes it's reading from the gear numbered 68 : https://www.ferrariparts.co.uk/Diagram/Ferrari/328-GTB-GTS-(1988)/027-Gearbox-Differential-Housing-And-Oil-Pump

So there is no adjustment possible - The speed reading is what the speed reading is.

 

Yes that is what I understood looking at the parts schematic. So the speedometer itself is out of adjustment. Do they have some internal potensiometer to adjust them ?

 

The ones on the Mondial have a pot for adjustment, using the same sensor.

 

… you could always buy tires with a different circumference.

 

Ok thanks that was my wish.

 

There are several threads here on updating the speedo/sensor.
I thought mine always over reported the speed.
I had to send it off to Atlantic Speedometer(if memory serves) for a new tub.
They also calibrated it.
now, 40 is the sweat spot.
Under 40 it under reports the speed. A local sign shows me I'm doing 25 when the speedo says 20
40 is dead-on
over 40 and it over reports the speed. I can be doing 55 (per any of those highway speed signs) and the car shows 60 or 65
and of course when I'm stopped, I'm still going over 10MPH.