Speedometer/Tachometer Question

The gauges are very frustrating to me... there is a police radar speed check unit that is often placed on a nearby street. When I drive by it, I hold the speedo at 30mph. The radar reading shows 24mph. So the actual speed is 20% less (speedo speed is 125% above actual speed). Wondering if this in a linear error rate? I wonder how much it overstates my odometer mileage?

 

My car is the same on both counts and I had planned to ask on here. I just got in Dec. I drove it over to the next town and back. There are several speed radars that show you your speed. My car was showing about 5 MPH faster then the radar no matter what speed I went. It did not seem to be liner rate of change. I know it is translating to the odometer because the town is 45 miles away so I went 90 round trip but the odometer was showing I went over 100 miles.

 

Is everyone running OEM tire sizes? I have never even though about checking this but it sounds like something that is worth checking out.

 

I don't know about 355's, but on the 348 there are very tiny posts sticking up from the guage faces that are right where you are seeing the indicators coming to rest, so I suspect they are on your guages too. Not sure why they have needle rests, or why they don't put them down where "zero" should be.

 

That was the first thing I looked at. They are factory size on my car so it is something else.

 

Yes. The same thing as mentioned above has held true for both 355s I've owned.

I guess that's means our cars all have 10% less miles!

 

+1 Yep tiny little posts that are difficult to see. My speedo reads 10mph slower than I am actually going and it is constant across the speed range. Would be nice if there was an adjustment pot in the gauge somewhere.

 

As far as I know, the instrument's precision is too low below a certain input signal value so the manufacturers prefer to rest the needle at a value above 0. When it starts to move off that threshold, it's supposed to be within tolerance.

Ferrari is not the only company doing this by the way.

In Europe, the speedo tolerance is 0 to the bottom = it is illegal to display a speed lower than your actual speed. You may legally display a higher than real speed, but not higher than 0.1 x true speed + 4km/h.

If your speedos are out of that range, you should have them adjusted...

 

If your speedo is really consistantly 10 mph off and you want to fix it, it's possible ( I had mine apart to re-face it. See entry #33 on "gauge face project" from 11/11/12). You have to struggle with the 2 bolts under the instrument pod, then unsnap the electrical connectors in the back and remove the pod. There are 3 screws holding the speedo in place... remove those. Then the scary part: the push button for the trip odometer on the front is pressed onto the shaft.. just grab it with you fingers and give a slow steady pull and it should come off. It takes a fair amount of force. Undo the 3 screws holding the front half of the speedo to the back and should now be looking at the speedo face. Gently lift the end of the speedo needle enough to clear the little post it rests on.. don't pull the whole needle off the center shaft yet! To get the tip of the needle to clear that tiny post you may have to pull the center of the needle up just a bit on the main shaft. When you pop the end of the needle over it's rest post it will swing to rest. Now remember where it's sitting, estimate based on the scale where it should be pointing to correct the 10 mph error you have. Now pull the center off the needle off the main shaft, then push it back on so the tip of the needle points to where you figured it should be. Now gently move the tip back over the little needle rest post and park it there. Now just reverse your steps.

 

All F cars do this. Even my old 1969 365GT 2+2 the speedo stopped at 10 mph.

 

What thing?

 

Ask your engine to run at below 500rpm...

 

BOTH tach and speedo have the little posts sticking up from the gauge faces, so they both do not go down to zero. The needles are blocked by the little posts sticking up. You'ld have to ask Enzo why they do it that way. You would think they would put the posts at the zero mark, but nooooo

 

I have an electronic tachometer on my Factory Five Cobra replica and it will stay wherever it was when the ignition is turned off. It took me a while to realize why, but then it made sense. As soon as the power is cut to the gauge (i.e., ignition is turned off), the needle freezes and that's usually at idle which is around 700 or 800 RPM.

 

In fact, it is "normal" on virtually all modern cars -- you just don't notice it because they just got smarter and put the "0" label on the speedometer dial face where the needle rests (not really at zero mph like your 355) -- to fool you into happiness (and it works ). If you look closely at this modern Subaru example, note that the "0" label is really at about the 5 mph position on the dial:
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hi guys

i have a tacho issues on my 348, it functions just fine below 5k rpm but above that the needle movement is very erratic. i haven't looked anything yet, but i did major recently and measured those crank sensor air gaps and any other stuff related to subject, didn't found anything odd

so has anyone experienced anything like it? its not engine related as it performs beautifully in any rpm

thanks