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Discussion Starter · #1 ·
Anyone calibrated their speedo? I ask because mine seems to runn about 5% optimist. Consistently get a speed reading from roadside radar signs that work out to 19 real mph per 20 indicated mph. I other words, I’m doing an indicated 40, the radar sign shows 38...20,is shows 19...80, it showed 76.

Wondering because that means the car is logging 100 miles for every 95 miles I drive. Honda faced a lawsuit for a similar issue, and had to extend their warranties. If we are all running like this, the 50k warranty is really only a 47.5k warranty.:surprise:
 

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Because of European laws the speedometers on European cars are biased to report a slightly faster speed than what the actual speed is. I have to set my QV on 82 mph to achieve a speed that is slightly below 80. Japanese cars tend to have the most accurate speedos, which is definitely true for my 2000 Nissan Maxima. I think its stupid, but there's nothing you can do about it. And it cannot be "calibrated" by a certified mechanic. In regard to the odometer, it counts miles correctly even though the speedo is off.
 

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Anyone calibrated their speedo? I ask because mine seems to runn about 5% optimist. Consistently get a speed reading from roadside radar signs that work out to 19 real mph per 20 indicated mph. I other words, I’m doing an indicated 40, the radar sign shows 38...20,is shows 19...80, it showed 76.

Wondering because that means the car is logging 100 miles for every 95 miles I drive. Honda faced a lawsuit for a similar issue, and had to extend their warranties. If we are all running like this, the 50k warranty is really only a 47.5k warranty.:surprise:
That's normal across the board of all manufactures.
-It tells you that you are going slightly faster than really are. This way if you get a speeding ticket, you can't claim the speedo was in error and sue the manufacture.
-It also accounts for replacement tires, that don't have exactly the same diameter even though the tire size is the same.

But, as a whole, those roadside speed signs are usually wrong. I've seen them tell me I'm doing 40 MPH, while at a full stop with nothing moving any where.

Download a GPS speedometer for your smart phone. Set the cruise control to 60 and see what the phone has to say. The phone should be telling you that you're doing about 2 MPH slower than what the car shows. That is industry standard.
 

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2018 Q4 with Fiamenghi Ti exhaust, Race Mod, and Tecnico wheels.
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I find that my Giulia's speedo is spot on according to known good roadside radars. At least that is assuming that the speedos in my other vehicles are accurate. I believe that some forum member posted that it appears that Giulia's speedo is "self calibrating", presumably using the GPS as a reference.

If a speedo reads slow it can be used as an excuse to fight a speeding ticket.
If a speedo reads high you get a falsely high MPG value, a false sense of performance, and also your warranty expires early (HMMM, I wonder about left lane Priuses...).

In the lawsuit of mostly Japanese mfgs, speedos were off by as much as 20% high. My 1973 Honda 450 had such a speedo, 65MPH indicated was 55MPH actual. My Desoto had a similar error, but I don't know if that was because of tire differences, old age, or error by design.
 

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I guess that explains why the speedo in my QV goes to 200, but the manufacturer publishes a top speed of 191.
 

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I guess that explains why the speedo in my QV goes to 200, but the manufacturer publishes a top speed of 191.
In the early 1980s, there was a rule to limit speedometers to 85MPH max since that was higher than the speedlimit anywhere in the USA. This had the opposite effect of what was wanted: lots of people driving 90MPH so they could say that they "pegged the speedo". Since then most if not all cars come with speedometers that read higher than the expected maximum speed of the vehicle.

In the high 100MPH range tire expansion and slippage makes the speedo pretty unreliable, unless you have the exact tires, tire pressure, pavement, and wind speed that was used to calibrate the speedometer.
 

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I would not trust those roadside radar signs to be particularly accurate. Since the signs are to the side of the road they see your car from an angle (not straight on) and will therefore always indicate a slightly slower than actual speed. A GPS on your phone or the old method of timing mile markers on an interstate will be more trustworthy.

Warning, the following is excessive detail that you may regret reading:

Which made me think about why the signs may not be not accurate. First because they are likely not calibrated often, but more importantly because they sit to the side of the road and not in the road. The signs measure how fast a car is approaching it. At 0º angle (the car moving directly toward it) the speed will be measured accurately. But as the sign is moved further from the center line of the road the more inaccurate the speed will be measured. Moving the sign 5 degrees off the center line will result in about 5% under-reporting of speed.

In theory the sign could be calibrated for the angle if the road crews would measure how far off the road the sign is, but that seems unlikely for a convenience sign. So the sign might be accurate, but the same sign on a different road on a different day might not be accurate.

Thanks, this was an interesting thought-problem.
 

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What I find interesting is the difference (on a level road) between cruise control setting and digital speed readout. Is synchronization too much to expect?
 

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Has anyone compared it to a GPS?
I'll go out later today and do a GPS comparison and a mile-marker calculation. It will be an excuse to take a drive.

UPDATE: I did a 12-mile measured route and at an indicated 70 mph my true speed was 68.6. That number should get closer to 70 as the tires wear and the wheels turn slightly faster to go the same distance. I think 1.4 mph is acceptable measurement error.

I tried GPS but the speedometer in the Waze app fluctuates too much.
 

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What I find interesting is the difference (on a level road) between cruise control setting and digital speed readout. Is synchronization too much to expect?
I expect that the speedometer is an analog instrument, which makes it amazing that the digital readout and speedo are even close to matching. For precision use the digital readout. However, I never use it since my brain is trained to read a gauge.

The speedometer could be made digital by using a stepper motor to drive the needle. I have no idea if any manufacturers do it that way. I would think that would either cause the needle to move slowly or to move in visible steps. Stepper motors also have a characteristic sound, although it might be too soft to hear inside a running car.
 

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Discussion Starter · #13 ·
So, I intended to ask if anyone had done anything to validate the speedometer reading via a calibrated distance and time. I went ahead and used a speedometer app and it validated the roadside radars. It seems somewhere in the range of 5% low. Now, as for those who find the speed right on, perhaps tires are the answer. I have the 18” Pirelli P7’s...might those with less error have the Bridgestones? As the tires wear, the error should slightly increase, it perhaps the Bridgestones are slightly larger? The specs show identical diameter, but no RPM on the Pirelli, while the 17” Bridgestones are very slightly larger (803 vs 804 rotations per mile).
 

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So, I intended to ask if anyone had done anything to validate the speedometer reading via a calibrated distance and time. I went ahead and used a speedometer app and it validated the roadside radars. It seems somewhere in the range of 5% low. Now, as for those who find the speed right on, perhaps tires are the answer. I have the 18” Pirelli P7’s...might those with less error have the Bridgestones? As the tires wear, the error should slightly increase, it perhaps the Bridgestones are slightly larger? The specs show identical diameter, but no RPM on the Pirelli, while the 17” Bridgestones are very slightly larger (803 vs 804 rotations per mile).
18" Pirellis, too. My speedometer reading shows above app reported speed, too (used Waze).

Last time I paid attention:
Waze: 120km/h
Car digital reading: 124km/h
 

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I compared my speedo to my digital read out and find that they match very accurately at 30 , 60 and 70 MPH. At 20MPH the speedo seemed to be a little low, but who cares?

Next time I am there I will try to remember to run a constant speed from one side of my property to the other. It is extremely close to 1 mile (within 30 feet or so) and the speedlimit is 65MPH.

There used to be measured miles marked on the highway, but it seems that they were taken down 25 years ago or so.
 

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I expect that the speedometer is an analog instrument, which makes it amazing that the digital readout and speedo are even close to matching. For precision use the digital readout. However, I never use it since my brain is trained to read a gauge.

The speedometer could be made digital by using a stepper motor to drive the needle. I have no idea if any manufacturers do it that way. I would think that would either cause the needle to move slowly or to move in visible steps. Stepper motors also have a characteristic sound, although it might be too soft to hear inside a running car.
No, giulia (and most modern cars) do in fact use stepper motors for the guages.... Much more reliable and accurate. Speed (and distance) info is derived from the ABS unit and sent via CAN bus in digital form to dashboard. This also allows other systems to use the speed info (infotainment, cruise control, climate control etc.)
 

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This thread wont end....I guess I'll add: 1. If one changes tire size to try to slow down the speedo the odometer will be affected (but I suppose in a good way). 2. There is (at least was) an aftermarket company which sells a unit which can be inserted within the CANbus which will translate the wheel pulses and can be used to slow-down or speed-up the speedo reading. I think it required splicing wires however..... 3. This is nothing new. Chances are your last car did this to and you didn't notice. BTW go read Road and Track from as far back as say 1981; they were checking speedo calibration on the cars they tested. Some were way off....
 

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I've noticed more in my QV than any other vehicle that it's readout is over actual speed.

Verified by GPS devices while on track, radar detector with GPS, and speed limit signs.

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