Cruiter

I know, it's minor but what else is there to talk about until the line starts and some of us get our own on the road. I was watching the Motor Trend video report again and at the 10:40 mark you can see the HUD in the w/s at the same time as the digital speedo in the center dash. One reads 77mph and the other 78mph. But the same thing from time to time happens in Lexus so I guess it's just a lag somewhere in the chip.

Travelor

It is all about how the computer software "rounds" the decimal MPH. On my C6 the cruise control on the DIC and HUD can vary also by 1 mph.

CPhelps

My C5 does the same, I can have the DIC say "cruise set at 60" and the HUD say 61.

ldepalma

I can also tell you that the HUD is the accurate one. In LA there are many school/construction sites that have a speed device and whatever number the HUD displays is what the speed actually is. The auto cruise is almost always 1 mph off when setting it (it's usually 1 mph higher then the car is going....

Michael A

I'm really hoping the tach in the HUD doesn't lag, like it usually does.

Michael

Gearhead Jim

On our current 2009 C6, and our previous 2006 C6 (100,000 miles total), the HUD typically reads 1 mph slower than the analog needle on the dash or the Cruise Control speed on the DIC. I checked both cars with traffic radar and different GPS units multiple times.

Radar is not perfectly accurate, and a sign that is not used for writing traffic tickets may not get calibrated once it leaves the factory. There is also the error induced by target angle. Of course GPS may also be a bit off, but I did enough comparisons to believe that the HUD reads 1 high and the needle/DIC are correct. This is with tires that have some wear but not a lot.

Cruiter

I just got the new Passport MAX from Escort and you can read the GPS speed on it. As it works by time between grids I'd consider it the most accurate. I noticed in my Lexus the speedo from about 35mph up, reads about two mph faster than the MAX is indicating. I think most cars do read slightly fast. I've no experience yet in the Vette tho.

Michael A

I have the free Waze app on my Android phone (also available for iOS), and if you go into Settings, you can turn on a digital speedo.

Waze is a great app. In addition to navigation, people can send in road hazard reports, police location reports, and traffic info for others to view. Highly recommended.

Michael

Gearhead Jim

I too think most cars do read slightly fast, and I've heard that BMW's are particularly high.

OTOH, our C5 and both of our C6's are dead nuts on at the needle, but usually 1 mph slow on the HUD. Amazing.

CaryKen

As you can see from the different reported experiences here, there is no such thing as a standard and universal measurement discrepancy across all cars of a single make/model or cars in general. Speedometers are not intended to be high-precision instruments and are not calibrated to roundoff errors of 0.5 MPH. Any individual vehicle may be a bit high or a bit low. The best you can do is to independently verify your gauges with other devices (as several posters have given examples of) and find out what your gauge errors are.

Cruiter

If you've got a passenger and stop watch you can set cruise at 60mph and click off at the mile markers to be really precise.

CaryBob

Speedometers in the US are permitted to read plus or minus 5mph at 50mph. Manufacturers strive for more accuracy than that, but speedometers are not precision instruments since they depend on wheel rotations. Tire inflation, tread depth, and tire diameter can affect the speed that is displayed.

In general, speedometers will register a higher speed than the vehicle is actually traveling by about 1-2 mph. Several tests have shown that result.

European cars are not permitted to register anything less than the actual speed, so most European cars will register much higher-than-actual speeds, sometimes by several mph.

Gearhead Jim

We've done that in Illinois, usually it works well but occasionally you find a single marker that's seriously out of position, usually but not always because of an on or off ramp.

We solve the problem by running for several miles at 60 mph and checking the time at each mile.

If the first 3 markers show you are doing 59/59/59 mph and the 4th one suddenly 55 or 65, just disregard that one and the next will usually be the correct distance from your beginning.

larkin

In the C7 both the HUD and spedo are digital, so why would they bother with two separate sensors when you can feed a single sensor into the two digital displays? I think the answer is that they wouldn't, it's the same speedometer source but the screen is probably lagged due to using different LCD panels which have different input lag (http://www.tftcentral.co.uk/articles/input_lag.htm). The HUD LCD is likely pretty cheap.

Michael A

I'm sure they are using the same sensor data. The input lag they are referring to in the article is not related to the LCD panel itself, but the video processor inside the monitor.

In many respects, I find the entire discrepancy surprising. You would think they would do all their speed, rpm, and other computations in one processing point, and then send the results to the various display processors.

Michael

larkin

You're understanding was right but then you contradicted yourself. The input lag is due to the difference in video processing built into the LCD screens which are probably sourced from different manufacturers.

The stats probably are calculated just once then sent to both the displays which take a different amount of time to process the DVI/VGA/whatever signal.

This could be annoying...

Michael A

The reason it sounds like a contradiction, is I am not so sure they are computing the actual speed on the same processor.

I don't know if people could detect say 10-20 msec of difference in delay between the two display processors alone.

Michael

larkin

10-20msec is noticeable enough during gaming. Here's a side by side example of just 16ms delay:

It might not be a deal breaker because people aren't making millisecond decisions based on feedback from the HUD, unlike the video game where the display is all you have to react to.

I don't know how much signal processing is needed to calculate speed but the processor probably computes it in less than 10ms.