Which speedometer is more accurate . . .

[snowbird]

. . . the one built in the car or the one in a GPS unit?

I notice the GPS units show a slightly lower speed than the car speedometers . . . I've seen this in several cars and GPS units.

Any thoughts on which is more accurate?

[GradyPhilpott]

I get the same effect, but Honda admitted to errors in my car's odometer, so I'm not sure if that affects the speedometer or not, but my observation is that on flat surfaces the GPS is more consistent with what those radar signs report and also what the police have told me when I've been stopped.

Unless the GPS modules have been coded with some algorithm to compensate for hills, there will be an error introduced when a vehicle is going uphill or downhill, because the rate of movement relative to that on a flat surface will be longer, thus indicating a slower speed.

How significant that difference would be would be dependent on the rate of incline.

I'm sure we have a mathematician here who can explain it better, but until one comes along maybe this will help.

[snowbird]

Interesting point about hills. I'm going to have to observe more closely and see what speed is on the GPS vs. the speedometer on hills.

[esm]

I'm pretty sure I have read some where before that that speedometer on the car actually displays a higher reading than the actual speed... This may have to do with some sort of "safety issue"....

Originally Posted by snowbird

Interesting point about hills. I'm going to have to observe more closely and see what speed is on the GPS vs. the speedometer on hills.

Probably a good idea to keep your eyes on the road as well....

[Grumpy Badger]

I think car manufacturers err on the side of caution so that their instruments can't be blamed for your speeding ticket!!!
That could get VERY expensive if a precedent was set.

[bayerische]

Originally Posted by esm

I'm pretty sure I have read some where before that that speedometer on the car actually displays a higher reading than the actual speed... This may have to do with some sort of "safety issue"....


Probably a good idea to keep your eyes on the road as well....

+1.

The GPS is likely more accurate, if it has a good signal.

[Cru Jones]

my GPS always shows a faster speed.....

[swatty]

GPS is the most accurate traveling along a straight flat piece of road

[RolexWatcher]

Your speedo will also be affected by non-standard tyres you have (if any). So GPS is likely more accurate.

[robertneville]

Originally Posted by RolexWatcher

Your speedo will also be affected by non-standard tyres you have (if any). So GPS is likely more accurate.

X2. very common issue.

[Pix]

If you figure in the time it take the GPS signal to reach the satellite then return it's pretty much dead on.

[snowbird]

That's a good point about car manufacturers intentionally setting the readings a little low, also about the speedometer readings being affected by any non-standard tires. I guess the speedometer readings could also be affected by air pressure in the tires and probably other factors.

[NYC6]

Many car speedos are optomistic. My BMW registers about 4mph faster than actual while my Corvettes is about dead on. My other cars/bikes Ive owned in the past also registered high on the speedo. This is based on several radar speed indicators you sometimes see on streets, usually around schools.

[CashGap]

Car speedo ALWAYS overstates speed.

1 - To give you the impression of speed. "Wow, this thing is zippy!"
2 - To give misleadingly high MPG. "Hmm, not the 29 on the sticker but 27 isn't too bad (it's 24.2!)"
3 - To consume warranty more quickly. 60,000 mile liability ends after 55,000 miles.
4 - To give you the "time to trade" feeling sooner. "Eh, 100,000 miles, time to trade..."
5 - To lower resale value at trade in time. "Would love to give you more but with this many miles...".

EU mandates that the speedo can never be "slow" ("ticket getter") and can be up to 10% fast. Manufacturers can now deliver a speedo with +/- 1mph, but prefer to be at the 10% maximum for the five reasons stated.