What's More Accurate

My sat nav was telling me i was doing 70, speedo said I was doing 79 / 80.

I know speedo's are usally out by a couple of mph, but nearly 10 miles out seems a lot ....I am assuming that the Sat nav is more accurate, unless a delay between satellites could fudge the speed???????

SatNav is more accurate, i tested the gps app. on my iphone and is was 99.9% of the time the same as my Garmin Nuvi or 1mph out, the speedo on the car was accurate upto 80mph above that it was out generally by 3-6%

My sat nav was telling me i was doing 70, speedo said I was doing 79 / 80.

I know speedo's are usally out by a couple of mph, but nearly 10 miles out seems a lot ....I am assuming that the Sat nav is more accurate, unless a delay between satellites could fudge the speed???????

There shouldn't be THAT much of a difference in my opinion. Let's say your sat nav is right ... in that case you'd be getting done for exceeding the 70mph speed limit if you were following your car speedo!!!

I'm guessing they're both a bit out. It's tradition for car speedos to be over so I guess your sat nav is under a little.

The sat/nav may be more accurate, in theory, but it isn't often acutely accurate in practice. It depends on the quality of the chip, and its requirement of satellites, and other variables. Often in the past, a sat/nav has exchanged speed for accuracy. As sotonjoe says, they're both out a bit. The speedo almost certainly more so than the sat/nav. But don't automatically condemn it.

I wonder IF car speedos are made to read fast so that the driver can brag about how fast his car is

and also maybe it is so people don't exceed the speed limit by as much as they think they are doing ?.

Mind you as a lot of people think the speed limits are just to get even more money in fines perhaps

the speedos are actually slow

Generally car manufactures make the cars speedo 3mph fast at 30mph.

Frequently they are also 10% fast throughout the speed range.

Generally car manufactures make the cars speedo 3mph fast at 30mph.

Frequently they are also 10% fast throughout the speed range.

Yes, which means that when people moan about getting done for speeding at 32 in a 30 zone their speedo was probably telling them they were doing 35 anyway!

My sat nav was telling me i was doing 70, speedo said I was doing 79 / 80.

I know speedo's are usally out by a couple of mph, but nearly 10 miles out seems a lot ....I am assuming that the Sat nav is more accurate, unless a delay between satellites could fudge the speed???????

Weird, I was wondering similar the other day ... my car speedo was reading 70mph, yet the digital speed reading on the car's trip computer said 67mph.

I assumed the digital reading was correct?

Car makers generally calibrate speedos to show 5%-10% above the actual road speed. This is for 2 reasons, firstly to make the customer think they drive a faster car than they really do (wow, I'm doing 80 and I'm hardly pressing the throttle!) and secondly as part of the tolerance stack, so that as the car gets older and parts start to become worn, the speedo will always display equal to or above the actual speed. The last thing a car maker wants is for someone to have an accident in a 20 year old car and find out that the speedo was showing 10% less than the actual speed.

GPS is accurate to with +/- 1 mph on a flat road.

If you are on a gradient it can mis-read your speed, See Pythagoras for further explanation.