gps speed vs dashboard speed

Usually the GPS would be more accurate, but if it matters to you sufficiently to figure this out then you'll need to consult another reference.

For example, find a highway that has mile markers and time how long it takes you to proceed from one marker to the next while traveling at an indicated 60 mph. If it takes you exactly 60 seconds then you know that 60 mph is an accurate indicator of your speed.

 

And make several of these measurements, not just one or two. Some of the posts are offset from their correct survey position due to obstacles or installation convenience. A few spans may have altered lengths due to road realignments performed after the original road was built. But the majority of the spans should be at, or average to, a correct mile.

 

It appears my 2013 Prius shows 2 mph faster than the actual speed - assuming you can believe the roadside radar speed safety signs.

 

The gps will not let you get a speeding ticket
The dash will make you slow in traffic

 

gps is accurate. I have slightly larger tires and now the gps does match the speedometer (until they wear down again).

 

Could the speedo not have been calibrated to the 17" wheels which I have (Option 5 package)? Is this something a dealer should have done? Or does Toyota do this at the factory depending on the wheels? Or doesn't it matter what size wheels?

 

The size of the wheels has no bearing on the accuracy of the speedo. It is the rolling circumference of the tyres or TPM "turns per mile" that affect the accuracy. The turns per mile figure on the prius is around 850.
The manufacturer can choose different size wheels "in the case of the Prius a ranging of 15, 16, or 17 inch wheels, but the sidewall height of the tyres reduces as the wheels get bigger. The TPM of the tyres remains within a small margin the same.
All car speedos over read the true speed, and most countries have regulations governing this allowing the speedo to over read by a given margin, but not under read. The speedo over reads to prevent you getting a speeding ticket, as if it under read your speed you would be suing the manufacturer when you got a ticket and the car speedo said you were not speeding.
As your tyres ware your speedo will gradually become more optimistic about your speed as the diameter of the tyres get smaller the wheels have to go round more times to cover a mile, so the turns per mile figure is not static.

John (Britprius)

 

Some simple calculations show how much difference this affect can make.
Choosing a turns per mile size that make calculations easy to understand:- 880 TPM, or circumference of 2 YDS (not to far from the Prius 850 TPM).

Every time the tyre goes round it travels 2 YDS or 72 inches (880 x 2 = 1760 YDS = 1 mile)
72 inches divided by pye (3.142) = 23 inches diameter (rounded figure).
If you ware 1/2 an inch of tread of your tyre the diameter will reduce by 1 inch.
23 -1 = 22 inches dia x pye (3.142) = 69 inches circumference. (rounded)
72 inches - 69 inches = 3 inches less distance covered per turn.
63360 (inches in a mile) divided by 69 inches = 918 turns per mile (rounded)

So the same tyre when worn will have to go round 918 times to cover a mile against the original figure of 880 a difference of 38 TMP
If the worn tyre goes round 880 times it will only go "880 x 69 inches" = 1687 YDS or 73 yds short of a mile. Or 4.3% short.

John (Britprius)

 

I normally see it listed in tires specs as RPM - Revolutions Per Mile.

 

Means the same thing. In the UK the rating is shown as TPM. The point I was really trying to get across is the substantial difference even a small change in tyre diameter can make.
RPM can be confused with revolutions per minute.

John (Britprius)

 

Confusions R Us. That is why we still don't use metric measurements, or the same date format as the rest of world, or ...