Yes, those are very serious problems. I think we should put you on suicide watch.

The actual speed per GPS will likely be different than what the car says, because of several reasons:

1. The diameter size of tires will vary as they wear down, and not all tire diameters are exactly the same, even for the same car (based on trim level, etc).

2. The manufacturer would rather err on the side of overstating your speed for safety reasons and so you don't get a speeding ticket.

 

Tire size is probably the biggest variable. When I put new OEM size on after the last worn set it was +2 for the new over +4 with the worn ones

 

Like you, I find different readings between the dial and digital speed readouts to be very odd. Both are also wrong (6% for the needle, 4% for the digital in my car).

Now, here's the kicker. If you plug a code reader into your OBDII port you'll find the MPH readout is far closer to your GPS speed. Mine is within about half a mph so, like Mark888 mentioned, probably a tire size issue. Close enough since my phone and Garmin disagree by the same amount and I'm not sure which is right/wrong.

Cheers!

 

Do intend to get an OBDII reader. Mostly, I found it interesting that the factory 'displays' didn't agree with each other.

As for: "As for the cruise control, I would not be surprised to see it vary a bit from wherever it was set." I get there is going to be some drift, that the system isn't going to LOCK at XX MPH up/down te road. However, it is almost consistantly 1 MPH slower than the SET speed. Figued the "set" was the compromise between the ANALOG and DIGITAL displays! :nerd: (I am joking, dont really figure it splitting the diff.).

Drive safe,
Smokier

 

I have a ScangaugeII in my car and it mirrors the GPS with +/-1 mph. The analog speedo is +2 mph all the time.
I have brand new tires which are within 1/2" of the circumference of the original tires.
Right before I changed them I noticed the analog speedo was +3 mph.
I have no digital readout that I know of.

As for the cruise control, I would not be surprised to see it vary a bit from wherever it was set.

 

For wiw My Garmin is very accurate ZUMO 550 for the motorcycle. It shows a difference of 3 % after 20 or more measurements and comparisons with mile markers and time and everything else.

In three hours of driving it's only a few minutes time and only a few miles of distance, so it REALLY doesn't matter much unless your underwear is REALLY too tight.

 

With OEM 215/55-17 tires, the digital speedometer is very close to the speed reported on my very accurate Garmin GPS. The analog speedometer reads 2 mph higher than the digital speedometer.


After I installed 225/55-17 tires, which caused the digital speedometer to underreport vehicle speed by 2 mph, I started using the analog speedometer because it was now the accurate gauge with the larger tires. So sometimes there is a benefit in having a choice and what was wrong yesterday may be correct tomorrow.

 

I'm going to agree with mark888 regarding the manufacturer erring on the side of a higher than actual speed because if a driver got a speeding ticket and proved in court that the speedometer was reading low, Toyota would have a class action on it's hands for every person that got a speeding ticket in that year/model car.
Ray A.
And yes, I find it disconcerting that the analog and the digital don't agree.

 

Gauge accuracy is popping up with multiple threads.

Automaker doesn't want the liability of owner speed stupidity.

Your tires wear out, can be at different PSI among owners, can have brand size variance, and can expand with speed. So, the automaker err's on the side of 'liability.