Did I miss something?

The speed on the dash is high by ~4% but the actual speed is in the system and can be read with a scanner. The odo is accurate in both places.

 

Ah, a little sanity.

 

Accuracy will also depend on the tires.
With the bridgstone tires the speedo was only .5 mile high.
The tires I have it's about 1.5 miles high.
This is by all the speed signs all over the place.

The odemeter is pretty close also. I can go way over 100 miles and only
gain a tenth....

 

Of course the tire circumference will affect all the readings, including the odometer and the un-fudged speed reported to scan tools.

What the combination meter shows for the speedometer display starts with the un-fudged number and then fudges it in the meter itself.

But what's the biggest difference in tire revs-per-mile between different tires people use on a Prius?

 

You realize the odometer and MPG indicator are integrated together right? I guarantee you the OP will thank me for helping him get his MPGs back. That is if he's willing to do a harmless 3-minute procedure and doesn't listen to the naysayers here. By the way, I've been in the same position as the OP where he's done everything mechanical as possible. Fluids, atfs, coolants, egr, everything and MPGs were still down. It wasn't until I performed the reset procedure that restored my MPGs. I have three gen-3s, a 2010, 2012, and 2013 PIP.

Here's another guy that restored his MPG by performing the reset. And this wasn't a short-trip test, he went on a road-trip. Lost 10mpg After Replacing 12V Battery | PriusChat

 

For anyone tuning in late, what nicoj36 just called "the reset" again is the very well-documented process for reading trouble codes from the car without a scan tool, which can be read about here:

Blink (a/k/a Flash) Codes – How to. | PriusChat

It's quite a useful procedure to know, but for other reasons, not that it resets anything.

There are also a few recalibration and code clearing procedures that can be useful to know, but those procedures are different. The recalibrations are rarely needed except after replacement or disturbance of the things they calibrate, and code clearing can sometimes have a use as part of a diagnostic approach, but gets misused a lot as a way to keep driving without fixing something.

And the creator of that video on youtube specifically believed it was a steering angle sensor reset, which was doubly mistaken because the steering angle sensor is one of the things in the car that doesn't have a calibration procedure or need one.

I am not so much into naysaying as just wanting people who come to PriusChat to be able to find out what these procedures really are, what they are really for, and when they will really be helpful and why.

 

I saw about a 1% shift in odometer correction factor when I replaced tires with a different brand, both of which were the original nominal size.
The "un-fudged" data reflects whatever tire effective circumference Toyota assumed to be typical for the specified size designation.

 

Not that it really matters, but most cars with obd2 a few years in now have a 2% excess speed reading for the actual speedometer read out. Its to keep you from blaming that Toyota was the reason you were speeding. Somehow they consider this a safety and legal thing. If you plug into obd2 you will see 70mph on the dash is actually 68mph on the obd2 port if you have the correctly sized tires. But the odometer is 100% accurate to the actual speed. I've tested it to be accurate on multiple cars with a simple scangauge unit plugged in logging miles and speed and using a gps speedometer/odometer.

The thing the OP really needs to try is pulling his sliding pins on his brakes front and rear. Get a boot and grommet replacement kit. Make sure they are cleaned and lubed and reassemble. A lot of priuses forgo brake lubes since they hardly put any wear on the brakes and people don't think about the caliper sliding pins needing to be lubed up every couple years regardless of brake wear.
If the nickel coating is worn off of the sliding pins i would go ahead and replace those too.