My attempt to re-calibrate the speedometer

actually....

the only part that matters is the RADIUS of the tire. the circumference is neat to calculate and all, but depending on the psi , the radius us going to change.

sooooooo, you'll want to measure the radius at the axel , to the ground.

but really none of it matters. I've watched as people obsessed about speedometers (motorcycles) for decades... and haven't used one myself in at least 12 years. pace with the traffic is always the right speed. higher or lower speed differentials appears cause problems.
. but hey, have fun with the project !

 

First of all, the circuit will work....well if it's designed correctly.
I can't speak specifically for Toyota, but I can report that there are commercial products out there that diddle with the VSS - taking power from the 12v bus. You're probably already using one of the 100's of buck-boost converters out there, and I'm thinking that it's drop-in compatible for YOTAs - but this is an "I think."
This is common both in the modding and M/C worlds where people diddle with their tire sizes and want their speedos to be "real-world" accurate.....and yes, for those defenders of mattress tags out there......it's illegal.
However (comma!) most of modders in this case do not have evil intent, UNLIKE the selfish jerks who disable VSS lockouts...
They just don't want to get a speeding ticket.

So...why is all of this this a thing for stock cars, with stock tires?
Easy.
It's not math, and it's not engineering but rather an function of government bureaucracy and corporate greed....mixed in with a dash of AR in some cases.

OUR Dot.gov (actually dot.DOT.dot.gov) probably has a standard that says that speedometers HAVE to be accurate to within X at X speed....and this is probably based on a 20-year-old rule.
Since most humans are born with ten fingers it's probably something like +/- 5MPG at 50mph.
This is probably an old rule....
Before iPhones...
Back in an long ago time when politics was just a little nasty.
Back when BEVs used lead acid batteries, and roamed golf courses.
I mean...we're talking about a LONG time ago!

One of the (few) good things about the EU is that they follow more or less a common standard....and even though they're unshackeled....so does the UK.
Because: inertia.
OH...and let's not forget the Japanese.....since they actually build and drive many of these cars.
Some nations out there (IIRC, the UK is one of these) state that a speedometer absolutely CANNOT underreport the vehicle's speed over ground....for pretty obvious reasons.
Even though some nations still drive on the wrong side of the road, the Prius is more or less a world car, and the more things you have to change from one market to another the more complicated and expensive your one car has to be....and BIG car makers who want to stay big will also use some system components for multiple models.

While it is true that in 2021, it's super easy to design a VSS (s) that's accurate to well within +/- .5%.......tire sizes do not remain constant and will vary over their life by as much as 1%.
THAT part IS math, and math doesn't care about your feelings OR your conscientiousness about changing out the various brands of tires that Priuses come equipped with before they're tread-less. Tires are all MORE or LESS the same size - even when they're NEW - right?

So the way all of this works out in the real world is that OEMs have a +/- range for VSS accuracy of "probably" 5-10% plus and zero minus....BY REGULATION across their various markets.

So.....what would YOU do if you also had to provide a vehicle warranty based on months/miles?
If you want to see an example of Toyota's corporate climate as expressed by their car's self-reporting it's actually pretty easy.

Hand-calculate your fuel efficiency and compare it with the house numbers.
Integrity - FAIL!
(But HEY...at least they're not Vee Dubbaya, right? )
There are some people out there who are foolish enough to buy extended warranties.
Overreporting the vehicle's real-world speed by 5% will save the company something like 6200 miles of repair exposure for as many cars as they can con people buying that warranty for....AND sometimes the base warranty doesn't have enough exclusions in it to prevent the OEM from having to pay a stealer for a repair.




As always.....
ACTUAL mileage WILL vary.

 

Radius, diameter, circumference: take your pick, they can all work.

As the car rolls out of the factory it has a speedo that reads slightly high, and an odo that is (or should be) accurate.

 

Wait, the ODO doesn't line up with the spedometer? I've never noticed that, but not like I was checking....

moto g(7) power ?

 

Maybe it does....
maybe it doesn't.....

If they're independent systems then it's probably a band-aid fix for past corporate greed....or maybe the engineers just applied a +5% bias to the VSS readings before they display speed to make sure that they don't have to recall and repair 100,000,000 cars in 5 years.

THAT's an example of corporate greed too.
It's a force that can be used for good AND evil.

 

You’re right on this, apples to apples, oranges to oranges.

your mission I don’t get though, lol.

 

With the one caveat that, as Elektroingenieur clarified in #15, the speedometer doesn't rely on the VSS. So the circuit will work, just not for the originally intended purpose. It will work to alter the speed seen by other accessories that rely on the VSS, so you might get a couple more miles an hour starting off from a stop before the radio and nav touchscreen buttons gray out, or before the audio amp boosts its adaptive volume, and so forth.

 

Well NORMALLY we're talking about a +3-5% bias, but in the OP's case we're probably also talking about something closer to a 10% change based on the numbers reported.

This tells me that the OP might not be the original owner of the G3 in question and that there's something else going on with this car.
I've got stick time in 2 G3's and they-both reported the typical +2-3MPH speeds.
This matters to me specifically because my beloved company is self-insured and they wisely equip all of their vehicles with data loggers, and if they report that I'm 10 over I want to make SURE that I'm really 10 over and NOT "VSS 10-over."

It turns out that they use GPS, but even THAT system is NOT as accurate as some people think it is!!!

GPS.gov: GPS Accuracy.

Fortunately.....my beloved company just like my adopted home state will let people go a little over the "limit" before controls are implemented.

 

I've made these measurements, and lined them up with engine RPMs and manual transmission gear ratios. And the numbers just didn't work out right.

Then years ago someone here clued me in to "revolutions per mile" on tire specification sheets. And then the numbers did work.

Folks should take a very close look at those tire specification sheets, and carefully compare the radius (or circumference) to the revs/miles spec. As Chapman points out back at post #18, the effective working radius is less than the measured physical radius.

Under load and placed on a flat road surface, the rubber squishes and squeezes around. Seen from the side, the contact patch is several inches long, not a single point. The effective rolling circumference that matches up to the revs/mile spec is not at the actual outside surface of the tread, but rather at some "neutral axis" (is there a better term? "neutral layer"?) buried some small distance beneath the tread.