Speedometer almost 10% off - fix?

I'm driving on the highway, and I start to get a weird feeling. My speedometer says 75mph, but people are passing me on the right giving me weird looks, flashing their lights at me, etc. Even at 80mph (indicated) I'm barely keeping up with traffic.
Something has gotta be off.

I download a GPS speedometer app on my phone. It reports ~8% lower than my speedometer. When my speedo says 70, I'm actually going ~64mph.

I used my scanner to read the ABS system and wheel speed data and vehicle speed. The indicated vehicle speed was 100% spot-on with my GPS.
So the ECU knows the right speed, but it is incorrectly reporting it to the speedometer.

My truck is a 4x4 AC with the 2.7L and a 5 speed.
I put new tires on it (the size indicated by the door sticker, and multiple tire vendors) - 245/75R16.

This is by far the most inaccurate speedometer I've ever had in a vehicle. I searched many forum threads, and asked on the Facebook group. Almost every reply is "put bigger tires on" or "Toyota does this as a margin for error for speeding".
I did not come across a real solution.

1. Is this a common thing across all Tacomas? Or has mine been edited?
2. How can I fix it? I know in this era of auto technology, anything and everything can be edited through the ECU. Is there something easier/cheaper than a Hypertech?

 

I used a Dakota Digital on my truck to adjust the speedo. They're about $80. Mine is a 1st gen, but I would think you could use one on a 2nd gen as well. Mine truck has the speed sensor wiring just behind the glove box. You just tap into 3 wires: Power, Ground, and Speed Sensor. If you're interested in that, maybe try searching around for these being used in a 2nd gen.

 

I don't know of a fix, but it is very common. I have the same issue with mine.

 

Mine is a little of at certain speeds, but not horrible. And I believe the allowable margin for error is only 5%.

 

I wouldn't worry about it. Use the GPS as your speedo if it bothers you and you don't want to spend money. Probably 90%+ of truck owners go up in tire size as soon as the factory tires are worn out. Some don't wait that long and Toyota probably has the trucks set up for the size they know will probably end up on them. My truck was off by about 3 mph with factory tires, perfectly calibrated with one size bigger. A 265/75/16 isn't enough bigger to hurt performance and will probably get everything calibrated. The guys who go to much larger than factory tires all deal with this, except their speedo's are reading slow.

They are never going to get it 100% accurate. Tires wear down and become smaller as they are used. A set of 265/75's with 40,000 miles on them will be about the same overall diameter as 245/75's when new. Your speedo will be more accurate when the tires are new than the day you replace them.

 

i swapped out my tires to 265/75r16 which corrected the speedo but now my gas mileage calculates out to mid 15s which is what i get on my Jeep with 35s...

 

The closest reference I could find is that it can be off as much as 5 MPH at 50 MPH which is 10%.

Reference:

49 CFR 393.82 - Speedometer.
The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph).


So to the OP as long as it is within 10% it is within the federal guidelines.

 

https://www.tacomaworld.com/threads/dakota-digital-install.59104/

 

Thanks for referencing that.
That is a federal regulation you just cited? Or a Toyota standard?

Thanks for going through and digging this up for me.
I feel silly doing such a primitive 'hot wire' fix on such an "intelligent" vehicle. I know it would take 5 minutes to properly edit the ECU because the ECU knows the proper speed, and it's using the proper speed for it's own calculations regarding ABS, traction control, etc.
It's just someone, somewhere along the line (likely at the factory) decided to have it spit out a wrong value for the speedometer.
I know this setting is very easily changed on GM LS1 vehicles. Even as far back as 1998, you can plug in a Hypertech "power programmer" and change certain parameters. While the Hypertech does absolutely nothing to produce any power, it actually does change the speedometer output so you can adjust for gearing and tires. It's so simple. Just go into the file, and change the value from a 3 to a 4 - Or something along those lines.

The Dakota Digital has me interested though. I have seen it very frequently when browsing the internet for years. It comes up with motorcycles/dirt bikes all the time. Now that I'm a bit more educated, I see how simple this thing is. It's just a variable resistor box. I'm considering the idea of making my own out of an Arduino or something similar. It's kinda fun to get creative with those things.

I wonder if there is a demand for it?

 

That is by Federal regulation.

I changed my tire size as suggested above for more accuracy.