Speedo Correction

Hey I actually learned some things here so was worth the read

update: I’ve also read that manufacturers typically have speedometers set to display a little higher than actual speed and that’s certainly true with my stock SR, speedometer reads about 2 mph high at 65-75mph with stock wheels as determine by radar alert signs and gps. I simply deduct 2 mph from the display for my actual speed because math is just not that hard lol.

 

It ain't worth the trouble or expense to try. It would take a pretty significant upsize in tires to cause you to be off my more than 5 mph at 70. If the speed limit is 70, just keep it UNDER 70. No one is going to write you a ticket for doing 3-4 MPH over the limit. At slower speeds you're only going to be off by 1-2 mph and who cares about that.

Even if you could get it calibrated perfectly it won't stay that way. As you're tires wear they get smaller in diameter. A new 33" tire will be 32" by the time it has 40,000 miles on it. You can choose to have your speedometer accurate when the tires are new, or when they are near the end of their life. But you can't have it both ways with today's technology.

 

Your wheel hubs are way before your differential gears? Your wheel and tire essentially mounts to your wheel hub

 

I see the problem is @Key-Rei has a 2nd gen where the speed sensor is in a different location than on a 3rd gen which is why I’m asking this question in the 3rd gen forum

 

Haha @RushT has faster internet.

 

Okay here's a 2020 and the sensor's description.



It doesn't matter, the actual sensor that reports vehicle speed to the speedometer is still located in the same place on that trans.

A 3rd gen is just a pretty (or pretty ugly depending on your point of view) heavier, more electrified 2nd gen with a mini-van motor.

The "speed sensors" at the hubs are only for the vehicle stability and ABS systems the report speed discrepancies from wheel to wheel but do not reflect on the speedometer.

When a wheel(s) is not moving (speed being sensed) compared to the other(s) that are the computer will activate ABS.

But the actual speed sensor is still at the trans.

 

I think that’s the disconnect here is that you think speed is read at the trans on newer trucks where we’re saying it’s read at ABS. Would I be correct in saying that?

 

You are correct that I am standing by my statement that VEHICLE SPEED is measured and reported to the speedometer by the VEHICLE SPEED SENSOR on the trans as shown in the dealer EPC diagrams.

And that WHEEL SPEED FOR THE ABS SYSTEM is read and reported to the SAFETY SYSTEM by the WHEEL SPEED SENSOR at the hub.

Your wheels are not your vehicle.

But hey. I ain't perfect, I just have a little time working for Toyota. Maybe I'm wrong and owe you an apology and if so I ain't afraid to do that.

I ain't even mad, I just want the correct info out there, and if I don't have it then I'll learn and adjust.

Perhaps resident vehicle electrical engineer @6 gearT444E can clarify for us.

 

I’m glad we found the disconnect. The speedometer is fed speed via the abs on all the automatics and also the newer manuals as well. I was getting ready to tag him as well because I knew you’d listen to him.

You can see here where he mentions it.

Which leads back to OP’s point, gears would not impact his speedometer readout.

 

I don't know when the change happened, but I can only speak from my experience as I didn't pick the design from the factory. My truck (2010 manual trans 4x4) takes its speed reading from the transfer case output. I believe at least up until model year 2010 there were two choices:

1: speed via transfer case sensor (M/T)
2: speed via skid control ECU which uses the wheel speed sensors (A/T)


It is described here:





To muddy the waters more (and maybe where people are getting confused), there ARE speed sensors in the transmission which feed into the ECM, but they aren't used for the speedometer indication, they are used for transmission controls.








To answer the question, yes if you re-gear your truck and have the speed sensor in the t-case, the lower gear ratio will cause your indicated speed to be higher than actual. I have witnessed this on my truck when going from 3.73 to 4.56. The counter effect to that is to put on larger tires and you will go more miles/rev of the wheel, and for a given speed it will indicate slightly lower than actual, and vice versa for smaller tires (but who the hell is gonna do that ). For me, re-gearing to 4.56 and putting on 36" tires put the speedo back to almost stock reading by verifying with a GPS.

If you re-gear a truck that uses the speed sensors at the hubs, re-gearing will NOT affect speedometer indication. Changing tire size will slightly change the indicated speed, just as mentioned above.

I don't know when Toyota changed it, but apparently the newer trucks no long use speed sensor in the t-case, and so it's easy to check. If you have a manual transmission, and don't have that speed sensor in the t-case and your speedometer is working it must use the speed sensors at the wheel hubs. If you have an automatic, it's always used the speed sensors at the hubs. I imagine that the skid control ECU must take a median signal of the sensors, as one or more can fail and your speedometer will still indicate.

 

I’m hanging out waiting for the definitive answer to the ultimate 3rd Gen question. Okay, maybe right behind ‘what’s the biggest....’

 

Speed for my 2nd Gen, 2014 is picked up at the LR ABS wheel sensor. It is fed into the TCU and not the main ECU. This allows the torque management control system in conjunction with ABS and also the BS electronic Limited Slip Diff to function properly. You cannot change a gear cog at one point as with older second gens. The speedo gets the reading from the TCU signal. It cannot be programmed and must be piggyback interrupt signal fed, as with the Hypertech system. Even with programming systems like HP Tuner, it only accesses the ECU and not the TCU and won't allow you to adjust the speedo signal. Your Scan Gauge will adjust a signal for you, but not display anywhere else but the scangauge. Hope this helps, and yes, third gens are the same. My vote is Hypertech, only because I have had good success and no experience with the other. Good customer service and easy interface.

 

That explains a lot. A little weird how they would use 2 different methods for different transmission types

 

Thanks for confirming what I was saying with documentation.

 

"significant upsize" isn't a hard number.
I'm only on 34" tires (285/75/17) and that places me at just under 10% off on the speedo vs actual.
luckily 10% is easy to math so I haven't bothered to get a speedo calibrator black box, especially since (to add even more to the confusion) the speedometer itself may be driven by the ABS sensors on the 3rd gen, but the speed setting on cruise control is not. so the speedometer calibrator box will fix your speedometer, so in my case I can get the speed to say 70mph when it is actually going 70mph (where right now the speedo would be reading 64mph), but the "set to" readout for cruise would be reading 64 mph.

(I am running 5.29 gears currently if anyone cares to add a data point to the argument that seems to have been resolved by now)

Also if you think noone writes tickets for doing 3-4mph over the limit you haven't been driving in ohio with out of state plates much
(if i go 70mph according to my speedo today I'm actually going to get clocked at 77 anyways)

 

Figuring out how fast isn’t that hard. But I just want to look at the odometer and know when my next maintenance interval is instead of doing some math

 

It's worth the cost IMO. I never bothered recalibrating and lived with a ~9% off speedo for awhile. Once I regeared, I HAD to correct my speedo due to reading speed at t case output. Once I got it installed and calibrated, it was one of those things that I didn't realize what I was missing out on. Having an accurate speedo is so basic but it was so nice.

 

I run an AEM gps speedo. Nice at highway speeds where I’m off 7+ mph. Only if you like having extra gauges though.

 

I got a speeding ticket in Ohio too! Cop got for me for going 80 in a 70. A). my truck can't do 80 unless it's downhill with a tail wind, and B). here in NY, 10 over is nothing. In a 65 if you're not doing 80 you can't even compete in the right lane.

 

For the speedometer, does it take readings from all the ABS Speed Sensors or just a certain one?
2011 Tacoma, automatic transmission.
My speedometer is out, odometer is out, cruise control does not work, CEL light is on, VSC light is on. Napa scanned and got code P0500, vehicle speed sensor. The ABS sensors are easy to replace, although expensive. However, the Napa scan did not specify an particular sensor and I'd like avoid paying Toyota $150 for a diagnosis.