How are speedometers calibrated? With the vehicle GPS?

Nuke

Moderator
Staff member
๐Ÿ“› Founding Member
Jul 26, 2023
253
142
Rexburg, ID
In modern-day vehicles, how is the speedometer/odometer calibrated to tire-wheel size? If one goes to a larger diameter tire, how is this compensated for? It would seem like a simple matter for the vehicle to use GPS, as virtually all vehicles have nowadays, but this may be asking for a bit too much?
 
GPS uses height above the ellipsoid for elevation, so it would not be suitable to calibrate the speedometer.
You have lost me Wanderlost, but does that mean the GPS receiver in the vehicle cannot accurately calculate speed over the ground, at least to the degree necessary to calibrate a vehicle speedometer?
 
My appologies Nuke. GPS can not measure altitude with an accuracy required to compensate for vehicle elevation changes over time because GPS altitude is referenced against a geomentric model of the earth (the ellipsoid) instead of actual height above sea level.
 
My appologies Nuke. GPS can not measure altitude with an accuracy required to compensate for vehicle elevation changes over time because GPS altitude is referenced against a geomentric model of the earth (the ellipsoid) instead of actual height above sea level.
Having worked with the military capabilities of GPS in regard to aircraft, I am aware it is very accurate positionally. One would think the civilian variant could at least calculate accurate MPH of a vehicle traveling at a relative snail's pace.
 
Having worked with the military capabilities of GPS in regard to aircraft, I am aware it is very accurate positionally. One would think the civilian variant could at least calculate accurate MPH of a vehicle traveling at a relative snail's pace.
Very cool, we have similar backgrounds. The most modern commercial transport aircraft use ground based augmentation systems for GPS position error. GPS speed is only used in conjunction with high integrity inertial navigation systems for back-up airspeed in an emergency.
 
Your tire size is in the ECU and it measures revolutions per mile. If you change the tire size you have to adjust the value in the ECU for your tire size.
 
Your tire size is in the ECU and it measures revolutions per mile. If you change the tire size you have to adjust the value in the ECU for your tire size.
How is this done? A gadget one attaches to the OBD plug? Recommendations?
 
I had an 02 Chevy Durmax HD2500 whose ECU could not be adjusted as I went from stock 245/70 tires to 265/70. Chevy dealer verified it could not be done. I think they changed that on the 2003 models. I just had to remember the speedometer read about 4% lower than actual speed.

OTOH I went from 16" to 17" wheels on my 99 4Runner and the larger wheels now agree with GPS readings @3000 ft elevation at 70mph. Toyota and maybe other manufacturers purposely introduce some error in the speedometer at the request of the legal department to avoid law suits on speeding tickets.

With cruise control on going uphills my 2020 4Runner downshifts and goes above the cruise setting. I am looking forward to an improved transmission in the new 8 speed on the 2024 LC. The old 5 speed just does not cut it.
 
Last edited:
Agreed, the 5-speed on our 4Runner seemed to all too frequently be shifting/hunting between 4th and 5th. I suspect the new 8-speed on the LC will do plenty of shifting on slight grades also, but probably not as perceptible. My technique is to put the transmission in manual mode to overcome some of the shifting. However, the 4Runner 5-speed was tough! They are putting the 10-speed in the Lexus twin of the LC. An old guy, would have never dreamed of a 10-speed auto tranny, and all to squeeze a tad bit more MPG.
 
Agreed, the 5-speed on our 4Runner seemed to all too frequently be shifting/hunting between 4th and 5th. I suspect the new 8-speed on the LC will do plenty of shifting on slight grades also, but probably not as perceptible. My technique is to put the transmission in manual mode to overcome some of the shifting. However, the 4Runner 5-speed was tough! They are putting the 10-speed in the Lexus twin of the LC. An old guy, would have never dreamed of a 10-speed auto tranny, and all to squeeze a tad bit more MPG.
I think they also went to 0-20 weight oil and lowered cross bars (making them less useful for attaching sky boxes) to improve gas mileage. I would love to see a 5 or 6 speed manual (makes it more theft proof), but I don't think that's going happen though some of the new Tacomas will have 6 sp manuals I read.

I will try the manual mode trick, but I thought that locked out 5th gear.
 
Not sure about the new LC, but from other Toyotas in the past, the answer is no, even tech stream can't change tire size to compensate. Most people just drive with inaccurate speed and odometer and not worry about it, or invest in some kind of plug and play gadget such as Hypertech
 
Not sure about the new LC, but from other Toyotas in the past, the answer is no, even tech stream can't change tire size to compensate. Most people just drive with inaccurate speed and odometer and not worry about it, or invest in some kind of plug and play gadget such as Hypertech
Thanks carpool. Not want I wished to hear, but no surprise.
 
Thanks carpool. Not want I wished to hear, but no surprise.
I get why Toyota doesn't even allow any form of ECU modification/recalibration, it's directly related to odometer reading so messing with it is tantamount to altering odometer. If one can easily set the tire size parameter, it'll make it too easy for the bad actors to cheat odometer by "selecting" smaller tire size and under register the true mileage.
 
With cruise control on going uphills my 2020 4Runner downshifts and goes above the cruise setting. I am looking forward to an improved transmission in the new 8 speed on the 2024 LC. The old 5 speed just does not cut it.
With my 4Runner now, the cruise control is just not very pleasant unless on relatively flat roads. It is a big, heavy box that has just enough power. So anytime it is loaded or on a grade or has draggy things attached, it has no choice but to kick down to 4th or 3rd gear to maintain speed. And it doesn't want to just kick right back down after upshifting, so it holds that lower gear over the crest of a hill where I'd normally have let off the accelerator. The vehicle can't see that we are cresting a hill. It's much more pleasant to just drive without cruise control on anything hilly.

This is the main reason I welcome the turbo, the hybrid, and the 8AT. The engine can build boost to keep the exact same RPM and make more torque on small hills. The electric motor can kick in to give more power. The gear spacing gaps are going to be lower on the 8 speed, so it won't be as noticeable when the vehicle downshifts or upshifts by one gear. Being able to blend these three things will really improve the way the vehicle drives, IMO. Right now, gear and throttle (and to lessor degree, variable valve timing) are the only tools the 1GR and 5AT have to work with.

I'm honestly just worried about weight. The GX is 1000lbs heavier than my 4Runner. That's over 20% heavier. Hopefully a lot of that weight gain in the GX doesn't impact the LC. 5300lbs would be OK.
 
With my 4Runner now, the cruise control is just not very pleasant unless on relatively flat roads. It is a big, heavy box that has just enough power. So anytime it is loaded or on a grade or has draggy things attached, it has no choice but to kick down to 4th or 3rd gear to maintain speed. And it doesn't want to just kick right back down after upshifting, so it holds that lower gear over the crest of a hill where I'd normally have let off the accelerator. The vehicle can't see that we are cresting a hill. It's much more pleasant to just drive without cruise control on anything hilly.

This is the main reason I welcome the turbo, the hybrid, and the 8AT. The engine can build boost to keep the exact same RPM and make more torque on small hills. The electric motor can kick in to give more power. The gear spacing gaps are going to be lower on the 8 speed, so it won't be as noticeable when the vehicle downshifts or upshifts by one gear. Being able to blend these three things will really improve the way the vehicle drives, IMO. Right now, gear and throttle (and to lessor degree, variable valve timing) are the only tools the 1GR and 5AT have to work with.

I'm honestly just worried about weight. The GX is 1000lbs heavier than my 4Runner. That's over 20% heavier. Hopefully a lot of that weight gain in the GX doesn't impact the LC. 5300lbs would be OK.
I hear you about the cruise control. I find my gas mileage improves when not using cruise instead using the accelerator to minimize downshifting on steep hills. I figured Toyota would have a TSB issued for the constant downshifting and exceeding speed limits using cruise control. I look forward to test drive reports in the near future. None so far...
 
I hear you about the cruise control. I find my gas mileage improves when not using cruise instead using the accelerator to minimize downshifting on steep hills. I figured Toyota would have a TSB issued for the constant downshifting and exceeding speed limits using cruise control. I look forward to test drive reports in the near future. None so far...
Dream on re a TSB. It is the nature of the beast. Our 2005 4Runner had this issue. Pre-2024 Tacomas also have the "hunting" problem. I wonder if the current generation of 8 and 10-speed tranny's also have this malady? I suspect so.
 
Dream on re a TSB. It is the nature of the beast. Our 2005 4Runner had this issue. Pre-2024 Tacomas also have the "hunting" problem. I wonder if the current generation of 8 and 10-speed tranny's also have this malady? I suspect so.
The reason it is so clear on the 4Runner is because the steps between the gears are very large. At 70mph, your choice is to be at 2000RPM in 5th gear, 2500RPM in 4th gear, or 3200RPM in 3rd gear. If the engine can't make enough power to pull the hill in 5th or 4th, it has no choice but to jump down to 3rd gear. On an engine with a 5800RPM redline, jumping from 2000RPM to 3200RPM feels drastic. The 8 and 10 speed transmissions should have the ability to make small steps down more likely landing on a gear that "fits" better and, as I mentioned above, they will have the ability to lean on making boost or pulling energy from the battery to fill some of the engine's power gap in the higher gears. I think it will be more invisible thanks to the multi pronged approach.
 
Back
Top