How are speedometers calibrated? With the vehicle GPS?

Nuke

Nuke

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Jul 26, 2023
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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?
 
Wanderlost said:
GPS uses height above the ellipsoid for elevation, so it would not be suitable to calibrate the speedometer.
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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.
 
Wanderlost said:
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.
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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.
 
Nuke said:
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.
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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.
 
El Chorizo said:
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.
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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.
 
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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.
 
Nuke said:
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.
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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.
 
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