In the interest of persuing some energy savings and a softer ride, I followed the recommendation of a couple of different people I have met and swapped the stock rims on my '23 EUV Premier for remanufactured 15" Sonic rims and Hankook Kinergy ST H735 205/65 15" tires. The softer ride is immediately apparent, but the efficiency assessment will take a while.
New wheels and tires
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Toyotiac
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I have only owned my Bolt EV for a few days, but I have noticed that the speed and distance travelled displayed by the car is very accurate, and matches what my phone displays. I wonder if the Bolt uses GPS to adjust speed and distance.
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46 Posts
It used to be that if an ICE car came with different wheel sizes (probably more common on pickups and 4X4s) you could just buy the matching speedometer drive gear (a tiny cheap plastic gear) and install it in the transmission
I believe manufacturers have to certify the accuracy of speedometers to meet some regulation, and they are allowed to read high (showing you are going faster than you are) by some % (10%?) but there is zero allowance on the low side - so they tend to be either accurate (possibly gps verified) or high. I think allowing user recalibration is probably not allowed under the regs
The digital display in my Honda always agrees exactly with the GPS speed display on my radar detector. I've not checked the Bolt, but it seems accurate compared to the Wyze display (I'm less confident in the app's speed accuracy)
I believe manufacturers have to certify the accuracy of speedometers to meet some regulation, and they are allowed to read high (showing you are going faster than you are) by some % (10%?) but there is zero allowance on the low side - so they tend to be either accurate (possibly gps verified) or high. I think allowing user recalibration is probably not allowed under the regs
The digital display in my Honda always agrees exactly with the GPS speed display on my radar detector. I've not checked the Bolt, but it seems accurate compared to the Wyze display (I'm less confident in the app's speed accuracy)
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7,128 Posts
German/Euro cars have strict speedo requirements to never display a slower than road speed, thus every German car I've driven in recent years displays 69 MPH when my GPS speed is 70 MPH. Thus, with tires down to the wear bars, they are still not going faster than the speedo.
jack vines
jack vines
Speedometers aren't accurately calibrated, plus or minus 5 is pretty common and +-5mph at 50mph (10% as mentioned) is what I think the current law is in the US. They aren't set high or low on purpose and I didn't see any reference to high or low in the rules.
On older cars with an analog needle, even an electronically controlled needle, the speedometer and odometer don't always agree. The analog needle often points to a different value from what the ECU is reading due to the gauge aging or just starting out poorly. If the vehicle is new enough to have OBD2 you can check the speed with that.
Many trucks like Ram/Dodge have tire size adjustments in the ECU that can be changed with the factory scan tool. It makes sense, many full size trucks have different diameter tire options from the factory.
As far as I know there are no vehicles that use GPS to calibrate the speedometer. You don't want vehicle speedometers going crazy when you drive in low traction situations like snow and dirt and then transitioning back days later under normal conditions. Chrysler navigation radios at some point did calibrate their internal speed with GPS and use dead reckoning when the GPS signal is lost like when driving through a tunnel but that information was not relayed back to the ECU. It's possible some vehicles do this, I've just never seen it done.
On older cars with an analog needle, even an electronically controlled needle, the speedometer and odometer don't always agree. The analog needle often points to a different value from what the ECU is reading due to the gauge aging or just starting out poorly. If the vehicle is new enough to have OBD2 you can check the speed with that.
Many trucks like Ram/Dodge have tire size adjustments in the ECU that can be changed with the factory scan tool. It makes sense, many full size trucks have different diameter tire options from the factory.
As far as I know there are no vehicles that use GPS to calibrate the speedometer. You don't want vehicle speedometers going crazy when you drive in low traction situations like snow and dirt and then transitioning back days later under normal conditions. Chrysler navigation radios at some point did calibrate their internal speed with GPS and use dead reckoning when the GPS signal is lost like when driving through a tunnel but that information was not relayed back to the ECU. It's possible some vehicles do this, I've just never seen it done.
I used to work for a company that made aftermarket gauges and the electronic ones were typically calibrated to within -0%/+1% at half of full scale with the reference signal. Some customers had different requirements but it's not hard to calibrate them accurately.
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7,128 Posts
Would respectfully disagree. I've rented dozens of cars, owned several, checked them all against the GPS and can't remember any of them being off more than one MPH.
jack vines
You've tested what percent of cars on the road, 0% right?
Keep in mind these specs are for a brand new car, after 5 years things are much worse. The problem is, after one year and one +100F day your perfect calibration isn't anymore. Calibrating in a lab is easy.
For sure tire wear, changes in temperature, changes in pressure, etc. will certainly throw off the calibration, but the calibration is usually quite accurate from the factory which is the best the engineers want to do with the budget they're given by the bean counters.
My Jeep has different size tires and I have an electronic calibrator from the speed sensor. I deal with day to day fluctuations in diameter and tires. I've also seen tons of clusters that are off by huge margins. I understand these arguments about how easy it is to calibrate and how best people can optimize this and that, all I know is what I've seen. I've seen pretty poor calibration but I understand others have seen decent calibration. Clearly some people have experienced much better precision than the spec, that's great. The acceptable spec is a full 10mph swing at 50mph from 45-55, let that sink in, that's for a new car.
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88 Posts
I doubt either GPS, Speedos, or even Radar is totally accurate. They all seem to give different numbers!
2019 Chevy Bolt EV Premier, 2019 Ford Expedition, 2016 Mercedes E350 wagon
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67 Posts
GPS speed is quite accurate in a straight line averaged over a minute or so. Radar is accurate instantly, if it's properly calibrated. Automotive speedometers count wheel turns, which introduces a bunch of error.
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88 Posts
IF it's properly calibrated...but would be interesting to see their calibration routine and what they calibrate to. Perhaps I'll Google it!
The analog wheel turn thing can work without power or anything else...so the old speedos just worked...and if you really wanted to keep it calibrated you could...but it sure would be a PITA! 🤣🤣🤣
GPS speed is essentially perfect as long as the GPS receiver is attached to the vehicle, in other words you aren't moving your phone around. The speed is required to calculate your position and is reported as a separate value, it requires no calibration. There is no way to calculate position from GPS satellites unless you know your speed and heading.
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534 Posts
I love this, It looks better than I thought it would, would love to hear more data & experiences. What PSI do you go with I would assume lower for the taller profile?
2019 Chevy Bolt EV Premier, 2019 Ford Expedition, 2016 Mercedes E350 wagon
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67 Posts
I think that was the stock size for my wife's former 1994 Mercedes E320 wagon, which rode like a limousine. ok, close, the MBz ran 195/65R15
"There is no way to calculate position from GPS satellites unless you know your speed and heading."
If my GPS receiver can "see" the requisite number of satellites, why would it need to know my speed and heading (velocity) in order to determine position?
Velocity can be derived from subsequent GPS position readings, and/or inertial guidance (using accelerometers), depending on the sophistication of the GPS receiver.
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534 Posts
Why are big wheels still cool when everyone has them? I want fat tires, no more scratched or broken wagon wheels.
GPS calculates distance from the satellites using dopler shift of the RF, you can't calculate the shift unless you know your exact speed. Speed is used to calculate position, not the other way around. Calculating speed from changes in position is possible but not necessary, GPS chips output speed and heading/bearing data directly and most start sending this data before a position fix.
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534 Posts
Yeah but still
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