Speedometer Accuracy
12-07-18, 12:13 PM
#1
Driver School Candidate
Thread Starter
Speedometer Accuracy
I've got a 2013 F-Sport RWD. This month is "my" 3 year anniversary with it. CPO purchase and currently has 49k miles.
Wheel and tires sizes are all stock. Have moved form Dunlop to Michelin Pilot PS4's for longer life.
Living in Texas, we have current speed posted at most school zones. Speed limit is 20mph. A couple of months ago I noticed when I peg it at 20 mph on my gauge, the sign says I'm going 17mph. This is really consistent for me driving round town. I haven't had a chance to get posted at higher speeds. Is this something I should be concerned about? I'm wondering if I'm posting more miles than what I'm actually putting on the car. (if my brain is thinking in the right direction).
Appreciate you thoughts.
Wheel and tires sizes are all stock. Have moved form Dunlop to Michelin Pilot PS4's for longer life.
Living in Texas, we have current speed posted at most school zones. Speed limit is 20mph. A couple of months ago I noticed when I peg it at 20 mph on my gauge, the sign says I'm going 17mph. This is really consistent for me driving round town. I haven't had a chance to get posted at higher speeds. Is this something I should be concerned about? I'm wondering if I'm posting more miles than what I'm actually putting on the car. (if my brain is thinking in the right direction).
Appreciate you thoughts.
12-07-18, 02:19 PM
#3
Just use Waze or some other GPS and that'll get you pretty close, I bet its much closer to what your speedometer says...
-Mike
-Mike
12-07-18, 06:24 PM
#6
Lexus Test Driver
OP - I think you'll find that your speedometer is within ±1 mph with tires of the correct OE size that have full thread.
12-08-18, 06:48 AM
#7
Speedometer Accuracy
I’m glad you posted about this. Mine has been 1 or 2 miles off since new. Where I live there are plenty of those signs and they always say I’m going that much slower.
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12-08-18, 12:24 PM
#8
Instructor
Mine with 2/3 worn stock sized Continental ExtremeContact DWS 06 235/45-18 tires is about 3MPH slow at an indicated 70MPH so I'm going 67MPH. I figure I'm about 4% low.
When these tires finally wear out or we get a real winter, I'm going up a size with 245/45-18 tires to get the error a bit lower, from 4% to maybe 2%, which is more livable.
When these tires finally wear out or we get a real winter, I'm going up a size with 245/45-18 tires to get the error a bit lower, from 4% to maybe 2%, which is more livable.
12-08-18, 01:13 PM
#9
Lexus Test Driver
Not sure what you are comparing your speedometer to, but if you can get your hands on a portable GPS device like a Garmin you will know for sure your speedometer's accuracy.
12-08-18, 11:32 PM
#10
Intermediate
Here is my post from a related thread on this issue: https://www.clublexus.com/forums/gs-...0-f-sport.html
Thanks for sharing this useful thread. I have a similar issue as reported in the above thread by many.
My 2016 GS (purchased brand new) came from the factory with 225/50R17 tires. Most non-F-Sport 4GS come with 235/45R18 tires. The circumferences of the 17" and 18" tires are 81.24" and 82.71" respectively (ref. http://www.1010tires.com/Tools/Tire-...5R18/235-50R17), i.e. the difference in circumferences is almost 1.5" which is not insignificant.
To go one mile, the 235/45R18 revolves 14.3 times less than the 225/50R17. Thus, if the speedometer/odometer of my GS is tuned for the 235/45R18, it will show 1 mile has been covered when 14.3 times the circumference of 225/50R17 is still left to go. That is, my GS's speedometer/odometer is calculating 1.78% too fast. Therefore, when my speedometer shows 70 mph, I am actually traveling at 68.74 mph. However, I tested with two Garmin GPS systems and cell phone Waze app, and they showed even more difference. They consistently showed 66-67 mph when I am traveling at 70 mph on cruise control; this observation is in line with the thread above.
Last time when I went to my Lexus dealership for service, the loaner RX I received had a much more accurate speedometer than my GS. In the RX, I set the cruise at 70 mph and my GPSes showed 69 mph while my cell phone app Waze varied between 69 and 70 mph. So not all Lexus vehicles are equally affected.
If my odometer is off by 5% (i.e. shows 70 miles when it is actually 66.5 miles), I will lose 0.05x60,000 miles = 3,000 miles from my warranty which concerns me. I have reported this to Lexus corporate and they advised me to talk to my local dealership and get my odometer checked. When I requested my local dealership, they did not check anything (did not even touch my car) and said that what I am stating is within specifications and the odometer and speedometer get data from different sensors (so an error in the speedometer does not imply an error in the odometer) and that I am just being too nitpicky. At that time, my GS had about 2000 miles on the odometer, so tire wear is not a factor in the above observations. I had given up since that is the only Lexus dealership within three hours from my home until I came across the above thread.
To be continued ...
https://www.clublexus.com/forums/ls-...screpancy.html
This may be relevant
This may be relevant
My 2016 GS (purchased brand new) came from the factory with 225/50R17 tires. Most non-F-Sport 4GS come with 235/45R18 tires. The circumferences of the 17" and 18" tires are 81.24" and 82.71" respectively (ref. http://www.1010tires.com/Tools/Tire-...5R18/235-50R17), i.e. the difference in circumferences is almost 1.5" which is not insignificant.
To go one mile, the 235/45R18 revolves 14.3 times less than the 225/50R17. Thus, if the speedometer/odometer of my GS is tuned for the 235/45R18, it will show 1 mile has been covered when 14.3 times the circumference of 225/50R17 is still left to go. That is, my GS's speedometer/odometer is calculating 1.78% too fast. Therefore, when my speedometer shows 70 mph, I am actually traveling at 68.74 mph. However, I tested with two Garmin GPS systems and cell phone Waze app, and they showed even more difference. They consistently showed 66-67 mph when I am traveling at 70 mph on cruise control; this observation is in line with the thread above.
Last time when I went to my Lexus dealership for service, the loaner RX I received had a much more accurate speedometer than my GS. In the RX, I set the cruise at 70 mph and my GPSes showed 69 mph while my cell phone app Waze varied between 69 and 70 mph. So not all Lexus vehicles are equally affected.
If my odometer is off by 5% (i.e. shows 70 miles when it is actually 66.5 miles), I will lose 0.05x60,000 miles = 3,000 miles from my warranty which concerns me. I have reported this to Lexus corporate and they advised me to talk to my local dealership and get my odometer checked. When I requested my local dealership, they did not check anything (did not even touch my car) and said that what I am stating is within specifications and the odometer and speedometer get data from different sensors (so an error in the speedometer does not imply an error in the odometer) and that I am just being too nitpicky. At that time, my GS had about 2000 miles on the odometer, so tire wear is not a factor in the above observations. I had given up since that is the only Lexus dealership within three hours from my home until I came across the above thread.
To be continued ...
12-09-18, 05:54 AM
#11
Instructor
Here is my post from a related thread on this issue: https://www.clublexus.com/forums/gs-...0-f-sport.html
I've done this on trips from KY to FL and the odo does seem to be closer than the speedo.
12-10-18, 01:53 PM
#13
Intermediate
Testing that theory is pretty easy. If the speedo is off and the odo is correct, you lose nothing in warranty. Pick a stretch of road the next time you start a trip and at a mile marker, rest the odo trip to 0. After 10 miles see if 10.0 miles is with the 10th mile marker. If you can go 100 miles, your accuracy will improve.
I've done this on trips from KY to FL and the odo does seem to be closer than the speedo.
I've done this on trips from KY to FL and the odo does seem to be closer than the speedo.
12-11-18, 05:04 AM
#15
