Speedometer and Odometer


#1

I have a 2001 Lexus. When I pass a police radar device it shows my speed as lower than my speedometer says (e.g., 37 vs 40). If the car thinks it is going faster than it is, will it think it has gone farther and run up the mileage on the odometer?


#2

You cannot assume that the off-road radar display is more accurate than your car. It may read lower than actual because of the cosine effect, for example.

What you need to do is calibrate your car yourself, which you can do any any interstate with convenient mile markers. Your systems may be right on the money. Or not, if you have ever changed tire sizes from factory specs.

But the direct answer to your question is yes, odometer distance is linked to speedometer speed.


#3

Thanks for the response. The off-road radar displays – at numerous different locations – are always lower than my speedometer. If I were a gambler I’t put my money on the radar.


#4

I’m curious how you know the comparison between your car’s speedometer and the police radar. Can you compare the radar with other cars?
Could replaced tires, with just a slightly different radius, “fool” your speedometer? We’ll leave it to the Lexus experts comcerning the connection between the speedometer and the odometer and the wheel rotations.


#5

The best way to check you speedometer/odometer is probably to compare it with your GPS, otherwise, you can check them both against highway mile markers using a stopwatch (like the one in your cell phone) over about 10 miles with the cruise control set.

Speedometers do tend to be a little optimistic on many cars, if it is too far off you should be able to have it calibrated. There are a bunch of places that calibrate german (VDO) instruments, I assume there are places that specialize in asian cars.


#6

I’ve had both high and low readings with RAV4’s over the years compared to speed monitors…haven’t been off more than 5 mph. My current RAV '07 happens to be right on speed…go figure.


#7

I’ve noticed the same thing in a number of cars-- the radar thingee is usually 3-4 MPH slow. I’ve noticed this even in cars where I’ve double-checked the speedometer with a GPS and doing the time/distance measurements. You’d think they’d make those things read high, so people will slow down more. I definitely wouldn’t consider them an accurate way to test your speedometer.


#8

Use a stopwatch and the highway mile markers. 65 MPH will take 55 seconds to go a mile. That will be close enough to measure the speed. Don’t assume anything until you time it.


#9

Does the Lexus diagnostic scanner (like the GM Tech2 )have the ability to monitor vehicle while speed while the vehicle is driven? If it does you can compare the speed on the scanner to the speed on the speedo. Just trying to fiqure out if this can be done with a scan tool.


#10

All or nearly all cars are designed to show a higher speed than real, like about 5%. The idea is to reduce your chance of getting a ticket if they are off a little due to manufacturing variance or maybe tyre size change.

As noted GPS is a good tool for measuring speed independently, but they can be thrown off by turns in the road. A nice long nearly straight stretch should be correct. Also consider doing the mile markers on the highway. Do a couple of miles worth and compare that with your odometer and speedometer (1 mile = 1 minute @ 60 mph)

However it sure look like it is working as designed. BTW I have found that those road side police reminders are very accurate.


#11

I’ve disassembled mine and corrected them so they read correctly with my specific tire sizes (pretty easy on mechanical VDO speedometers). I don’t know why the manufactures do that, it’s very annoying.


#12

If you have some kind of GPS receiver that will display speed in miles per hour, that is the best way to check the calibration of your Lexus’s speedometer. The second best is a stop watch and mile markers along a rural interstate highway. Don’t rely just two consecutive markers. They aren’t always exactly one mile apart. You can calculate speed from time with this equation: S = 3600 / T. S is your speed in miles/hour. T is the number of seconds to go one mile.

Provided you go a subtantial distance, interstate mile markers are a good way to check the calibration of your odometer. Ten miles is the minimum for a decently accurate result.


#13

I’ve also used the trip computer function of my GPS to check my odometer by reseting it to zero and comparing it to the odometer after 50 or 100 miles of highway driving.