Elim Garak

Hi guys, long time fan and reader, first time poster since I got a 2008 G37 coupe about a month ago. Anyhow, I've got one minor little problem and have not come across a lot about it in the search function. I have read a couple threads indicating that the speedometer reading high is not terribly uncommon, but I would like to hear what kinds of errors others are experiencing. Also, some have indicated that an inaccurate speedometer would cause inaccurate odometer readings, although I'm having trouble reconciling it with my ScanGauge II results.

In any case, I got a ScanGauge II shortly after I bought the car. I noticed that going around 100 km/h according to the speedometer, the ScanGauge II would give me a reading of about 10% lower. Of course, I blamed the ScanGauge, probably since it's easy to adjust its readings.

Yesterday, however, I drove past one of those Highway Dept. radar trailers that let you know your speed as you're going by. I had an indicated speed of 110 km/h, and the radar billboard showed me at 102 km/h. So, it would seem that the speedometer is, indeed, inaccurate. It's not a big deal, since I know the magnitude and direction of the error. However, it would be nice if I could get a calibration done. Has anyone had any experience with this on a G37? I have only powertrain warranty left, of course.

Secondly, where does the odometer gets its data feed? If it's through the speedometer itself, there would obviously be a similar error. However, the OBD II port is reporting the correct speed through the ScanGauge II. I intend to time it or verify it over a fixed distance, but I'm wondering if anyone has any information on that. Again, it's certainly not a huge deal, but it would be nice to know.

Thanks to all in advance, and thanks for providing a great resource!

KLB

What size tires do you have?

spect2k

Tire Size Calculator - tire & wheel plus sizing

As KLB said, look at the tires that are on your car and determine what size they are. Then, using this website, compare the tires that you have to the original tire size for the car (they are more than likely different from the problem you're having). Once you've put the information into this site, it will tell you the percentage that your speedo is off by. Play around with it to get a good understanding.

Elim Garak

Sorry, guys. I'm aware of the issue with different tire sizes and have done such calculations on other vehicles. I just neglected to mention they are the factory installed 225/50 18 tires on the factory rims. The size matches the tire placard on the door jamb.

Anyhow, I just find it strange that the ScanGauge II matches the radar measured speed, while the speedometer is out. I will have to test the odometer and see how that turns out, too.

Elim Garak

Well, I did some testing. As I mentioned above, the ScanGauge II speed matches my speed through a radar trap. If I reset the average speed on the trip computer while going at a steady state, that speed also matches the ScanGauge II and the radar speed. The odometer is also correct. So, it only appears that the speedometer itself is out, while the trip computer, OBD II speed, and odometer all have the correct speed value available to them.

Itchytoe

Think of it as a safety feature. If you're going a little slower than you think you are, you're burning less fuel and driving a little safer.

It's just that the printed face of the gauge is rotated a degree or two counterclockwise. Just shift the whole thing in your mind and you'll be right again

Elim Garak

Yep, that's exactly how I'm treating it. I have had a couple vehicles that were the same way, either because of tire changes or it was just the way they were. A couple previous vehicles were right on, so I thought it was a bit strange. A Town Car I used to have had an actual certified speedometer. It must have been a shared piece between them and the Crown Vic police interceptors.

It's not the end of the world. The speedometer error is predictable and repeatable, so at the very least, it can keep me out of trouble. It might have saved my bacon going through a school zone on my first day driving it. I thought I was maybe 5 km/h over the limit (which probably wouldn't have warranted a ticket anyhow), but I was going basically right at the limit, while others whizzed by me into a nice LIDAR trap.

It is fortunate that the trip computer and odometer are accurate. If those were out, then I might want to do something about it. They're not, so I'm satisfied. All is well.