I've ordered a ScanGuage II and was wonder which end of the car does the speedometer read from? My rear tires are about an inch larger in diameter than stock, and I want to calibrate the SGII to reflect this if the speedometer reads from the rear wheels.

 

I think speed is measured via the VSS (Vehicle Speed Sensor) which is located on the transmission. It reads pulses from a shaft rotating inside the transmission then sends them to ECU where they are converted to an indication of speed. (This is the same data output through the DLC/OBDII port)
The only speed sensors on the wheels are for ABS.
With the ScanGaugeII you will only be able to see the same Speed indication as your speedometer in the cluster.

EDIT: If you can calibrate the scangauge II to adjust the displayed vehicle speed, use a handheld GPS to figure out the difference and dial it in.

BTW. The ScanGauge II looks really nice. I have almost broken down and bought one on a couple occasions. It seems like it maybe the easiest/cleanest way for a novice (not talking about OP) to install an ECT gauge on an AP1, not to mention all it's other many uses; DTC's, etc.

You should consider reviewing it for others after you install it and check it out. I have seen a couple similar products whose names escape me (apex i-moni or something) reviewed on here with pics and all. I have always thought the ScanGauge II looks superior to others that i have seen; size, display color choices, 4 parameters displayed, hookup via rj34 on rear or side. Anyway, I would like to see some pics of one installed in an S2000.

Here's a link for anyone curious:
http://www.telematica.gr/Product/OBD/gauge_en.html

FINAL EDIT: MY06 and up S2000's with VSA may have some other mechanism in place for measuring Speed (even at the wheel) I have no idea. Also anyone considering an OBDII scan-tool or other product that communicates via the DLC/OBDII port for a MY06 and up S2000 should think about contacting it's manufacturer to confirm it can communicate via the "extended CAN" protocol which is required by the newer S2000's (06 and up).
As I understand it:
MY00-05 S2000 = ISO-9141 protocol
MY06-08 S2000= extended CAN protocol (29bit header)

CAN=Controller Area Network
normal/old CAN = (11bit header)
extended/new CAN = (29bit header)

 

^^^ great info thanks!

 

Bascially, the rear wheel size will determine speed indicated since they are connnected to the output of the transmission. So, yes, the rear wheel size is what matters (in addition to any changes in the differential, of course). Now when you get in to VSA and MY06+, things may or may not change from this (as the previous poster stated).

 

Thanks for the info. I remember seeing the VSS wires on the transmission housing now that you mention it.

The ScanGuage II can compensate for the change in gearing or rear wheel diameter. That also means that it's measurements for distance and MPG are going to be a little more true. I'll do a write up on it after playing with it.

My goals are to:
1. find driving techniques to improve my fuel economy
2. create a baseline of intake and engine temps before I do some cooling mods.
3. to see my true RPM for some math problems, because the digitach increments are too crude to nail down precisely.

 

I didn't know the scangaugeII could do that! That makes them even more attractive.

Since RPM is one of the things you are interested in:
There is a blue wire with a cap/cover over it's connector in the engine bay corner near the brake/clutch master cylinders. Supposedly it's a hook up for a tach. I still haven't found a reference to it in the service manual. I read about it on the forums in a thread about adding shift light. I did confirm it's presence on my car.

Finally, I like that you are planning to do write up.

EDIT: I know you can get RPM via the DLC. I just wanted to share about the mystery tach hookup.
Found this thread with a pic:
https://www.s2ki.com/forums/index.php?showtopic=181388