peteski

Let me just jump in here with my 1st post.

I've noticed the same on my speedometer. I've hooked up a Autoxray scanner which gives me a direct digital speed reading through the OBD II connector. It shows that the dash speedometer is indicates 2MPH higher than the direct speed readout from the PCM.

This cannot be due to any tire diameter variation because the same speed sensor is used by the dash gauge and the Autoxray. Seems that the dash gauge is just calibrated too high. Possibly to slow the drivers down a bit.

guitarsurf

Welcome to forum peteski! :D

I did GPS mine a few weeks ago and it just said the same thing as everyone else. About 2-3MPH off, so I don't think I ever bothered posting it.

Can you imagine the class action suit if the actual speed was just a shade over the speedometer reading?

peteski

Well, this excerpt from the Service Manual shows that the speedometer is supposed to be calibrated a bit on the high side. That explains the discrepancy we all see.

Easyoboe2

I just installed a ScanGaugeII. It says I am going 65, when the car speedo says 70. My tires are 205/50-16 which indicates an error of minus 1.4 percent.

1anonymous1

OK everyone, prepare to get your panties in a wad.
The ODOMETER is correct, the SPEEDOMETER is off, like almost every car made in my lifetime.
The reason for this is to keep your silly a** from getting tickets. And then there's also that class action lawsuit thing that I'm sure all mfr's want to avoid.
The ODOMETER must be accurate, but dupeing you, the driver, is for your own good.
The easiest way to get around this is using 185/65/15 tires on your STOCK 15" rims instead of your 185/60/15 Goodyear's (Junk, but that's another topic) which came with your car. But then the odometer will be off (read slow).
Keep in mind that the speedometer is calibrated to take into account the wear that occurs on your tires (over 40 K miles of use), and the difference between the stock tire NEW and the stock tire worn out (or down to 2/32" tread) is about 3/16" on RADIUS.
The stock tires at their wear limit will read accurately, or even slightly fast (overspeed).
All tires are slightly off from their "advertised spec". They are ALL small - just a little bit - compared to their stated size on the sidewall. How much depends on the mfr. and profile.
This is why the actual speed has to be calculated on the loaded radius of the tire, which depends on other factors,
including the vehicle weight
and the tire inflation pressure.
and the rim size.
and the profile of the tire.
There are many resources available on the 'net' to help you determine exactly how many RPM your tires are turning, and by virtue of this, exactly how fast you are going. Companies even make both electronic and mechanical correction gears (for older vehicles) to correct your speedometer based on your wacky non stock tire choices.
(At this point someone is about to rip into me because Toyota makes 'overstock size' rims for the xB.)
The overstock size rims are designed for tires which have less wear life (read lower "/32 of tread than stock) due to harder compounds and less tread depth material... in order to improve handling. This means that these tires will have less deviation due to wear over their projected lifespan.
I will stop this dissertation here with the admonition to check your vehicle's loaded tire radius on level ground with a tape measure and use this measurement to calculate the actual RPM of the tire. This, in combination with any one of the tools available, will give you your real speed.
OBTW if you are within 2 mph on stock tires and are going above the posted speed limit... well, I hope there's not a helicopter over you...

upshift

'After skimming thru most of the previous post ...... Just hook up a gps handheld and get a really accurate mph and odometer reading by comparing speedo and gps. I hope you don't think a gps is off also.
upshift.