More about odometer - request for verification
 

OK, a few months ago I commented that there is an issue between the trip ODO and the full ODO. It's still there, but I have another issue now.

I think both ODOs are off. I was driving up from SD to Vegas a few weeks ago, and I watched the mile markers right at the Nevada border. In 25 miles, my trip ODO (which has tenths) was off by 1.3. In other words, it started at 1.3 (I got gas at stateline), and ended at 27.6 instead of 26.3. Since then I've verified similar discrepancies at the mile markers on the 8 from SD to Phoenix and the 5 between LA and SF.

Unfortunately, the computers at Mazda don't actually run the car, they simply test to make sure that the calculation is the one Mazda wants.

SOOOOO, I'm hoping someone else who drives long distances can help me verify this. If you drive a long truck-driver stretch (frequently on the interstates between major cities), and you come across one of these mile markers, please watch the tenths digits on your trip ODO. Does it reflect differently than what the markers show?

I hope your wrong
and I'm gonna keep my eye on it though

I will start paying attention & report my findings.

If it is that far off, couldn't you do a yellowbox to get it more realistic?

http://www.blackrobotics.com/yb_home.htm

I have one on my geared 2003 R6 Bike, and it works great.

YMMV on the RX8 though, literally!!

Also of note are lane changes and your line through twisties. Best bet are the long straights along I-15 through the valleys.

Most car manu's speedos are indeed off, but they account for tire wear, gravitational pull, karma, and how much you fed your pets before you left on your trip.

Never heard of Yellowbox before.

After the first few miles when I noticed the discrepancy, I made sure to drive in the same lane to avoid that exact question of added inches due to lane changes. Ended up being more than half the distance (maybe 18 miles). That stretch (along with most of the drive between Barstow and Vegas) is straight enough that twisties are essentially not a factor.

Another option if you have a friend is to start one directly behind the other and reset trip ODOs to 000.0, then drive in the same exact line (neither one changing lanes) for about 20 miles, then check the ODOs. That I haven't tried.

Yeah, it is straight for quite a few miles between the two. If you didnt' notice, I'm in vegas ;)

What tire pressure were you running? That might not be a concern unless they were WAY underinflated.

Your variation is almost 4%. Is that threshold within some sort of federal law?

I checked my tire pressure before the drive, in Vegas, and again afterwards. 32 the whole time. And with ~4400 miles on the car, I don't think wear should be a factor, either.

Test with GPS?
 

How about trying to validate the odo by using a GPS system to compare with. Most have a trip distance function and are accurate to within a few feet. It would likely be much more accurate than comparing to someone else's odo.

Mazda is good at that

I drove on three different highways tonight and my odometer was consistently off. Over the course of 40 miles I was off by 1/10th every 7 miles on all three highways.

I had a '91 Miata that did the same thing....I ended up learning to live with it.

1/10 over 7 miles isn't that bad, mine was a bit worse. Still, I'm glad to see that somebody else is seeing the same thing...

Started checked my readings this A.M. on the drive to work.

Mine was registering 1/10 of a mile more per 5 miles driven.

Checked the speedometer against my handheld GPS last weekend. Speedomoter reads high. Maybe I didn't get 323 miles out of that last tank afterall!

the last couple of days i have checked my speed against a local police radar point. it is one of those that has a sign that reads "your speed is" with a digital display under it. the speedo in my car has been 1 or 2 mph faster than what the sign says.

That sux. Guess I'll have to keep an eye on it.

I checked mine against GPS and the odometer was reading about 1.7% high. The speedo was about 1 mph high too. That was only over about 20 miles distance so I will want to check it against a longer trip, but that's hard to do in town so it will have to wait until my next road trip. Will post results then.

I would say you have stumbled upon the poor fuel economy cause.

Actually,
If the Odometer is reading high, that would make us think we are getting better MPG.
Example: If it reads 2% (.1 mile over 5 miles) high, then if we think we are getting 18MPG, then we are really getting about 17.6 MPG.

Still this is minor, and IMHO not the reason for the poor milage.

I've done the same thing - checked my miles vs the highway markers. I'll check on the Subaru tonight as comparison for total miles on my trip from work, and then over the next week to see how if it varies much. I think both cars are very close to 40miles from home to the same I-70 exit I take. I'll have to reset trip meters in exactly the same spot with no detours (getting gas or anything) to get some readings.

I don't think it would necessarily account for any specific problem we're seeing, but I do think it points to some software that needs to be tightened up and recalibrated, which might hopefully fix some other known issues.

Testing with GPS shows mine is 3% to 4% optimistic, depending on speed.

My speedo reads 26-27 when radar "YOUR SPEED" says 25. ODO, I noticed on trip odometer that gas station that for years has been 2.2 miles away on 2 Beemers and a 626 Turbo is 2.3 miles away for the RX8. (Given rounding, error could be much less than a tenth but still troubling to have any on such short distance.)

4% is not bad... christ my friends Porsche is off something like 10% (got nailed by a cop for doing 130 when his speedo really said 145... and its on video to verify it).

Tire pressure, tread depth, and tire manufacturer tolerances can easily account for 4%.

All cars speedometers overread. Autocar, a magazine in the UK publishes it's roadtests with true speed readings. I've never seen one yet where they agree completely.

This one was for the Audi A8