On a trip back from Chicago yesterday on the flatlands of WI I ran the following test:

Set the cruise at 70 MPH
Reset the AVG mph
Reset the MPG
Ran GPS on laptop
Ran for 45 min.

Results:

GPS showed 71.2 mph
AVG mph showed 68 MPH
MPG showed 32.5 MPG
And the speedo was steady @ 70 mph.

This pretty much confirms a recent article in Car and Driver and shows why the MPG is not accurate although not bad for the power to weight ratio of the 02 Max.

 

What was the difference between the GPS miles traveled and the vehicle's odometer reading?

BuddyWh

 

Damn good question; did not save that!

 

BMW and Porche are the worst for speedometer accuracy. They must account for the LARGEST possible wheel/tire combo that will fit without rubbing or exceeding indicated speed. Supposedly, it's because it's so common for them to run plus size rims/tires. Lexus/Toyota are the most accurate. Honda/Acura are next and Nissan/Infinity follow. Domestics are more accurate than imports due to Europes vague and somewhat odd regulations.

I get between 2-3 mph difference at 60mph just from the ECU and the speedometer. I think at 70mph it's pretty much right at 3mph.

It's built into the system to account for MANY variables such as tire pressure, mechanical degradation, extreme climates, and MANY MANY more variables. Each variable is given a +/- percent of the TOTAL mph of the speedometer NOT the actual speed. Therefore, it has NOTHING to due with "power to weight ratio" just our 160mph(?) max speedometer value.

Of course manufacturers also leave a margin to prevent lawsuits for speeding tickets.

 

Interesting, while I haven't done any comparisons w/a long steady cruise nor the trip computer, I've compared my Magellan Map 330 GPS unit speed indication w/the speedo on my 2K2 Max GXE. If memory serves, with the cruise control set at 70, my GPS unit seems to read around 68-69 mph. I'm not sure really which is actually more accurate. I don't have the accurate "5th wheel" equipment that car mags have.

The online version of the article you're referring to is at http://www.caranddriver.com/xp/Caran...peedometer.xml. They have some extra graphs in the online version but are also missing a couple nice charts that were in the print mag.

Maybe your results are diff from mine since you have an SE (which probably has 17" wheels) while I've got a GXE w/the stock 16" wheels.

 

I have had several GPS units. All have consistantly given the maxima at about 67 @ 70 on the speedo. Our pathfinder reads 3 mph slower as well. I realize that hills and turns would skew the GPS reading, but I've driven ALOT with a GPS and this is pretty consistant. I was in a Suburban that was reading 7mph too slow, and he was getting tickets ALL the time.

When I put on 2k2 SE rims (from my GLE 16's), I calculated that I would gain 2mph at 70. My GPS confirmed this. I'm dead on now. This actually a bad thing because for the last few years, I've been cruising a few mph over and knew I wasnt in danger of getting a ticket. Now when I set it on 75+, I'm really doing that. :/

 

And I called you a "PRUDE"!

 

Mine reads a consistant +1mph/30mph.

I figure if I go +1 on my tires next time around, my speedo will be right on...