Speedometer accuracy?

[Ethan Blank]

I just bought and installed an Escort 9500ix, which has a satellite-based GPS system. In addition to the radar/laser/red light camera/speed camera functions (which comes in handy in Montgomery County), it also measures your MPH.

The Escort GPS system tells me I am about 5 MPH slower than the car's speedometer, at highway speeds.

Has anyone looked into this with BMWs? Is this unusual?

In particular, this could be an issue as I lease this car, and am currently over (at best close) to the mileage limit. At 5 extra miles on the odometer for every hour, that does add up.

[1996 328ti]

Has anyone looked into this with BMWs? Is this unusual?

Completely normal. BMW speedos are conservative.

[Nick325xiT 5spd]

BMW specs for the speedo are that it should read somewhere between high, and REALLY high.

[Reach]

However, it is my understanding that there is not a similar correlation with the odometer. The speedometer reads high in ever BMW I've been in, but your ODO should be right on track.

Odometer accuracy is something the DMV's take very seriously. Honda lost a huge suit a few years ago over a <2% inaccuracy on certain vehicles they produced. If there were widespread ODO issues, someone would have made a stink about it by now. Of course, this takes as an assumption you're running OEM wheel / tire sizes, which is what the car will read mileage off of (and will effect the speedo as well).

[Ethan Blank]

Doesn't the odometer drive off the same mechanical device as the speedometer?

I'm no expert, but I thought that was the case.

[ldsbeaker]

IMO, the only thing mileage is good for is resale.
I rarely use the odometer to measure anything nowadays anyways. I have other internet tools for that.

In other news, are you in love with the 9500ix yet, or what?
Man, I love mine!

[Ethan Blank]

The 9500ix seems good, and fairly flexible. I've only been using it since yesterday.

I used the V1 for many, many years, and only replaced it because it was stolen.

The big selling point for me to not get another V1 was the GPS. From the test reports I read online, the 9500ix is as sensitive as the V1, and I can live without knowing if the radar is coming from in front or behind.

Since I drive in areas with a lot of red light and speed cameras, that feature is very valuable. Not that I plan to run red lights, but, I do try sometimes to speed up to "make the light". Where I know there's a red light camera because of the warning, I won't do that and take a chance. Plus, with the GPS, I (hopefully) won't get that same X-band noise when I pass the same door opener every day on the way home from work!

[Ethan Blank]

Just found this article on the web, very interesting and informative.

http://www.caranddriver.com/reviews/...candal_feature

It's good to know that the MPH are that overstated, at least in my car.

And, it appears that the odometer is more accurate, but still something like up to 4% off.

[1996 328ti]

Since I drive in areas with a lot of red light and speed cameras, that feature is very valuable. Not that I plan to run red lights, but, I do try sometimes to speed up to "make the light". Where I know there's a red light camera because of the warning, I won't do that and take a chance.

Sounds safe.

[ldsbeaker]

The 9500ix seems good, and fairly flexible. I've only been using it since yesterday.

I used the V1 for many, many years, and only replaced it because it was stolen.

The big selling point for me to not get another V1 was the GPS. From the test reports I read online, the 9500ix is as sensitive as the V1, and I can live without knowing if the radar is coming from in front or behind.

Since I drive in areas with a lot of red light and speed cameras, that feature is very valuable. Not that I plan to run red lights, but, I do try sometimes to speed up to "make the light". Where I know there's a red light camera because of the warning, I won't do that and take a chance. Plus, with the GPS, I (hopefully) won't get that same X-band noise when I pass the same door opener every day on the way home from work!

I know exactly what you mean with the red light cameras. It's really cool how "Smart" mode works. You pass the same one three times, and it knows that it's a bogey. Locks it out for you... Now for that same cop at that same speed trap... ??
I don't even use my speedometer for speed much anymore.

[Ethan Blank]

Allways within the posted speed limits, of course! And safety first, too!

[ldsbeaker]

Allways within the posted speed limits, of course! And safety first, too!

Yessum. Of course. ALWAYS.

[Ethan Blank]

ldsbeaker, I actually was responding to 1996 328ti, but the response inadvertently came after yours. Sorry about that.

Just wanted to make clear that I was referring to the circumstance where you can either brake hard to stop for a yellow light, or take a chance and go through the intersection on yellow.

If I know there's a red light camera, I will go for the hard braking to stop (which actually might be less safe!)

[Jim Dunlap]

Driving back from the Florida purchase of my '05 M3 Competition w/19"ers I found it hard to believe the I 95 traffic was running 80-85 as the speedo said we were doing. Sure enought, per my son's GPS the car's speedo is at least 5 mph too fast. What I haven't determined yet is if the reading varies with speed. This is an issue since we live in Alexandria, and often drive thru Falls Church, both of which have main streets with 25 mph limits and police with radar hiding in driveways and behind hedges in hopes of nailing "speeders," assuming one can apply that term to someone doing 31 in a 25 zone.

[Reach]

Driving back from the Florida purchase of my '05 M3 Competition w/19"ers I found it hard to believe the I 95 traffic was running 80-85 as the speedo said we were doing. Sure enought, per my son's GPS the car's speedo is at least 5 mph too fast. What I haven't determined yet is if the reading varies with speed. This is an issue since we live in Alexandria, and often drive thru Falls Church, both of which have main streets with 25 mph limits and police with radar hiding in driveways and behind hedges in hopes of nailing "speeders," assuming one can apply that term to someone doing 31 in a 25 zone.

Yes, on our cars it does. There were some metrics done on this a few years ago, and here were the results:

Indicated----Actual

40-----------38.2---1.8
45-----------42.9---2.1
50-----------47.6---2.4
55-----------52.3---2.7
60-----------56.8---3.2
65-----------62.2---2.8
70-----------67.0---3.0
75-----------71.9---3.1
80-----------76.5---3.5

I found mine to have perhaps a slightly higher delta, about 4mph at 72+ and 5mph at 81+. The point is, the ratio can be taken to be slightly off and therefor the difference is not constant. Below 50 MPH I take my speedo's speed to be accurate (and of course, its necessary to be conservative on northern VA roads due to enforcement) and at 'Highway speed' I shave 4 mph off to estimate actual speed.

Of course all this varies by model.