[ewald88]

I don't know if I'm missing this discussion somewhere else, and if I am please just point me in the right direction, otherwise I am now beyond curious and would enjoy some feedback.

Ever since my wife and I bought our '04 SL AWD we've been saying that it's a speeding ticket waiting happen, and now it has.....twice in 4 days.

The first time was in town, and my wife was nailed in a speed trap coming down an overpass (54 mph in a 35 mph zone), and while she said she knew she was going faster than 35, she almost fell out of her seat (thank goodness for those seat belts after all!) when the officer told her she was doing 54 mph.

A few days later I was pulled over in a rural area by an officer who claimed I was doing 73 mph in a 65 mph zone....I was a little annoyed I was being pulled over as I had the cruise set dead on 69 mph and couldn't believe it when the officer did a U-turn to run me down. Now he did let me off with a warning, and I did notice that he had a "trainee" or passenger with him so I don't know if it was a "this is how it's done" deal or the speedometer is off by 4 mph or more. As I routinely go 4-5mph over the posted limit due to frequent travel and longer distances on those trips I'm a little leary of my speedometer now.....has anyone else had similar experiences with the accuracy of their speedometers? We have the OEM tires on aftermarket rims (supplied by the dealer as they were on the vehicle when we test drove it)....so I don't think there is any reason to not have a correctly calibrated speed readout.

Thanks as always for any insight everyone!

 

[ekaxel]

Mine has an error of 3% slow.

 

[ewald88]

Hmmmmm,

if my math is right 73 mph when the speedometer is saying 69 would be 5.5% slow......that might explain why we always are passing everyone on interstate when using cruise control. Cruise on 80 mph would actually be almost 85 mph.....now I'm wondering how in the [email protected]#$ I haven't been picked up to date by the highway patrol while I've been thinking I'm only doing 5 mph over when I'm pushing 9-10 mph over. I'm definitely going to have to buy the next one a drink or something as 9-10 mph over would be a hell of a ticket in MN (not so bad in ND).

Now I'm going to have to go find one of those little white radar on wheels they set up around construction zones and see just what the MO is really doing before the insurance rates cost us more than the MO does at the rate my wife and I are going. Especially since the MO garners enough attention from just the design, let alone giving any officers any extra need to check it out!!

 

[zofsuvs]

I posted a long while back on the other forum that the trip computer reading for average speed would be about 2 mph lower than the cruise control/speedometer speed. Someone posted back that it was normal for speedometers to read slighlty higher than the actual speed. I'm surprised, then, that yours actually reads lower. Doesn't seem normal.

 

[zofsuvs]

Here's are links to a Car and Driver article on the topic dated April 2002:

Full article

Summary of results

 

[hfelknor]

We were given a ticket for 61 in a 55 in a SC radar trap.
Our cruise was set on 60.

We also have OEM tires on aftermarket wheels.
I must say our aftermarket wheels were exact repacments tho.
They are 18" by 7.5" with a 35mm offset as are the OEMs.

Homer

 

[zofsuvs]

The article says that some speedometers are set to read lower, but most higher...

When traveling at a true 70 mph, as indicated by our highly precise Datron optical fifth-wheel equipment, the average speedometer (based on more than 200 road-tested vehicles) reads 71.37 mph. ...

Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph. Are our cars trying to keep us out of traffic court?

(See previous links for full article.)

I think mine reads higher, but I should really check.

 

[Eric L.]

Echoing the article, I would think the MO's speedometer would read higher, not lower. In either case, being off by a few MPH is not surprising when receiving a citation since those few MPH often bring in more revenue for whoever wrote you the ticket. I know in California, a speeding ticket carries a base fine plus something like $10/mph over the limit. So adding 5mph to your actual speed = another $50 for the CHP.

 

[jwmarlowe]

I have a portable GPS that I use on a regular basis. My GPS says I am going 2 MPH faster than my speedometer indicates.

 

[zofsuvs]

Hmm. How much variation in speedometer readouts could we expect between different Muranos?

 

[pcs15394]

Speed

I have to echo jwmarlowe. My iWay 500c usually shows that I am going 2 to 3 mph faster than my speedo. I just picked up on this on the way to my parents last weekend.

 

[zofsuvs]

Curious as an experiment, for those with the GPS, when you're on an open stretch of road sometime set the cruise control and re-set your trip computer. Then after a few miles see what the trip computer reads for the average speed in comparison to the speedometer and the GPS unit.

I don't have the GPS, but my trip computer displayed a lower average speed after several miles of driving.

 

[ekaxel]

I did this repeatedly with my Magellan 700 on my recent trip. It has a speedometer in addition to the various odometer displays. The MO speedometer, odometer, and fuel economy readings with half worn stock tires were all 3% low. When I put the snows on (235/65-17, a little smaller), readings are almost spot on. Looks like the 3% low number is quite consistent among all who have reported here.

 

[zofsuvs]

I did this repeatedly with my Magellan 700 on my recent trip. It has a speedometer in addition to the various odometer displays. The MO speedometer, odometer, and fuel economy readings with half worn stock tires were all 3% low. When I put the snows on (235/65-17, a little smaller), readings are almost spot on. Looks like the 3% low number is quite consistent among all who have reported here.

You didn't mention the "AVG MPH" on the trip computer. That's the one I'm wondering about, since my average speed reads lower than the speedometer.

 

[ekaxel]

I suspect it would be affected the same way. I use thr Roadmate trip computer (s), as they are accurate.

 

[zofsuvs]

Just saying it might be worth testing the AVG MPH readout to see.

 

[Eric L.]

Just saying it might be worth testing the AVG MPH readout to see.

The only way to compare the AVG MPH reading with your speedometer is to reset the trip computer while you already have the cruise activated. Otherwise, the minute it takes you to get onto the freeway and get up to speed, your avg speed will never hit your cruise speed.

 

[zofsuvs]

Yup. And the more miles the better.

 

[ewald88]

Hi All,

Just wanted to thank everyone for the input. I'm just going to have to cut back on the cruise control until I can find a portable radar or maybe a friendly officer over the 4th of July (small hometown in rural area....maybe get a speed test w/o getting a ticket!). I still think the officer was just showing his passenger the ropes as he was very quick to give just a warning when I told him the cruise was set on 69 and I was surprised to be pulled over. In ND there is a base for ticket, but it's peanuts (MN is much stiffer base penalty), so to be pulled over even for doing 71 in 65 mph zone would be rare so I'm guessing he was fudging his numbers to get a reaction/info (whether I really knew what speed I was going).

I found the article that was posted to be interesting....although if I'm reading it right and I was a (so I thought!) bad ass Porsche/BMW owner getting away with speeding I might be a little irked to find out I might actually be going below the speed limit or barely making the speed limit.

As the second vehicle is a Buick (GM right?), maybe I'll take it out and run it side by side with MO and get a correct/accurate reading off the cheaper GM speedometer which should be more accurate according to the article.

 

[Gonzo]

If you have a portable GPS just use that and compare that while you have someone driving (disclaimer)