Nope...they do it all the time. Magazine road tests usually include a column for speedometer accuracy in percent. It's nothing new.

 

You can always take it to AAA and arrange to have your vehicle calibration checked. At least you will know how much to speed up or slow down to stay within the speed limit.

 

my current car has always been off... when i go by one of those 'You are driving THIS fast' signs it will say 4-6 miles under what i'm actually doing...

 

My wife's '07 MCS speedo is way off. GPS speed & distance measurement is spot on. I've been using GPS for many years. I used it in my wife's for a few miles and commented to her how far off it was. 10% seems about right. That's about the same as most Asian motorcycles (I have many).

Those roadside radars are about the most INaccurate way to measure speed.

 

My 07 MCS is only about 3% off.. not 10%

:\

So when I'm going 60 I'm really going 57-58

 

A second thought I had reverting back to the Washington State Patrol, they typically won't pull over folks on the highway for going 5-7mph over the posted limit (of course depending on circumstance). I figured that was because they'd be pulling everyone over, but perhaps it's because they realize the discrepancies in speedometers?

 

I think there are several reasons. It is difficult for the average driver to maintain a precise speed. Giving latitude means it is more difficult to contest a ticket. They are mainly interested stopping people who are considerably over the flow of traffic speed. The two most dangerous groups of drivers are those going 10 mph or faster than the flow of traffic, and 10 mph (or more) slower than the flow.

 

I saw and add for some type of engine program kit for Fords. The thing that caught my eye was the item -recalibrate your speedometer to match any tire size. Cool. That's what we need. After all, it's all electronic. Not like we'd have to swap out little gears on the end of cable to make the change

 

Quite right Robin, I was thinking more in a legal sense though. If an inaccuracy in the MINI speedo caused us to get pulled over (unlikely I know) for going say 65 in a 60 zone, then perhaps that won't hold much water legally in court because of the speedo discrepancy. Especially if they are as common as has been indicated.

I would say that if a speedo was off by 10%, that should be fixed by the dealer. That is far too much difference legally let alone for safety sake.

 

Most auto companies do not manufacture the speedo/odometer assembly. A separate company makes this. Most cities have special shops that will test your speedo. They certify this and will issure you a statement. If it is off by a substantial amount you take the vehicle to your service department. If they are unable to correct this and the auto is new, it would fall under your states lemon law.

Keep in mind that if you take your vehicle in and they have to replace your speedo/odometer assembly, it will impact your vehicles resale value. Do not do this unless you have a lawyer in your back pocket.

 

Ok I did the stopwatch testing tonight. I set my cruise control so the speedometer said "75 mph" on the digital readout, then timed how long it took to cover distance between mile markers. I did it three times to check for consistency. Every time, it was 51 seconds to cover 1 mile.

Converting 1 mile per 51 seconds into MPH by diving 3600 seconds per hour by 51 seconds, we get 70.6 MPH.

1 Mile / 51 seconds X 3600 seconds / 1 Hour = 70.6 Miles / 1 Hour

Taking displayed speed of 75 MPH and dividing by actual speed of 70.6 MPH, we get a percentage of 6.2%. This means my speedometer is displaying 6.2% faster than actual.

Now, I have to adjust this error by taking out the circumference difference of my tire size versus stock. I'm running 215/35-18's, which have a diameter of 23.9". This is smaller than the stock tire diameter of 24.4". Rollout of my tires is 75.1" versus stock rollout of 76.8", meaning my new tires will only cover 97.8% as much distance for the same RPM.

Can I subtract the 2.2% slower speed due to the shorter tires from the overall speedo error? Does this mean the speedometer is reading only 4% high for stock tires? My brain just shut down as I was writing this, LOL

Off to watch the Red Wings game,
Ryan

 

Thanks for the test info, Ryephile. I'll do some testing this weekend.

Assuming of course, that my car gets an OK bill of health from the dealer after I take it in tomorrow morning to assess why I'm getting a big pool of oil collecting under the car

 

Assuming your speedometer is accurately calibrated for stock tires, a readout of 75 MPH corresponds to an actual speed of about 73.6 MPH. That's 3 MPH error from your actual speed of 70.6 MPH, which is 4% of your speedometer's readout. So...yes.

 

um..a little ****?

 

WOW! I thought I was the only person that noticed this.

Last night while driving to work I was cruising along another car where I could read it's digital speedometer. My car said it was cruising at 82mph while the other car, which I was right up there with cruising at the same speed, was reading 76mph. WTF?!

 

Just did a check on this subject searching through the internet. What I learned is that this optimistic speedometer issue is universally a gripe with owners of just about every type of car. There was one Mercedes owner who said his speedometer was right on but everyone else is finding that their speedometer always runs higher than gps readings (typically 3 to 7 %). One fellow wrote the following:

I had thought BMWs were notorious for this; I believe it has something to do with German industry standards; if they advertise a BMW motor rates 300HP, they need to be sure that the worst motor built on the worst day has 300HP. If the speedometer reads 100KPH, the car must be going no faster than 100KPH.

Tomorrow I'll have my Garmin out on the highway with me. From what I understand the speed accuracy on these is pretty good as long as you hold a constant speed and are on a long straight highway that's free of satelite blocking obstructions.

As for the portable roadside radar displays, the consensus seems to be that they can't be relied on to be accurate. My own experience bears that out. Often one of these will be set out by one of our local parks, a 40 mph zone. I've had days when it was pretty close to what my speedometer says, on other days it's been 4 to 5 mph different (same speedometer, same speed).

 

I've road tested my Garmin GPS numerous times in my Dodge Caravan and Saturn SL2. It has been a common kids game during some of our drives. It always tracked closely to the speedometer. I'll be interested what you learn.

 

All Boxsters are off by 3 MPH (they read high). According to the Boxster forums everyone is that way. Both my 911 and BMW are right on the mark. This is all per my Garmin GPS.

 

I tracked my speed with my Garmin 660 GPS. The speedometer consistently reads around 3% above the GPS, which is 1-2 mph, depending on speed. The GPS speed readings are quite accurate at higher speeds and errors should average to zero.

 

Seems to me the consistent higher than actual reading is another case of being forced to address litigation.

 

Why? The automakers are allowed a certain amount of error in their speedometer readouts.

 

Could this be the manufacturer trying to keep us out of troulbe by fooling us to drive 3 mph slower?

Having seen the post concerning the accuracy of today's GPS...CAUTION! WE ARE ABOUT TO GET TECHNICAL!!

Garmin, and, others, have been incorporating WAAS (Wide Area Augmentation System). The FAA and the DOD have developed the WAAS program for use in precision flight approaches and now used for earth-bound GPS in cars. Currently, GPS alone does not meet the FAA's navigation requirements for accuracy, integrity, and availability. WAAS corrects for GPS signal errors caused by ionospheric disturbances, timing, and satellite orbit errors, and it provides vital integrity information regarding the health of each GPS satellite.

A few years ago you would have an accuracy of about 300 feet. Now it is within 7 feet so you can expect some accurate MPH readouts.

 

Here's some sampling I did showing a discrepancy in the 6% range. The data isn't perfect, but the trend is obvious.

 

I set mine to say how fast i am going in the tac. It is usualy 3-5mph off. The faster i go the more it is off<

 

I haven't tested mine 07 MC yet. But I did get a letter on my 2004 accord. By law the speedometer and odometer should be within +/- 5%. The letter i got on my accord was that there was a class action lawsuit against Honda for adding up miles on the odometer too fast (which is the same error as reading too high on mph). The letter informed me my warranty had been extended about 600 miles to take the discrepancy into account. Not a big deal as on our CRV, the dealer fixed something for free 20,000 miles after the warranty was up. (tie rod) This type of thing might be a bigger deal for lease vehicles who start paying per mile after so many miles.