Speedometer Calibration

There is nothing I have heard of to correct speedo errors. I'm in Subaru next week, I'll ask the instructor. Steve

Questions: Are you running the original tire size and wheel diameter? If your 2003 OB is the same as my 2000 OB, it's 225/60R16. I don't know which year they bumped up to 17" rims on the OB. Over or under sized tires/rims will cause speedometer error.

Is it +4 MPH or -4 MPH, and at what speed did you check it? I'm guessing you checked it at an indicated 60MPH as the seconds directly translate to MPH at 60.

I'm not familiar with the exact type of speedo the 2003 uses. I'm not sure of the small details, but the 2000-2004 Outbacks are almost identical. My descriptions are not Subaru-specific, they are generic automotive.

There's the classic "dial pointer" style that's driven mechanically by a rotating speedometer cable from either a wheel hub or the transmission. This one is almost always accompanied by a "rolling dials" odometer. I doubt that this is what you have.

The next version looks like a "dial pointer" style but is actually an electronic meter that's driven by the vehicle computer/ECU. The odometer is almost always a separate digital display. This is what my 2000 has.

The last is a fully digital electronic display for both speedometer and odometer, driven by the computer/ECU.

None of these are even remotely easy to recalibrate by an owner.

Most vehicles with electronic speedos driven by the ECU can be recalibrated with special software by a dealer shop. They do the adjustment by measuring the wheel/tire outside diameter and plugging that number into the software to reprogram the ECU. This may take more than one try to get acceptable accuracy as the only way to check the work is to repeat the same tests you have made by driving. Most dealers usually farm this out to specialty speedometer shops as speedo work is a PITA this way (it's uncommon, so the mechanic usually has to spend some "re-education" time with the manuals).

Speedometer specialty shops use dynamometer-style huge rolling drums to accurately simulate road speed and then proceed quite quickly through the recalibration since they are very familiar with this work and can verify their work on the spot there on the dyno rollers.

There's a small snag here. AWD vehicles (like ours) can only be done at shops with two sets of dyno rollers, for front and back wheels. Some shops still have only one set of rollers for 2WD vehicles. Definitely stress "Subaru" and "AWD" when you contact them.

Look for "Speedometer Service" in the Yellow pages or Google "Speedometer Service"+(your city).

Same here, 2005 Forester XS, 72 mph indicated, 68 mph actual speed. Service writer at my dealer said nothing they could do. Mine has been off since day one. I drive a different rental car each week with my gps, and most new cars are dead on with their speedometers. Seems Japanese cars are the most off. I think they have issues with kilometer to miles conversions, or a problem with using different tire sizes in their domestic market vs. the U.S. as the

2005 Forester had 17" rims in Japan vs. 16" in the U.S.

Blair Baucom

2005 Forester XS

All cars, I repeat ALL cars have an error built in. It is intentional. There are codes that cars are built to. Ever notice that your gas tank indicates full before it is full. It practicaly has to. If it is full, ir has to indicate full. If it indicates under full, then people might try to put more in and spill gas. (at least that was the reasoning. auto shut-off nozzles, and jsut the manner in which people fill their tanks pretty much negate the regulation, but it stands.)

Along the same line a speedometer can not indicate that you are going slower than your actual speed. Because of manufacturing variance, the speedometer will be designed to indicate high so that the outliers that indicate lower than designed are still indicating higher than actual speed. The standard deviation for the error might be .5mph at

70mph, so very very few speedometers will actually be far enough off to get a car company in trouble if they are designed to indicate 4 or 5 mph high.

A few years ago some news organization tried to blow it all out of proportion. At the time BMW had about 10mph error at 70mph. They were very very optimistic. The media tried to paint them as cheating the customer. Maybe that is a little too much safety margin built in. I don't know.

Fact is your car is normal, and if this is the first time it has bothered you, maybe knowing that your other cars were probably about as far off despite you never catching on hopefully will make you feel better.

If you know it is there, and the error works in your favor as far as speeding tickets, I dont' see why it should bother you. But then again my 2 year old gets bothered when she is missing one of her markers in her marker tray. Different people obsess over different things.

The speedometer is a needle and dial. The odometer is digital. I have timed it out on the highway with a stopwatch using the cruise control on flat straight highway. The speedometer consistently reads 4 mph higher than the true speed. For example, with the cruise control set steady on 75 mph, the stopwatch and the gps confirm I am truly doing only 71. I realize no gauge is going to be 100% accurate, but everything else in this car is very high quality and quite satisfactory, so I am surprised by this amount of discrepancy. Other cars I have owned have had 1 or 2 mph errors, but 4 is a bit high. Sure would be nice if there was a way to adjust it. I suppose I could open the face of the speedometer and bend the needle 4 mph to the left. LOL

On Thu, 2 Jul 2009 08:12:11 -0700, against all advice, something compelled "Gunny_2009" , to say:

When it's time for new tires, get taller ones.

Yeppers, that's the plan. Lot's of tread left on these so it will be a while.

Mine is also off by about 4 or 5% Has always been. It's a 2003 OBS. It's canadian, so I don't think it's a miles-kms conversion issue. My Honda was the same way. I agree with the poster who said it was deliberate to ensure stupid drivers don't get themselves into trouble.

Well that does not explain why 95%+ of the rental cars I drive are dead on. The last two were a 2009 Chevrolet Cobalt and a 2009 Honda Civic which had a digital speedometer. Both indicated dead on against the gps and radar. My Subaru has been the worse I have seen since buying my first gps two years ago, and I drive a different car every week.

Blair Baucom

2005 Forester XS

That is a good point. Maybe Honda is confident that their standard deviation on error is so low that designing them to be dead-on is safe. This reminds me of something I was told about eight or ten years ago, which would actually work against my argument. But I'll share anyways.

Years ago I used to buy used tires for my cars. I asked the owner how it was that he would wind up with perfectly good tires that still had the nubbies on them. He claimed that rental car companies would bring in their new cars and trade the tires out for worn or lower profile tires so that the odometers would spin faster and they could charge more for mileage.

If your recent rentals had this done, they'd be off. Although, I doubt any rental company would do this any more considering most of them have unlimited mileage, and spinning the tires more also translates into more wear on the rest of the drivetrain.

Sure, almost everybody in the world JUST KNOWS that you can pass a 50 km/h speed trap with the needle on 60! Then you have really 56, take off the tolerance and the trap will do nothing. BTW, is it really that hard to subtract 4 from a number?

You'd be surprised...

No, but you shouldn't have to. That's the point. They don't intentionally mislabel the channels on your TV and tell you to add or subtract to get the real channel and they don't intentionally put the wrong price on a menu and tell you to subtract a given amount. Things are so high-tech in cars these days there ought to be a way of syncing up the gauges occasionally so that they read true.

Sorry, I have to disagree. To me, it doesn't matter whether - say - the motor coolant has a temperature of 85.354=B0C or 87.831=B0C. It's enough to know that it's not too hot. Likewise, why should I care whether my speed is 70 or 74? You better look at the street, weather conditions, traffic conditions and choose your appropriate speed based on that data without even looking at the speedometer. (If you can't do that, then you probably should not drive.) Having done that, a (short) look at the speedometer tells you with sufficient accuracy whether you are in accordance with existing speed limits. Sure it would be technically possible to have a speedometer that gives you the speed with any accuracy you want - but not only is this more or less useless (for the reasons above), it also would make the car more expensive. I am pretty sure the majority of drivers is not willing to pay extra money for this kind of perfectionism.

"Ingo Menger" wrote in message news: snipped-for-privacy@c1g2000yqi.googlegroups.com... On 6 Jul., 22:02, "Gunny_2009" wrote:

You make several personal value judgements in your posting. That is fine. They are your personal preferences, no more valid or less valid than mine. :-) That doesn't mean either of us is wrong. We just have different standards. Just out of curiousity, at what point would you no longer be satisfied? 4 mph doesn't bother you. What about 5? How about 6,7,8, 10, or 15? You see my point. All of your comments about road conditions, weather conditions, etc are irrelevant. We are discussing only how much inaccuracy in your instruments should you have to tolerate. Some of us personally prefer the instruments to be "more" accurate (no perfection). As far as cost, I don't think it would be all that expensive. With all of the computerization incorporated in the modern cars I don't think it would cost hardly anything for a technician to enter a +4 or a -4 into a menu field while he has the car in for periodic diagnostics if the speedometer function is currently or some day controlled by one of the onboard computers. In that case, if you are tolerant of the noted inaccuracy, say nothing to the tech. If not, you just tell him,"By the way, while you have it hooked up, add/subtract X mph to the speedometer please." My original question was based on me wondering if that technical procedure is currently available. I know it isn't directly available to the driver, but I wondered if it could be done when it is in for service. I can live with the situation, but if I don't have to, I won't.

Right.

Sure I do. But it was told here in the thread that up to 5% is absolutely usual in most cars, and this is in agreement with my own experience.

As are your comparisions with discrete values, such as channel number on TV or price of a menu.

I understand. But have you considered that this is may be more a legal problem rather than a technical one. It is entirely possible that car makers make sure that the speedometer does not get past to save themselves troubles with speed trap victims. Moreover, check your local laws: here in germany it is the law that the speed displayed must not be lower than the real value. I guess this will not be much different in other countries. (Otherwise, speed trap victims could argue that it is not his or her fault ...)

I see your point, but I counter your point with a similar question. There must be a tolerance since it can never be absolutely accurate. What error is acceptable to you if not 5mph? 0.5 mph? 0.05 mph?

0.005 mph? 0.0005 mph? 0.00005 mph? See my point? All of these accuracies I listed are achievable, but at what cost? Slippery slopes work in both directions. As we slide down your slope the consumer accepts lower accuracy. As we slide down my slope the manufacturer goes bankrupt trying to produce a car to Aerospace-program-like standards, and for what reason? Apparently the car companies(or at least subaru) has decided that they are satisifed with 5 mph. And since car companies have been doing this for a long time, I'm sure someone at one time or another has said,"I bet the consumer will never know the difference." and for most of us, they are right. I know my speedometer error above 20 is about 2mph +1/15 of indicated speed. I'm anal enough and curious enough that I figured it out from several different experiences with the "Your speed is" signs and by timing it on the highway. However, it is a matter of curiosity for me. I'm an engineer, and mildly OC. I like numbers. I don't let it bother me though. Not that I am saying that you are wrong to let it bother you.

If you have GPS navigation option, invoke the "Navigation Set Up", go to the 2nd page and touch the symbol labelled "Vehicle Signal". There you get the speed as computed from the GPS data, which should be quite accurate. I checked yesterday in my 08 Outback, and it shows a speed 6 to 10 km/h lower than that of the speedometer.

I appeqars that the speedometer is calibrated to exaggerate the speed a bit, in order to comply with (almost) world wide regulation.

On Fri, 10 Jul 2009 00:41:00 -0700 (PDT), against all advice, something compelled Ingo Menger , to say:

I checked the speedometer in my Audi against a Tom Tom, and it looks like it's pretty much dead nuts on, just like the rest of the car.