Outback Spedometer Error

[Jim Davis]

First of all, let me introduce myself, and let you know I am new to this site, and this is my first post. I have been searching the web for a Subi site that would be informative and have settled here.

 

My post concerns my 2003 Outback H6 VDC Wagon. After 18K miles of trouble free driving, I have finally found somthing to gripe about. Ever since I started driving my Outback, people have been riding my ***** and constantly passing. I thought I was traveling the posted limit. At first I attributed this to good old lead foot Montana driving habits, then noticed that it got worse out of state. Finally one day, my son inlaw commented on the possible inaccuracy of the spedo. This past Christmas I received a Garman StreetPilot GPS III and after learning to use the thing decided to check the spedo against the GPS readings. Sure enough the spedo was reading 3.5 MPH fast. Better "fast" than "slow" when it comes to the law, but obviously irritating to those who follow. I might add that I was using the OEM tires. To double check my GPS vs spedo, I checked the GPS unit in a Mercedes and Dodge Viper and both showed within .03 MPH of the indicated speed. Thus this leads my to think my spedo is wacked. Sure I could drive a little faster to compansate, but heck, this is a new, top of the line Subaru, with low miles. Not the biggest problem to ever hit this site for sure, but worthy of asking about.

 

My question: Has anyone else out there had similar experiances with their spedo. What would be considered acceptable error. Do you think this is a warrantable issue? Anyone ever tried? Thanks in advance. I will be interested in what you think.

[NOMAD327]

My 99 is about the same, a bit more than 3 high at 60, that's not too bad, It's also about 3 high at 30 which is a 10% error. I usually settle on a speed where I am passing a couple of cars for each one car that passes me, and that seems about right. My speed check is by the big outbreak of "your speed" radar speed signs at all the work areas in Pennsylvania this spring. I believe they are pretty close, other sources I have to check speed seem to concur. the state probably recycled all it's older speed enforcement guns into the new signs.

[Commuter]

I can't recall the exact numbers, but speedometer readings are one of those things where they run the statistics such that out of 10 cars, one will read (slightly) low, 3 will be right on, 5 will be high and 1 might be 'quite' high. (Don't quote me on that, but you get the idea.)

 

I think they have to be within about 5%. It would seem that you are right on the boarderline.

 

Tire wear alone can make 1% or 2% difference. And be sure you have the proper tire pressure.

 

I doubt that you'll get anywhere with Subaru on the matter. It's probably within specs, but at the limit.

 

You may want to carefully select the size of your next tire. One way to compensate a bit.

 

Commuter

[Rich in NEPA]

Speedometer inaccuracy is probably my biggest pet peeve, as well! Both of my previous OB's read about 2½ to 3 percent fast, now my '04 OB L.L. Bean reads about 4% too fast—meaning the speedo shows 75 mph when the vehicle is really doing only about 72 mph.

I've used two different 12-channel hand-held GPS receivers (Garmin GPS-12XL and GPS-map76) for determining my correct highway speed, and I engage the cruise control in order to maintain consistent speed on level sections of open roadway. GPS accuracy typically has less than 0.1 mph error when there is a good view of the sky. My tire pressure is 32 psi all the way around (a 1 or 2 psi change doesn't amount to any significant change in speedometer readings, perhaps less than 0.5%). I have even used the stopwatch timing method between mile markers and got similar results. The GPS method is more convenient and reliable because it's closer to real-time.

It's certainly a PIA constantly having to factor in the error while driving on the Interstates. I wish there was an easy way to correct this without having to go to a speedometer repair & calibration shop! The strange thing is that the odometers have always been pretty much dead on!

Every once in a while one of the major U.S. monthly car magazines runs an article on speedometer accuracy. The latest one that I recall said that, in general, the speedometers in Japanese vehicles tend to read higher, in domestic vehicles they read fairly close to actual speeds, and in European vehicles they typically read lower. I believe the magazine article also said that less than 5% error was considered "acceptable."

Cheers ...

[Hondasucks]

I wonder if there is a resistor pack in there somewhere, I know on most GM vehicles with electronic speedometers, since they only use one speedo unit (different face though..) for most of their cars, there is a resistor pack under teh dash somewhere that tells teh onboard computer what size tire is on the car. So a Cavalier (yeech!) and a Silverado can have the same speedo unit..

 

Anyhoo, you can get aftermarked boxes to re-calibrate those, I wonder if the Soobs have a similar setup (since they prolly share speedos, but not tire sizes..)

Also, my auto instructor said that he heard somewhere that GM was purposely calibrating their speedometers to read faster than the actual speed, to get the car outta warranty faster. The recalibration boxes are kinda hard to find, since GM had sued several of the manufacturers, fearing people would set their Cavalier to have 40 inch tires, so the warranty period would last longer.....

[Beausabre]

Hey there. New Subaru owner - just bought a Baja two weeks ago. Noticed a similar error this week on a long trip also when using a GPS. The error seemed to be a constant 3 MPH, regardless of the speed. Called the dealer and they said it was set that way on purpose as a "safety factor." Also, the odometer seems to track with the GPS accurately.

[Gnuman]

In general, it has been my experience that the cops will give you +-5mph for "speedometer error" before they even look at you for speeding. More likely, however, is that less than 5MPH is not worth thier time to stop you when there are idiots that will do 20 and 30 over the limit all the time. . .

[cookie]

officer. He told me he never bothered anybody with 10 MPH of the limit. I habitually drive about 9 over and have only gotten a ticket when I neglected my own advice. Pink Floyd on the box, sunny day, etc....

My Subie is about 3% slow and my BMW is about 4% slow and not linear. Treat it like you do a gas gauge, you can never really trust one.

[99obw]

The SAE dictates that speedometers should be accurate to within +-4%.

 

Our suby is about 1 MPH slow with it's latest speedo head and the jeep is about 3 MPH slow.

[99obw]

As a follow up, this is a response a friend of mine got from SOA on this issue:

 

Thank you for contacting Subaru of America with your concerns. The automotive industry generally follows standards for speedometer and odometer accuracy as set forth in SAE (Society of Automotive Engineers) Standard J862. According to this Standard, the odometer records actual distances traveled within plus or minus 4 percent at 20,40 and 55mph. Tolerances such as this are acceptable due to the finite usable number of gear teeth on the ring and pinion drive assemblies and other measurable factors.

 

With regard to road speed, a tire may experience as much as a 3 percent change in revolutions per unit of distance from 30mph to 90mph due to a change in rolling radius caused by centrifugal force. Yet another variable to consider is that a speedometer is calibrated at room temperature by instrument manufacturer. SAE Standard J678 outlines recommended calibration tolerances for the instrument itself. But, consider how often a vehicle is operated outside of the parameters of room temperature. Considering the factors which affect the odometer readings (takeoff pinion design limits, tire make, tread and construction, inflation pressure, tire growth, tire size, centrifugal effects, operation temperature and load increase on rear axle) a certain degree of tolerance is to be expected.

[Strakes]

Hey guys, this article may help you:

 

http://www.caranddriver.com/article.asp?section_id=4&article_id=1906&page_number=1

[mtsmiths]

Well, Jim Davis, whether you speeding or blocking traffic (so long as you're in the right lane) welcome to the board. Nice to see another Montana member.BTW, what's with all the hot weather in GF this winter? You guys got a permanent Chinook, or what?

[outback_97]

Wow, this was an old thread, but I missed it first time around. Very interesting read. When we put WRX wheels / tires on our TS, I was concerned that the speedo would read too low since they're 3% bigger than the stock tires. Actually, it is now much more accurate according to roadside radar displays.

 

Steve

[forester2002s]

With all the technical advances of recent times, maybe it is time to rethink how a speedometer should work.

 

Could a speed-signal be taken from the road surface below the car, e.g. using a laser beam perhaps? That would eliminate all of the variables listed in the above response from SOA.

 

Just thinking out aloud...

[mtsmiths]

Hmmm, interesting thought. I wonder how it would read from: wet pavement, grass, Black Ice, a foot of fresh unplowed NW powder (aka Montana Peruvian Flake), one of those 'humming' bridges that you can see through to the water?

[cookie]

record speed by GPS. It would not be much trouble to develop a GPS based system I think.