Odometer Accuracy; Speedometer Accuracy '09 1.8L Automatic

[jake75]

On a recent trip I checked my odometer with the mile-marker signs. When driving 20 miles by the mile-markers my odometer read 19.5 miles. SO for every 100 miles racked up on the odomenter I actually have driven 102.5 miles. SO - if miles were a factor my warranty would actually be 36,900 miles. Also, my calculated gas milege would be understated by 2.5%. (30 mpg would actually be 30.75 mpg).If those road side speed meters are correct the speedometer seems to be pretty accurate. Looked to me that it was over-reading only about 1 mile per hour. I would have set it exactly on 60 mph and timed how long it took to go 1 mile by the mile markers but I would have been hit in the rear if I tried. In my 195 mile drive from Lexington, KY to Columbus, OH via I-71 and I- 75 on July 5th I noted that the cops were making their speeding ticket quota and then some.

[808 Vibes]

Oh nooooooo...he's giving us a math test!

[jake75]

That's not a math test - I gave you the answers! I always thought that the manufacturers would set their odometers to over read - that way you think you are getting more mpg than you really are, and also your 36,000 mile warranty arrives sooner. I had a 1977 Buick that registered 10 miles for every nine miles actual. Therefore to find the Vibe under reading miles surprised me. Maybe when the tires wear down it will be accurate. The circumference is less because the radius is now less. Math test: How do you calculate the circumference.(a) PIE (3.14) r squared(b) PIE (3.14) d(c) PIE / d(d) none of the above

[BlueCrush]

(e) PIE(3.15) R round

[lowincash]

lol yeah to be honest when i first read this thread it gave me a headache...i was never good with numbers hahabut all i know is that my speedo reads 4-5mph more than what i'm actually driving

[lovemyraffe]

My speedometer is off too. I have checked with mile markers and with GPS. When my speedometer says 80 I'm actually doing 82. I get about 9.5 miles on the odometer while mile markers go 10 miles.

[star_deceiver]

My GPS says I'm doing 102km/h when the speedometer says 100. (I wonder if the cops'll buy that excuse???)

[Old Tele man]

...according to our local speedometer shop, most current GM digital-analog speedometers read "2 mph" higher than actual speed....had my '09 Vibe 1.8L 4-speed automatic done and it was consistant from 10 mph all the way up to 80 mph, ie:10 mph actual = 12 mph speedo...40 mph actual = 42 mph speedo...80 mph actual = 82 mph speedo

[lowincash]

do you think the stock 18's on my GT makes the speedo off more? mine is off by like 4-5 compared to your 2.

[gwm]

lowincash,My 2009 GT's speedometer shows 64 mph when I'm actually doing 60(according to Interstate mile markers). So I think the 18" wheels make the GT read a little more optimistic.

[Ace]

A lot depends on the tires. With my winter tires (studded snows), my actual speed is about 2 mph higher than on my summer tires. All this is according to my GPS, though the main difference in speed reading seems to occur at over 60 mph, under that and it seems there isn't a big difference.

[ned23]

I calibrated mine with a GPS and it's reading about 2-3 mph over the actual speed. The reasoning behind this isn't as arcane is it might seem. All they do is run basic statistics on the accuracy of the guage. Let's sa, for example, that the tests show that the speedometer is 98% accurate with a confidence interval of +/- 2 mph. So, they will set all the speedometers 2 mph high. That guarantees that for 98% of the drivers, the speedometer will read no lower than the actual speed they're moving. The speedometer then will show the speed as anywhere from 0-4 mph over the actual speed.

[Old Tele man]

...do you WANT your speedometer/odometer to perfectly accurate?...if so, then GOOGLE "VAGdashCOM" by OBD Factory Auto Electrics Co., Ltd. (China!)...they're the same people who manufacture the MaxiTrip TP100 Auto Computer (very similar to ScanGauge)....the VAGdashCOM is "...unique advanced diagnostic tool to read out the internal memory of your instrument cluster. You can display the login code, recalibrate the odometer and rewrite the..." [emphasis is mine]

[joatmon]

Quote, originally posted by Old Tele man »...do you WANT your speedometer/odometer to perfectly accurate?...if so, then GOOGLE "VAGdashCOM" by OBD Factory Auto Electrics Co., Ltd. (China!)...they're the same people who manufacture the MaxiTrip TP100 Auto Computer (very similar to ScanGauge)....the VAGdashCOM is "...unique advanced diagnostic tool to read out the internal memory of your instrument cluster. You can display the login code, recalibrate the odometer and rewrite the..." [emphasis is mine]500 -700 euro? Ouch! Do they even support toyota clusters?

[jake75]

Quote, originally posted by Old Tele man »...do you WANT your speedometer/odometer to perfectly accurate?Nope! I'll just assume reads 2 mph high and spend that money on something more fun!

[BahamaTodd]

But you also have to remember that your odometer is also reading too quickly meaning you're putting miles on quicker. Your warranty and overall value is therefore going away quicker.

[jake75]

Quote, originally posted by BahamaTodd »But you also have to remember that your odometer is also reading too quickly meaning you're putting miles on quicker. Your warranty and overall value is therefore going away quicker.Per my earlier post, there appears to be a disconnect between the mph and the odometer. The odometer is under reading - a fact I verified via checking against mile markers. 20 miles via milemarkers = 19.5 on my odomter.In comparing a mph to one of those roadside speed meters in a construction zone, I was about 61 mph on my speedo and the roadside speed meter said 60. But that is probably not as accurate observation as the milemarkers. On a level roadway I need to set the cruise on 60 and see how many minutes/seconds it takes me to go 5 miles by the mile marker. Should be exactly 5 minutes. If it is over 5 minutes then the speedo is under reading - I am not really going as fast as the speedo says. If under 5 minutes, then I am actually going faster than the speedo says. As for the warranty - I will never drive 36,000 miles in 3 years or 100,000 in 5 year as so that is not an issue. A few miles one way or the other is probably not going to have any appreciable effect on the sale value especially since I only drive 4,500 miles a year tops.

[BahamaTodd]

Really? Hmmm, I would think that the speedo and odometer read off of the same signal. Interesting...

[JetBlack09VibeGT]

By all accounts the tire size is wrong on the 2009 Matrix and 2009 Vibe with the factory 18" wheels. They are 215/45/18 and they should be 215/35/18. The only problem with that equation is the ride would be way too harsh with the much lower side wall. The dealer should be able to make an adjustment with the OBD Computer and correct this issue as it is a software issue. I will be finding out if this can be done with my dealer soon.

[joatmon]

I don't know about the 09's, I expect it works like it does on the 03s. On my 03, there is a vehicle speed sensor on the transmission that puts out a square wave, a series of pulses. If the vehicle is going fast, the pulses are shorter and packed closer together, if the vehicle is going slow, the pulses are wider and spaced farther apart. The odometer basically just counts the pulses. each pulse represents a certain distance when I was messing about with my cluster, I found that a pulse train of 140 cycles per second was about 120 mph, works out to be about 4200 pulses per mile. If the tires are the wrong size, then the number of pulses per mile will be off and so will the odometer.The speedometer does work off the same signal as the odometer, but instead of counting the pulses, it effectively measures how fast the pulses are. It uses some kind of electronic circuit to convert that frequency to a voltage to drive a motor that rotates, moves the speedometer needle. If the tires are the wrong size, then the frequency is not accurate, so neither will be the speedometer reading. There are also variances in the motor, and exactly where the needle is placed on the shaft of the motor. I noticed when you first apply power to the cluster (like when you first connect it to the car) the speedometer needle does a sweep. It's possible thats a calibration cycle to remove variances in exactly where the needle is on the motor shaft. But I don't know.The signal that feeds the cluster goes through the ECU, you can hook up a scangauge or similar tool and read out what the computer thinks the speed and distance are, compare them to the cluster. That would show you if the cluster is off, or if the signal is off.The signal from the sensor goes throught he ECU, and not directly to the cluster, so there may be an adjustment that can be flashed into the ECU to calibrate the signal going to the cluster, but that's just conjecture on my part.The key thing is to figure out on your individual car if the speedometer is accurate, and if not, which way and by how much, so you can avoid getting a ticket.

[jake75]

Very interesting how that works - I enjoy learning about things even when I have no reason to know.

[Kincaid]

Resurrecting this old thread because I finally borrowed a GPS unit to confirm my speedo is off too. I have the 2.4L engine, manual transmission and 17" alloy wheels.At an indicated 60 mph on the speedo I'm actually moving at 57 mph. I've no idea if the odometer is off by the same margin. I'll need to do the mile marker test on that.

[engineertwin2]

No one seemed to mention the +/- accuracy of the mile markers yet in this thread (2.5% would be 130 ft - how precise are the mile markers? How good are we at getting the exaxt same marking location - at 60 mph its less than 1.5 secs).I think all indications (both speed and ODO) should be considered approximate regardless of how accurate we think they are.As for them being under or over, I would expect that more and more, the ODO under reports mileage given the risk of Class Action lawsuit for some claim such as "deliberate attempt to void warranties prematurely."Anyone checked the trip meter to the ODO? after having logged every tank of gas for the last 77k miles, I can tell you that when entering data into the spreadsheet, I have an ~500 miles difference between the sum of the trip meters and the reading on the ODO.

[vibedrivermatt]

Quote, originally posted by engineertwin2 »No one seemed to mention the +/- accuracy of the mile markers yet in this thread (2.5% would be 130 ft - how precise are the mile markers? How good are we at getting the exaxt same marking location - at 60 mph its less than 1.5 secs).I think all indications (both speed and ODO) should be considered approximate regardless of how accurate we think they are.simple. just say wait for it... wait for it... wait for it.... NOW!!! thats how you get less than 1.5 second accuracy in between the mile markers.but really shouldn't the GPS be a pretty good indicator of how accurate the speedometer is? When using the navi, it was saying my car was going 1.5 to 2 mph slower than what the speedo was reading. and if speed should be considered approximate then why should we trust a police officers radar gun when he says we're speeding. Surely he has some accurate way of determining our speed right? otherwise I want all my money back from the city/state.

[engineertwin2]

Well, to counter your point regarding police officers documenting your speed, typically you are cited or fined at some amount over the speed limit. This is usually greater than a couple miles per hour (10 mph is the sterotype). When its 10 mph over the speed limit you can argue the accuracy of the instruments all you want, but the fact remains you are speeding regardless. You have a legitimate argument at 1 or 2 mph over.Regarding your "wait for it, wait for it", you may get it within 1.5 secs, but how can you ensure you are in the right spot? It will vary by several feet. Additionally, perspective is different as mile markers may vary in their distance from the road. We've already established you may or may not be going 60 mph...The point of my argument was that there is a lot of variability in the measurement, but in the long run, its just an approximation so lets not put all the faith in the world that these instruments are accurate down to the tenths and that some "error" should be accepted.

[jake75]

I forgot about my original post of a year or so ago. I just drove from Columbus to Wilmington, NC in the Vibe. I wondered why the distance on the trip odometer was less this time (compared to driving my T&C). This thread reminded me why.

[Bikerscum]

Quote, originally posted by vibedrivermatt »but really shouldn't the GPS be a pretty good indicator of how accurate the speedometer is? Absolutely. And, your car knows exactly how fast it is going too. Connect a scanner like a Scangage to the OBD 2 port & it will agree with the GPS, cop's radar, etc. The problem is with the instrument cluster. They just read fast. Had mine replaced.... still reads fast. GM's bottom line.... they just read fast, deal with it.

[Kincaid]

As long as the odo reads accurate I'm cool with it. I am thinking up going up in tire width 10mm when it's time for replacement, which should get things closer.

[jake75]

Quote, originally posted by Kincaid »As long as the odo reads accurate I'm cool with it. I am thinking up going up in tire width 10mm when it's time for replacement, which should get things closer.But my odo is reading 97.5 miles for every 100 actual miles. Not a big deal now that I know about that.

[Old Tele man]

...if you're wanting accurate MILAGE, simply divide that "97.5/100"-ratio into all MILAGE numbers you read from the dashboard speedometer/odometer, like this:UNCORRECTED: MILES = 150 miles (indicated)....CORRECTED: MILES = (150/0.975) = 153.8 miles (true)...or, you can do the samething for MPG values:UNCORRECTED: MPG = (150miles/5.0gal) = 30.0mpg (indicated)....CORRECTED: MPG = (30.0mpg/0.975) = 30.8mpg (true)