Speedometer and Odometer error corrected easily

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I'm glad someone posted this up because I've noticed about a 4.3% offset in my speed when comparing my speedometer vs GPS, so thanks OP. Now, and please forgive me if this is a dumb question, but why not just maintain a steady speed by GPS, say 60mph, and then reposition the actual needle at the 60mph hash mark?

 

I have 245/75/r16 and the odometer reads 7mph slow @ 65mph according to the gps.

 

If I understand correctly what Ive read before it cant be recalibrate or adjusted unless you splice in a device that alters the signal going into the odometer.

 

Hm. I neglected to consider the actual odometer reading.

 

As you said, your speedometer is off by 4.3%.
That translates to about 2.5mph at 60, and 4.3mph at 100.
Moving the needle down by 2.5mph will make it accurate at 60, but it will still be fast above 60, and the real problem, it will then read slow below 60.
Not a huge problem if you're doing 26.5 in a 25... but your 4% error is so close that it's really not worth correcting. You'll get a 2-3% variance as tires wear down, particularly on large off-road tires with thick tread.

But if someone is 10% off, now you're up to doing 30 in a 25 if you just move the needle... and while it falls under the common rule of "under nine you're fine, over nine you're mine", it can be a problem in a school zone.

The gear change, or electronic correction, corrects by a percentage, so it corrects the error consistently through the range.

 

Odometer and speedometer receive their signal from the same sensor, but the error may or may not be the same percentage because the two indications do to different processors.
My Kawasaki Vulcan is off by just over 10% on the speedo, but the odometer is off by about half that. I installed a SpeedoHealer and corrected the speedo to be dead on with the GPS, which left the odometer about 5% slow.

 

Who's to say the GPS is correct?
The correct way to do it is to use the mile markers on the freeway.

 

Who is to say the milemarkers are correct either, your logic is flawed...

 

Milemarkers are close enough when measuring over the distance of a full tank, assuming you don't get off the highway. There might be some discrepancy from one marker to the next due to the impracticality of installing a milemarker in the middle of an offramp, but over 200-300 miles, even an error of a full mile is going to be less than 1%.

GPS is theoretically accurate, but depending on where you are, there can be variances, as well as software issues.
I reset my GPS trip odometer same time I reset my Ultragauge and dash Trip-1 when I refill my tank. The GPS will vary by as much as 10 miles over or under compared to the other two over a 350+ mile tank.
That is most likely due to the fact that I will get rolling before the system boots up, and at work I'm staring up in an enclosed garage with no coverage so the system can't even begin to seek the birds until I clear the structure.
That accounts for the GPS being under. Today it read 9 miles high... most likely it was an accurate reading.

But gassing up in Vegas and hitting the road toward Salt Lake, I'd say that resetting the GPS Trip would get you what you need to know.




BUUUUTTTTTTTT...........

As I mentioned above, while the same sensor feeds the speedo and odometer, they most likely are not running the same error.
Just because your odometer, GPS, and milemarkers all agree doesn't mean that 60mph on the speedo is 60mph in the real world.
Likewise, adjust your speedo to match the GPS, and that's no guarantee that 100 miles on your odometer is 100 miles in the real world.

 

I have driven multiple police cars that have certified calibrated speedometers and compared the GPS mph reading. They have never been right.

It is all trivial anyway.

 

Actually checking the gear is stupidly easy. Using a 12 mm wrench, the bolt just above the speed sensor comes out. Then the driven gear and speed sensor come out as a complete unit. You can count the number of teeth or just read the molded number on the end of the gear shaft. N29 is 29 teeth. N30 is 30 teeth. I assume any number molded in would indicate the number of teeth but since I've not handled any others all I can say is 29 = 29 and 30 = 30.

You'll need a small oil pan to catch about a quarter cup of trans oil. The whole unit slips out so easily you'll be surprised at how easy it is. It's easier than changing a tire and just a little more difficult than opening and closing the tail gate. SERIOUSLY!

 

"Right" is a relative term. Would have helped if you gave figures. Say "At 65 MPH on speedo the GPS said I was doing 66 (or 64 - or worse).

Based on time trials between mile markers I've found that they can vary in distance (mile markers) by a few feet to several feet. However, on the Over All distance they are pretty accurate. So by going 10 miles based on mile markers you can get a pretty accurate time check. Doing that I've found my GPS to be relatively accurate. Well, there's that word again "Relative". Here's what I mean: My GPS reads in full MPH increments. It doesn't give you the TENTHS. So you might be going 65.4 MPH and the GPS will say 65. But go 65.5 and it will say 66. So in that respect it's not accurate.

As to driving a police Tacoma - I bet their speedometers would be more difficult to calibrate. It might take the addition of a "Dakota Digital" signal processor to get the speedo "Accurate". But then again, just how accurate? What is the acceptable tolerance? Is it (at 65) 64.9 to 65.1? Or 64.0 to 66.0? Until you know the level of accuracy, it's all "Relative". In my business, accurate can be 0.0002. Or it can be 0.06. 60 thousandths is considered a rather loose tolerance. But then again when I worked for MacDonald Douglas I learned that a jumbo jet can be as much as THREE FEET different in overall length when you add up all the tolerances. Still, 3 feet at 155 feet long is only 19 thousandths per inch.

Nevertheless, all are welcomed to share their viewpoints and experiences. Thanks to all who have posted.

One final thought about GPS: Cell phones with the feature are not true GPS units. Some people consider them to be true GPS. They depend on tower location and triangulation. NOT on satellites. You can call a Turkey a Duck but that doesn't make it one. And GPS are Digital. Depending on their refresh rate they can appear more or less accurate. Mine updates every second. I don't know this but I assume some higher end units may update 10 times per second. Though I don't see the need for it doing so.

 

Gotta keep in mind that a GPS is only going to be accurate on speed if you are maintaining a constant speed on level terrain.

GPS is a "lagging indicator"... it tells you where you WERE a couple of seconds ago and how fast you WERE going a couple of seconds ago.

Navigate a boat by keeping a compass on heading, and the GPS will show a straight line that will drift off course laterally due to currents.
Navigate a boat using only the GPS heading and your course will look like the path of a drunk leaving the bar at 2am.

Same for speed. Set the cruise for 60 on level road, and you'll see the GPS "wander" a bit until it locks in to your actual speed. Accelerate or decelerate, and the GPS will lag a bit.
Climbing a grade may or may not introduce an error. Newer software compensates and corrects speed for the distance covered on the hypotenuse. Older GPS units/software would read slow on a grade.


Unless those speedometers are "certified and calibrated" ever week to allow for tire wear, my money is on GPS.

 

Kudos on this thread!

My ride is an 08 2.7L manual 4X4 base model rolling on stock size tires.

My speedometer reads 5 mph faster at 70 mph than what my GPS is telling me.

I ordered last week P/N 33403-39365 30 tooth gear and picked it up today. Set me back a meer $27.49 with tax.

Plan to replace it this weekend. Da gum! 5mph difference!! Anyway, as long as it gets me closer than what I"m at now I'll be happy.

Over 143k miles, I wonder how many of those are actual? lol

Thanks goes out to ya"ll that figure these things out, then post the results for all to benefit from.

ROCK N ROLL

 

I want to implement this fix, where can I buy that N-30 driver gear? And how did you move the speedometer needle below zero mph? Thanks.

 

Did a test using GPS and found that my speedometer is slower than my actual speed but my odometer is perfectly accurate, how is this possible?

 

Due to your change in tire size which affects tire diameter and revolutions per mile. Truck uses same data (gear/speed sensor) but processes speedometer and odometer separately. Originally my truck was almost exact on odometer (only off .1 mile for 50 miles driven by mile marker) and the speedometer displayed 2-3 mph more than I was actually going. When tires replaced (had lost a full inch in diameter) odometer was off more - smaller diameter means it takes more revolutions to cover a mile, and the truck/software really has no way to know how worn the tires are (or if you installed a different diameter tire from oem) so both your speedometer and odometer error changes.

 

I understand everything you said. I have bigger than stock tires (265/75r16), so my speedometer should be slower than my actual speed, and my odometer should be registering less miles than what I actually travel. But for some reason when I verified with several different GPS, my speedometer is faster than the actual speed and the odometer is perfect. How is this possible, makes no sense.

 

Your experience is very similar to mine - speedometer is about 3mph fast, odometer is spot-on. Only explanation I have is software, ie the speedometer calculation and the odometer calculation are totally separate, and can/do have different error rates.

Another error-related topic = as tires wear, diameter becomes less, so error rate should change as all the speed sensor/drive gear does is count axle revolutions. Will save this topic for another thread.