A new take/perspecive on speedo error

Since getting my GPS set up I have been very interested in the various threads complainuing/commenting on the seemingly inhairent error in our speedometers.

This test invoved an '09 TRD/OR AC running the factory 265/70R16's.
I have installed a Scangage II and a garmin GPS.

This is what i have found over the last 10,000 miles - this data is from todays normal route and typical milage accumulated thru the day.

Analog speedometer needle is dead on with GPS and Scangage (NOTE that the Scangage had to be calibrated +4MPH to match the GPS and the Speedo needle). This is at all speeds thru the range I normally drive at.
This is very differant then what is generally reported on this site.

The interesting part is the Odometer: todays data -
Odometer: 100.2
GPS: 104.1
Scangage: 104 (does not report tenths; and this comes from the Today function)
This is a -4% error and has been the case since the truck was picked up new 4 months ago.

The GPS is accurate, there is no question- it is WAAS enabled as most are.
I would suspect and error in the speedo needle as it is a digital to analog conversion which is not a good situation for accuracy in any case, but to have a digital signal to digital display inaccuracy is, I think, noteworthy. It looks to me that there is a signal processing error band in the speedometer electronics that is corrected in the Scangage (but that does not answer the question of the why the Scangage MPH is low compared to the needle MPH)

It would be interesting to see what going to 285's would do to all this.
Would the needle be fast and the odmeter track better ??
No change ??
something else ??

 

The tire width (285) does not effect the Speedometer or Odometer.

The tire height (70 or 75) effects the speedometer / odometer.

This calculator should help you with tire size and relation to speedometer/odometer.

 

The height of a tire measurement is a ratio to the width.
So you can go with 285/70 or 265/75s and get nearly the same tire diameter....and if you go to a 285/75 it's a larger diameter than both. So if you change either number- You change the other all diameter and the distances related to them.

 

A larger tire will of the same aspect ratio will have a larger diameter. The aspect ratio (ie 70, 75, or 65) refers to the ratio of tire heighth to tread width. So a 285-70-16 will have a larger diameter than a 265-70-16.

 

And if I am not mistaken the vehicle gets its speed data differently than it gets its odometer data.

 

It doesn't matter what the needle says.... it's the data in the computer that's important. The needle is just a visual reference for the driver and might not be calibrated properly to the numbers.

My GPS & Scangauge are both reading the same. My needle is 2mph slow.

Changing to a different/larger diameter tire will change both the speedometer & the odometer data and it'll be different from the GPS.

 

So what is more accurate, the Scangauge or GPS?

Scangauge gets its data from the computer, so GPS is the only real accurate data, correct?

 

I would think so.

The only other way to compare would be using the mile markers on the highway if you could get a clean start/stop point. That would give you mileage comparisons. Without knowing otherwise, I would tend to think the speedo & odometer are all in the same system.

You could effectively check the GPS (reset trip data), againest the Scangauge (reset trip data), and the trip on your dash.

I'm not sure if the Scangauge pulls odometer information from the trucks computer, or whether the scangauge calculates this on its own. If you did a mile marker check, this would indeed answer that question.

Not to forget that it takes 1 minute to travel 1 mile at 60mph. You could do tests - on that same mile marker - that figures all that out.
You might want a 'buddy' in the car with you while you drive and they look at all the data on GPS/Scangauge/dash/etc.

 

I had speedometer errors in my 1st Gen (98), even had the speedo replaced by the dealer (no change). The speedo indicated 3-4 mph high, prompting me to wonder why I consistently seemed to be the slowest vehicle on the road. This was before GPS so my findings were based upon long streches of relatively flat interstate, mile markers, and a stopwatch. Later, the GPS confirmed all that. Since I want to actually go the speed limit, and law enforcement is so obscessed with exceeding it, I played with tire sizes to get it right. As I recall both speedo and odo were fairly close when I traded it.
Now with a 5 lug 2nd Gen 09 (utilizing GPS as a reference) I am back to the same issues. With stock tires and wheels (215/70-15) speedo shows around 3.5 MPH higher than GPS (at highway speeds), odo is 4 % low. Strange that the two are in opposite directions. There is some variation, like if the target speed was established from acceleration or deceleration.
Someone proposed in an earlier reply that the speedo/odo have different input sources. Any confirmation on that?
I would like to have acurate indications even though I can always use the GPS: what can be done to make appropriate adjustments?

Android Smart Phone
'GPS Speedo' App
'Speedview' App
Garmin GPS