Id beleive the GPS. Ive heard of quiet a few people whose speedos were off about 2-3 mph

 

GPS should be accurate to say 0.5 mph due to signal degradation. The cars speedometer is manufactured with a much higher error rate that could be 5 mph or more. The max error is specified by the federal government.

 

Generally, our speedometers show the truck going a few miles faster than it really is. Once I put on 275/70 tires the speedometer now matches what my GPS says.

 

so is the scangauge2 more accurate also?

 

so is the scangauge2 more accurate also?

Should be. My Scangauge (1) is dead-on with the radar signs the cops like to put up occasionally. My tacoma and corolla speedometers both read about 3mph faster than I'm going with stock tires according to those signs as well.

 

My SG says that I am 2 to 3 mph slower than the speedometer reads.

 

My SG says that I am 2 to 3 mph slower than the speedometer reads.

oh ok. i found the same true with mine, just didn't know which one was right though.

 

2-3 mph is really not that bad.

BMW is the worst when it comes to calibration error. The consensus on my R1100S motorcycle forum is that the speedometer is off by around 17% at 60mph (indicates 70 vs. GPS's 60). I'm pretty sure my wife's M Roadster is pretty far off too. I'm guessing this is done so services are done 17% more frequently, thus more $$$ for the stealerships.

I figure my R1100S with 11,800 miles on the odometer actually has more like 9800 miles on it. This definatly effects resale value, and most of us are not terribly happy about that.

 

I remember an article a few years ago in Car and Driver which tested the accuracy of speedometers. From what I remember of the article, the faster you go, the more innacurate the speedometer will be by overstating your speed. This is considered to be normal.

Your GPS will be more accurate.

A couple of times, on a racetrack, of course, I took my C5 corvette on a high speed run and my GPS recorded a max speed of 152 mph...no kidding...on both attempts (on a racetrack, of course) I maxed out at 152 mph. It seems that my brain has it's own internal govenor which will lift my foot off the accelerator whether I want it to or not. The speedometer, from what I remember from both occasions, was right around 160 or maybe a little over. I must admit my attention was on the road...of the racetrack I was on...

of course.

 

Generally, our speedometers show the truck going a few miles faster than it really is. Once I put on 275/70 tires the speedometer now matches what my GPS says.

That is one fix. But what is the down side? My concerns would be slower acceleration, worse gas mileage (or maybe better). I get paid by the mile at work. I could see it decreasing my take. But then it would also put less miles on the truck.

 

That is one fix. But what is the down side? My concerns would be slower acceleration, worse gas mileage (or maybe better). I get paid by the mile at work. I could see it decreasing my take. But then it would also put less miles on the truck.

I get paid for mileage too. I just round up a few miles here and there. I have 285/75 16's on that throw the odometer off about 7% so I add 3-5 miles for every 50 I drive for work. Sometimes more depending on my mood. :lol:

 

In most cases your speedometer will be off but not your odometer. With all the technology these days vehicle manufactures would be in a world of lawsuits if the odometer was not spot on accurate and you lost out on value of your vehicle due to the mileage being more that what is really on the there. Also I noticed that when I drove to my deer lease before I put the lift on it was about 92 miles. After I put the lift and bigger tires on it was still 92 miles. explain that! I will check it out and compare it with my sg that I got for Christmas and see if my odometer is the same as what my sg reads.

 

Your GPS specs should list an accuracy -but note that is best possible.
Will vary with number of satelites and their relative positions.
(6 directly overhead don't give very good triangulation data, whereas several spaced out at 40 deg above the horizon will give dood results.)

The other issue is that the GPS is giving you a "quasi instantaneous speed". It is computing position, then a fixed time later, computing another position, subtracting the 1st from the 2nd, dividing by the time increment. The time increment on all but really crappy GPS will be fairly small, improving the accuracy, but the speed is an average over that increment, and therefore doesn't account for curves (horizontal plane) or hills (which increase distance traveled vertically) except for straight line, point to point. So, if you've got a pretty good gps, a good selection of satelites (a good gps will have a display page to show you relative locations of the satelites it's using), you can assume that the accuracy is pretty close to the specs in the manual. For a straightaway run, it's a good comparison.

Checking my Tacoma and Odyssey odos against mile markers over 10 miles indicates about a 2% average error.

 

My '05 Taco speedometer consistently reads about 2-3mph faster than my portable GPS unit measures at the same time. I have stock wheels/tires on the truck, so I assume the speedometer should have been originally calibrated to reflect the correct speed for that tire size.

Does anyone know a GPS unit (which measures speeds to the 10th of a mph) should be more accurate that the stock speedometer?

.

I just got my 07 Taco and noticed this the other day when I decided to to see if the speedometer was off or not like my Yukon. And yeah sure enough I have the exact same thing as you, a constant 2-3mph difference when I had cruisecontrol set in the 65-75 range. Pretty sad considering its a brand new truck if you ask me

 

I've got a huge difference on my Regular cab 4x4 5speed. At 60mph, the GPS shows 56mph +/-.5. That is with top signal accurate to 3m. Almost 7%. No wonder everyone at work says I drive like a grandma.

Odometer is a little better. I had 1 mile difference after a 26 mile trip so almost 4%.

So when my truck gets 24.0mpg on a tank, I really achieved 25mpg.

Time for bigger tires!

 

So has anyone established yet if the odo and speedo are equally inaccurate or not? On my first drive home from the delaership I could tell my speedo was off. But when I checked it to my pre-measured distance by my house, I found the odo to be very accurate.

What is the best way to establish the level of inaccuracy and more importantly, what is the best solution?

 

By the way, if a simple tire size change will fix it (without screwing up the odometer), then here is a great tire size calculator that allows you to compare different tire size options and see all of the differences (revs per mile, % off- too fast/slow, etc)

http://www.1010tires.com/TireSizeCalculator.asp?action=submit

It's also good for guys looking to throw some nicer rims/tires on their truck without messing up the speedo.

 

So has anyone established yet if the odo and speedo are equally inaccurate or not? On my first drive home from the delaership I could tell my speedo was off. But when I checked it to my pre-measured distance by my house, I found the odo to be very accurate.

What is the best way to establish the level of inaccuracy and more importantly, what is the best solution?

It's only the speedometer that is off. One of the reason being is that the digital signal (from ECU) is converted to analog signal to turn the speedometer motor. The odometer is accurate because it is a digital display. I've compared the tripmeter and scangauge and they are the same b/c they are getting the same signal.

I've compared GPS and Scangauge and they are pretty close for the speedometer. Trust the GPS and scangauge. If you've changed your tires/wheels, trust the GPS and adjust the scangauge accordingly.