OK, so can somebody give the story.
I've read in passing on a few other posts that if your running the stock tire you acually getting inacurate reading on the Odo, speedo and hence MPG.

If somebody knows what the correcting percentage to multiple your miles by to get an acurate reading, please post.

 

I 2nd that^^^ Very curious.

 

1.03%

 

Does anybody know where this was first discovered? Did Toyota put this info out there?
I'm curious how this came to light.

 

Toyota published the accuracy of the odometer and speedometer in the Service Manual.

 

And I have posted a scan of that page in one of my older posts on the subject. Overall impression seems that the speedometer needle indicates speed bit faster than the actual speed. This might be done for legal reasons (to prevent lawsuits steming from speeding tickets).

Odometer is more accurate and also the ScanGauge shows speed closer to true speed indicated by radar traps. So the car is "fooling" the average driver on purpose.

There are several threads on this (as the origina poster already found out).

 

OK so that means your miles are 3% lower and MPG are 3% less than what you think your getting?
So Insted of getting 30 mpg, I'm only getting 29.1?

 

I could be wrong but I think you got better mileage than you think. According to another theread you would multiply the 1.03 by your mileage.

Example: 30mpg x 1.03 = 30.9

I hope this is right. Then we all do better!

 

I'm confused by what peteski said.
the input the computer gets for both speedo and odo come from the number of rotations of the wheel.
How can the speedo be overshooting but the odo be more acurate? It's all one input.

 

They may have the same input, but they're not the same measurement, plus one is a digital and one is analog. The short answer is they are just calibrated slightly different, maybe to reuse parts. The speedo isn't even marked in single mph increments, so being a couple of mph off isn't significant.

 

mine is DEAD ON with 17" and nittos. i recall it was very, very close with stock. within 1.5%.

 

this is very common especially in the motorcycle world. Speedos are almst always over what your actually doing. Motorcycles can be off as much as 5mph at 50. Yamaha's R6 tachs read redline of 17,500 when the actual rev limit is 15,500. It also gets you to change your oil sooner on some cars, saves the company a few speeding ticket law suits etc etc. They have a degree of accuracy to deal with so they stay on the safe side of it.
P.s. cop motorcycles are specialy(guess its a big deal) calibrated to be within 2+- mph accurate at all speeds.

 

i've watched my speedo when passing one of those radar carts (the ones that have the speed limit & display what you are currently running) and to have it display the correct speed my speedo shows higher.....i.e. 32 mph = 30 per radar.

 

The digital conversion for the speedometer appears to be still based on the ORIGINAL designed tire size for the bB/xB: 185/65R15, not the one finally settled on for the US model, 185/60R15 (BOTH sizes are listed in the repair manuals). That makes the speedometer read about 3% high.

The speedometer needle moves a certain specific distance (with some small amount of allowed error) in front of a printed scale. The printed scale is PURPOSELY printed so that the indicated speed will be progressively faster than the actual ground speed.



In fact, according to the repair manual, if the speedometer in the box reads 60 MPH when you are actually traveling 60 MPH, it is out of spec and needs to be repaired.

With my oversize tires my speedo is "right on" while my odometer reading is even farther off - 6% instead of just 3%. This means my warranty will last 6% longer, since by the time my odometer reads 36,000 miles I will have actually gone 38,160.

I am running tires that are slightly oversize, so my correction is 1.061 x [indicated miles] = [actual miles]. This means when I figure MPG I do the following:
[indicated miles traveled] / [gallons to fill up] x 1.06 = [actual MPG]

(There is NO one tire size that will make both the odo and speedo read correctly. The only actual fix would be to run a tire about 3% undersize, and put in a new speedo face in that is calibrated to actual speed.)

 

speedometers, like the lines painted on roads, are meant to be suggestions only!

 

What Tomas said. He da man on stuff like this.

Short version: with stock wheels and tires, speedo reads about 3% faster than you're really going, and odo reads about 3% less than you've really traveled.

Calibration doubtless varies from vehicle to vehicle, so if you want to know for sure, you'll have to compare against a gps, or do it the old-fashioned way using freeway mileposts for the odo, plus a stopwatch for the speedo.

 

Yes, there is only one vehicle speed sensor. This electronically feeds the car computer with a certain number of pulses per wheel revolution.

Everything else is computed from those pulses. The odometer digitally divides the pulses by some predetermined value to arrive at a 1/10 mile count for the trip milage or at a full mile count for the odometer.

Speedometer is just electrically driven analog gauge. The compter again takes the speed pulses and it then computes an analog voltage which is then applied to the speedometer to indicate the vehicle speed.

Both calculations are probably done independently. So, they both can be programmed to whatever the design engineers decided to do.

 

...and without changing ANYTHING in the electronics, one could make an accurate speedo face with a piece of paper, scissors, and a number 2 pencil...

Remember, it is not the analog needle movement that isn't "right," it is the markings on the face that are purposely in the wrong place.

Tom

 

there's only one issue with this theory guys.... if or if not toyota changed our speedo's when changing tires. I'm betting they did.

 

Well, they sure changed the odo conversion when they changed tires, but they seem to have erred: a 3% smaller tire and and odo adjustment of 6%, making the odo off 3% the other direction.

The speedo has a brand new face, in miles not klicks, so it could have been made right on, and IS right on with the original tire size, so I think they changed it to miles, THEN changed the tire size, and for some reason found it easier to just correct the manual to show how much off it should be. (Both the xA and tC are essentially right on...)

Tom