Spedometer inaccuracy?

[otterhere]

Mentioned in another thread:

I'd swear my spedometer shows me going 10-20 miles faster than I'm actually going; has anyone else noticed this, or should I report it to my dealer??? Not sure how to measure this accurately except that yesterday I was just moving with the flow of traffic on the interstate (70 mph; most do 75-80 mph), and it showed me doing 90!!! Not everyone else was also driving a Yaris, either...

:confused:

[contraband831]

Mine is off by about 5mph, i noticed it with my GPS, but i don't mind.

[tetzyamis]

Tire size will make the speedo off, but not that much off.
Let's say your original tire size is 185/60/15, to make the speedo off by 10 mph at 60 mph, you gotta put tires like 165/50/14.
I don't think tires that small even exist.

I would ask a friend to drive next to you and ask him how fast he's going.
If your speedo is off, then I'd take it to the dealer.

[otterhere]

They do substitute 15" wheels for the original 14" when they add the packages (I got convenience and all-weather, only because I couldn't avoid them)... Could that be it?

[tetzyamis]

They do substitute 15" wheels for the original 14" when they add the packages (I got convenience and all-weather, only because I couldn't avoid them)... Could that be it?

No, that will make the speedo slower, not faster.
If 175/65/14 says 50 mph, 185/60/15 would say 48.35 mph.

[nemelek]

Making the asssumption that the speedometer and the odometer are linked together, the odometer is easy to check using the highway mile markers.

[CKaelin]

According to my scan guage my spedo is pretty much dead on. Maybe a mile per hour fast.

[WeeYari]

Making the asssumption that the speedometer and the odometer are linked together, the odometer is easy to check using the highway mile markers.

Just as easy for speedo too. Hold constant speed between markers and time it.

[CTScott]

According to my scan guage my spedo is pretty much dead on. Maybe a mile per hour fast.

The speedometer and scanguage get their data from the same place. The Yaris' speedo is purely electronic. If you see a difference between the speedo and the SG, it is because of the SG's lag in reading and displaying data from the OBD port.

[voodoo22]

Speedometers on all cars are set to read faster than you are really going, and that's correct. Putting on 15 inchers in place of 14's would make your speedometer read about 3-5% slower depending on overall dimensions.

[b_hickman11]

It doesnt matter if you have the 14's or the 15's. If it came from the factory then the speedometer is set using the rims on it. I had a 05 Taco with factory 20's which was an upgrade. The regular rim size was 17. My speedometer was set for the 20's.

[tetzyamis]

it does matter , if the dealer doesn't reset the ecu. if you change your overall diameter the speedo will be off.

Hickman said "If it came from the factory", which means dealer did not change the rim.

[TinyGiant]

i'm about 5mph off with the summer tire/rims

[ddongbap]

Are you guys taking into consideration the overall diameter of the rubber themselves? It really doesn't matter how much off the metal rims are.

[IllusionX]

yes but if downgraded after to 17's without resetting the ecu the speedo would be wrong


No. It doesn't change a thing. A sales guy at Brossard Toyota (he doesn't work there anymore) did the test with 2 echos. a hatchback with factory 14" (175/65R14) and a sedan with factory 15" (185/60R15). They did many tests driving side by side... both at 100kmh, the 15" would over take the 14". The sedan would have to drive at 96kmh to stay equal with the hatch, and the hatch has to drive at 104kmh to keep up with the sedan.
The test was done to prove that toyota did not adjust the ECU to reflect the different overall diameter size of the wheel/tire.
In fact, to match the 175/65R14, you would need 185/55R15.

[Bob_VT]

Mine is quite accurate.

GPS, Speedometer and SCII all show exactly the same mph. I have noticed that the GPS unit has a lag in the reading.

I am running stock tires on steelies.

[otterhere]

Wow, y'all have given me a headache; did I mention that I'm math-challenged??? It doesn't bother me; I just move with the flow of traffic anyway, keeping a safe distance behind whoever's in front of me.

Certainly not going to take it back to the dealer and complain; too happy.

:smile:

[floydisrock]

Math is easy--> http://www.discounttire.com/dtcs/infoTireMath.dos

[IllusionX]

Wow, y'all have given me a headache; did I mention that I'm math-challenged??? It doesn't bother me; I just move with the flow of traffic anyway, keeping a safe distance behind whoever's in front of me.

Certainly not going to take it back to the dealer and complain; too happy.

:smile:


but you don't want your ODO to show 20% error if you are on a lease.

[tetzyamis]

No. It doesn't change a thing. A sales guy at Brossard Toyota (he doesn't work there anymore) did the test with 2 echos. a hatchback with factory 14" (175/65R14) and a sedan with factory 15" (185/60R15). They did many tests driving side by side... both at 100kmh, the 15" would over take the 14". The sedan would have to drive at 96kmh to stay equal with the hatch, and the hatch has to drive at 104kmh to keep up with the sedan.
The test was done to prove that toyota did not adjust the ECU to reflect the different overall diameter size of the wheel/tire.
In fact, to match the 175/65R14, you would need 185/55R15.

That's an interesting test they did.
I always thought Toyota would adjust the speedo according to the tire diameter.
I guess my speedo shows 3.3% slower than I'm actually going, but that's no problem.

[b_hickman11]

yes but if downgraded after to 17's without resetting the ecu the speedo would be wrong

Arent the tires on the 17's smaller than the ones on the 15's? I always thought the bigger the rim, the smaller the tire.... Usually you can stand them up side by side and they will be the same height. One has more tire while the other has more rim.

[IllusionX]

well then i'm not understnading something with my 195/55/15 while my speedo says 120km/h the gps says 123km/h. while on stock tires 175/65/14 the speedo is dead on. so there is an error, or am i missing something

I'm running on 195/55R15 in the summer.. and 195/60R15 in the winter.

That's interesting, i should grab a GPS to check this out again. I haven't really checked out my speed with a GPS since i'm not running on stock tire size.
And btw, we're allowed 10% error on the speedo, so I do not think Toyota would bother making another part for models with different tire sizes.

[tetzyamis]

To get diameter in mili meter:

Width/Height/Rim
Rim * 25.4 + Width * Height / 100 * 2

So,

175/65/14
14 * 25.4 + 175 * 65 / 100 * 2
= 355.6 + 227.5
= 583.1 mm

185/60/15
15 * 25.4 + 185 * 60 / 100 * 2
= 381 + 222
= 603 mm

And,

1 - 583.1 mm / 603 mm = -3.3%
which makes 185/60/15 to show 3.3% slower in speedo than 175/65/14.


I always thought Toyota adjusts this as PK198105 says.
But IllusionX's dealer's test shows that Toyota doesn't.
I guess I gotta find that out myself by testing mine since it came with 185/60/15.

[IllusionX]

they dont but the dealer will readjust the ecu, at least my dealer does. as a rule as soon as the diameter changes the indicated speed will be different from the actual speed. therefore if your wheel diameter changes but your overall diameter (wheel+tire) stays the same or close to the original then your speedo will be ok , but if the overall diameter changes from stock then the speedo will be wrong

which dealer do you go to ?

And i just don't think the ECU is THAT flexible in reprogramming ??? Because if the dealer has the resources to do it, then other people out there can do it. At least, the mechanics has to learn how to do it, and sooner or later, the information will leak.. People will use this to ALTER their ODOMETER readings, hence illegal... ???


haha btw, PK, i posted in the Quebec forum with the question.. let's hear the answers...

[IllusionX]

non ta vitesse est calculer avec les speed sensor dans le centre des bearing de roues pour un vehicule avec ABS et dans la trans. si pas ABS


translation: the speed is calculated with the speed sensors at the middle of the wheel bearings for cars with ABS, and it is in the transmission for those without ABS.



sooooo........... if you are 20% off, you might want to take it back to the dealer to have them checked. Because if your speedo is showing 60mph and you are really going 80mph!~ Hmm... it could be very bad! that's 250$USD speeding ticket in Champlain, NY

[Chrsto4]

Interesting thread. I just thought I was crazy because my GPS usually says I'm going faster than my speedo shows. Hmmm....I AM sane - who knew. :iono: