Spedometer inaccuracy?

[IllusionX]

Originally Posted by PK198105

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.

[tetzyamis]

Originally Posted by IllusionX

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.

[IllusionX]

Originally Posted by PK198105

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.