Hi guys,

I have a problem that's a bit baffling. Just got my '07 350z HR (JDM) base model and I'm finding that the speedo reads too fast. I have a feel for certain speeds and had a feeling it was quite off so I hooked up my TomTom gps and sure enough the speedo reads about 5-6 km/h too fast.

What size are my tires you ask? Fronts 245x45x18 Rears 265x45x18 which are just a bit wider than stock but no shorter. The Miata.net calculator says the speedo should be SLOWER if anything.

This makes no sense. I've been reading about ECU resets but don't know much about that. Is this an ECU thing or is it a physical sensor that is off?

Thanks

 

Im running 19's and mine is off 2 mph at slower speeds and 3 at higher. 245x35x19 is what I was told to run without issues. In for an answer.

 

Dial indication of a speedometer of a mass-produced car is set to read higher for several reasons; 5-6 km/h (3-4 mph) is fine.

 

265/45/18 is both wider and taller than 245/45/18

 

Are they? aw $%&* those are going back for 40's then. Cheers.

 

Use this when choosing correct tire sizes.

http://www.miata.net/garage/tirecalc.html

 

From my 1st post "The Miata.net calculator says the speedo should be SLOWER if anything."

The calculation of this wider tire says that when speedo says 60 you are actually travelling at 61.5

My problem is the reverse. When my speedo says 60 I'm actually at about 54-55