When I am traveling 100km/hr my speedo is showing 112 to 115km/hr. Is it possible to correct this with larger tires? Or would i need new wheels to make up such a huge difference? I have stock 2006 18" wheels with stock 225/45/18 and 245/45/18. From what i understand the speedo cannot be adjusted as it is tied into the stability control ABS etc. My speedo and the digital readout are the same.

 

Yes, the tire OD is what makes your speedo off. If you have the stock size tires on it, i don't see how it could be that far (12-15%) off.

 

What are you using to confirm actual speed?

 

Tw2 …You have the stock size tires with stock Overall Diameter (the wheel size is irrelevant) so your speedometer should be correct.

The tires’ OD is tied to your safety equipment on your Z, so the speedometer is correct. Anything else would be very unusual.

Why do you believe your speedometer is not displaying the correct speed?

--Spike

 

I first thought something was off when i got the car, suddenly everyone is going past me on the highway. I have followed people who have given me a signal when they are going 100 and seen that the speedo is reading way higher. Cops don't care that i am driving 15-20 over the limit and the gps agrees. These 4 things are not exact but there is definitely a problem.

Yes all stock specs which i agree is weird.

 

Only time I've seen anything like this is a friends car has 17's up front with 18's in rear. Abs lights and Speedo issues even nissan was dumbfounded, simple solution we put 18's on and issue was solved. I know you said you have 18s already.. But sounds like maybe something was wrong prior? is this speed sensor related?

 

The only other thing I could think of is different final drive gearing. Was the car modded by the previous owner?

 

Definitely 18's. The car has no signs of mods at all. Even the crappy stock cd player was left. It is a JDM import so history unknown. ABS traction control etc work fine. Would the final drive change do this?

 

As herrshaft mentions, changing the drive gearing in your differential can result in an inaccurate speedometer reading. The electronics in your car won’t know this since every value shifts the same amount. The safety systems will all work; you only see that your speed is not reported accurately by the car’s instruments.

A previous owner may have made the change to improve torque and acceleration (geared down) or mileage on the highway (geared up)

--Spike

 

Diff doesn't matter, the Z takes speed off the rear wheel speed sensor. Rear tire size is all that matters.

 

So i have to change wheels. I don't think 255/45/18 will be enough and any higher than 45 ratio will look like tractor wheels. I guess i will live with it or look out for some nice subtle 19's.