Will my speedo read faster or slower?

[NASCAR Mike]

I am upgrading from the 16 inch factory rims to 18 inch SVO rims.

I am installing Nitto Terra Grapplers in size 285 / 65.

The stock tires (265/75R16) are 31.64" tall. The new 285/65R18 tires are 32.58" tall.

I have tried two online wheel calculators and one tells me that my speed at 60MPH will be 1.8MPH too slow and another says at 60MPH it will be 2MPH too fast.

Which is the correct?

I think it will read too slow but I thought I would ask here.

 

[6LMonster]

Should read just a bit slower.

 

[JRJ04]

Should read just a bit slower.

yup yup. i took a gps and compared it and it was anywhere from 1.5 - 2 mph off at varying speeds. at some points it was actually the same? dont ask me why, but it was. but in any case, your speedo should read a bit low when going up in tire/wheel size, but in your case it will be minimal.

 

[big_rooster]

yes it will be slower but the faster you go will raise the difference in how much slower.

 

[NASCAR Mike]

So when I am doing 58MPH am I really doing 60MPH

OR

is it when I am doing 62 MPH I am really going 60?

 

[6LMonster]

So when I am doing 58MPH am I really doing 60MPH

OR

is it when I am doing 62 MPH I am really going 60?

If your speedo reads 58 MPH then you are actually going 60 MPH.

 

[Scooter]

My experience doing the exact same change is that I went from doing 58 when the speedo read 60 to being spot on. I have tested it with a GPS and radar cameras, and it is dead nuts accurate now with the larger tires.