"dont get bigger tires... it will throw off your speedo", i beg to differ Options

[lagos]

so, for years ive always had people say to stay with low profile tires, because anything bigger will throw off your speedo. im sure we've all seen the online calculators and countless threads. ive even had tirerack call me back and try to convince me to change my order from 225-45-17s to 205-40-17s because of this..lol ...

well anyway, i just wanted to say that all that, is mostly a myth. i recently got a gps for my car. my speedo is dead on with the mph reported by the gps. i was a bit shocked because i expected it to be off by at least 1-3mph. so go ahead, get bigger tires and enjoy the ride quality..your speedo will be just fine

This post has been edited by lagos: May 24, 2007 - 9:25 PM

[Supersprynt]

I was curious about this same thing. As odd as it sounds, my 18'' wheels with 35mm tires are millimeters bigger than my stock 15s & stock tires.

[Racer_J]

I was curious about this same thing. As odd as it sounds, my 18'' wheels with 35mm tires are millimeters bigger than my stock 15s & stock tires.

It's the rolling radius of your wheels that effects your speedo....

Say a Celica is supposed to have 195/55/R15 tyres

Its rolling raduis will be:-

195mm Tyre width
Profile 55 - 55% of tyre width (107.25mm)
Rim 15" or 381mm Dia (190.5mm for radius)

Rolling Radius = 190.5+107.25 = 297.75mm


My setup is 215/40/R17..

215mm Tyre width
Profile 40 - 40% of tyre width (86mm)
Rim 17" or 431.8mm Dia (215.9mm for radius)

Rolling Radius = 215.9+86 = 301.9mm

That's +4.15mm.


So for 215/35/R18's....

215mm Tyre width
Profile 35 - 35% of tyre width (75.25mm)
Rim 18" or 457.2mm Dia (228.6mm for radius)

Rolling Radius = 228.6+75.25 = 303.85mm

That's +6mm.