Wheel size affecting speedo

[mikeukrainetz]

This is the first car Ive put oversize wheels on and from what I understood it would throw out my speedo quite a bit. I dont know if its because the speed sensors, ecu, etc are all digital information and they dont know the difference like the old gear driven speedo's but my car is at most +/- 3km/h with 20's.

I drove through a construction zone yesterday with the radar telling you how fast your going and I was doing 39-40km/h and the radar said 41km/h.

Anyone with plus size wheels and tires notice a speedo difference over stock?

My stock wheels were 14" w/ 185/65r14

My new wheels 20" w/ 225/35r20

I think theyre almost 4" dia different.