Is my speedo off that much because of my rims and tires?

[Surfstang2020]

Okay as some of you know I recently got rims and tires installed on my car about 2-3 months ago. Well the other night I got my !st ticket ever. It was a state trooper that said I was doing a 80 in a 55 on 501. My speedo didnt show a click over 60 which is the fastest I go on the roads is 5 mph over just to try and avoid a ticket. But there was a toyota yaris or some small crappy car like that that was tailgating me and I saw him fly by me before the officer pulled me over. I think his gun caught him but the officer saw my car and pulled me over ( I may be wrong). But the officer wasnt an ass or anything he dropped my ticket by 1mph so it would be 4 points instead of 6, but he gave me crap about the rims and tires knocking off my speedo. He said if I go to the judge and show them that I got it fixed they should cut the ticket in half and the points would only be 2.
My question is could my speedo be off by 20 mph? Im new to modding cars and this is the first car Ive really modded and put a bunch of money into. Any input would be helpful. Thank you

 

[Kankakee]

What size are the tires?

 

[joedls]

What size rear tires do you have? What was your stock size?

 

[Surfstang2020]

Their the nitto 555rs out back which are 240 or 245 18 out back I believe I cant remember lol

 

[Surfstang2020]

I know my stocks were little the rims were 18by 8 the gt polished rims found on the premium coupe

 

[joedls]

Doesn't help. We need tire width, aspect ratio, and wheel diameter.

 

[stkjock]

Doesn't help. We need tire width, aspect ratio, and wheel diameter.

qft....

 

[cruisin]

I cant see your tires being that much bigger but hell, heres a calculator

http://www.performanceprobe.com/ind...nceprobe.com/misc/index.php?do=calc&page=tire

 

[Chrome61]

Do you have a GPS? Most tell you how fast your going and are accurate as hell.

 

[Surfstang2020]

no I dont have gps I'll have to do a little research and check on the tire sizes, I dont know I was just curious thanks though guys, Im gonna go to court not this monday but the next, he made the court date like 2 weeks after the ticket which I thought was pretty fast I guess their trying to make their quota for the month lol

 

[DraggnRoush]

Speedometer reading is proportional to wheel diameter (because circumference = 3.14159*diameter) - Your OEM S197 tire has a 27" diameter, so let's assume you went to the largest diameter tire that fits the car, say a 30" diameter - then you'd have increased speed by 30/27 = 1.11 or an 11% increase.

So then if your speedometer read 60, you'd really be going 67...

Therefore it is NOT possible for your speedometer to be off by 20 mph until it's reading about 198 mph... at which point you'd be going 220 mph.


So the cop's radar was picking up the OTHER car!