The reason I ask is becuase I just changed my rim and tire combo on my 2004 Scion xB from the factory to 18 inch rims with 40, R 18 and I just got two tickets back to back for speeding!

Can I fight this in cort becuase the dealer witch put them on did not change the settings to match this causing me to get the tickets?

Thanks!

 

Well changing the rims without getting the right size tires will throw off the speedo. Think of your rims and tires as together and you add them together to get a diameter. The new rims and tires should come close to the stock diameter. Therefore your new tires on your rims are either too short or fat. You should put 35's instead of 40's for those size rims.

 

At one point I had the same rims on my xB with 35's, but they had too short of a side wall for me so I took them off for a while and when I got the new tiers I put them back on and this was when I started getting tickets!

 

it's not the rim size that changes it.. but the entire wheel itself.

you can't change the "settings" for anything.. it won't hold up in court.

 

weird.. I have 19's on mine and I don't have that problem

 

I know man I have not gotten a ticken in over eghit years and now I got two of them in a month and when the cops told me that I was speeding I was kind of off with it becuase when I got taged I looked over and my speed o showed me going about two miles over the posted!

I give up on it and started to drive five under, but that is no fun driving five under!

 

The only thing that changes the speedometer is overall diameter of your tire.

 

I know that, but what is causing this then? I also have been getting others flashing there lights at me like I have my high's on after getting my rims back on with the 40mm tires on!

I have to say that my lights look very high now, and I did not change anything with my lights!

 

There's more to it. Post the whole thing, before the 40. 215? 225? 245?
Then someone can tell you how far out you are.

 

if your wheel and tire have a bigger overall diameter than the stock tires you replaced them with, then yeah, your speedo isn't going to read properly. i actually think there is a calculator online somewhere that tells you how off your speed will be. like 4 mph over or whatever.

as for court. just ask the judge for some leniency and plead about your 8 year streak and that your latest alledged moving violations were 100% unintentional and you were genuinely not intentionally violating a law of voluntary compliance, and maybe show that you have taken the proper measures to adjust your speedo so that it gives you a correct reading.

if you can convince the judge to give you the benefit of the doubt, then maybe you can get out of these two tickets.

 

215/40R18 tires are 3.5% larger in diameter than stock. Meaning that your speedo will read 3.5% low so at 60 MPH speedo reading you will be going 62.1 actual speed. A 225/40R18 is 4.9% larger. An easy to use tire calculator can be found at www.miata.net/garage/tirecalc.html

 

Depends how much you were over the speed limit.
If you were 15 miles over - and tire size difference is 2-3 mph you
have a tough argument.
Go plead your case - the worse thing they can say is no.
Ask the Judge - now that your speedo is corrected, If you go ticket free for the next year can the tickets you got be null and void? - Now slow down, Buy a V-1, and
Good luck. . .

 

That is great info! I am planing on calling them today and try and talk to the D.A. and tell him what is going on and see if he will reduce it down to a none moving and this will give him a win and the city money and it will not be as much to pay and not show on my driving record!

Thanks!

 

Have you changed your springs? When I added the Tein S-techs I had people flashing me also. I think for the most part we just have very bright lights. I think that with the new springs I bounce a little more also which might make it appear that I am flashing my lights. Good luck in court, but I don't think you can win that one. Go to traffic school if possible and get rid of one of the tickets.

 

thanx for the link. I have the same prob but the the other way around
I am running smaller tires than stock.
I am running 195/45/15
It says with my tires, that when going 60, I am really going 55.4.
But I have figured that it was more around 9 to 10 MPH off not 5. I do have a mild stretch so would that be more like a 40 tire?

 

Any luck?

 

The stock tire diameter (185/60R15)is 22.4 inches. The 195/45/15 is 21.9 inches without stretch. This is 7.7% small. This means your engine revs 7.7% more for a given speed and that your speedo reads 7.7% fast. A little stretch would likely result in a little decrease in overall diameter which would in turn influence your error by a similar amount. Measure your tire diameter. You must jack the wheel to do this so it is round. find the percentage difference between what you measure and 22.4 and you have your speedo error.

 

This photo helps understand things as stated throughout the thread.

As you increase the "rim diameter" you decrease the "section height" of the tire so the "overall diameter" remains fairly constant.

 

Nice diagram

 

WOW. I have a tC and i checked out that website. Stock tire size is 215/35r17. the ones i have on are 225/40r18 and it says that my car is 9.4% slower. so at 60mph, i'm really going 65.7mph. Huge difference!!!!