Does tire size affect speedometer accuracy/readings?
Thread Starter
Junior Member
Joined: Dec 2011
Posts: 51
Likes: 6
From: Abu Dhabi, UAE
2005 SL55 AMG, 2004 CL500, 2007 CLS63 AMG
Does tire size affect speedometer accuracy/readings?
Hello folks.
I have a W219 CLS 63 AMG.
Just replaced the tires with:
Rear Tires 275/30ZR20 97Y (should be 285/30ZR19 97Y)
Front Tires 245/35ZR20 95Y (should be 245/35ZR19 95Y)
Reason: the tire supplier did not have the exact tire size on hand at the time of replacement (it was not the MB dealer).
I then got zapped by radar (several times in fact) despite having cruise control set just under the speed limit.
Question is, do the tire size affect the accuracy and subsequent reading of the speedometer? If so, is there a formula for calculating this effect (i.e. by how much should I reduce speed)?
Appreciate the input.
I have a W219 CLS 63 AMG.
Just replaced the tires with:
Rear Tires 275/30ZR20 97Y (should be 285/30ZR19 97Y)
Front Tires 245/35ZR20 95Y (should be 245/35ZR19 95Y)
Reason: the tire supplier did not have the exact tire size on hand at the time of replacement (it was not the MB dealer).
I then got zapped by radar (several times in fact) despite having cruise control set just under the speed limit.
Question is, do the tire size affect the accuracy and subsequent reading of the speedometer? If so, is there a formula for calculating this effect (i.e. by how much should I reduce speed)?
Appreciate the input.
Super Member
Joined: Jun 2014
Posts: 884
Likes: 23
From: U.K.
sl600
Your overall diameter is way out of spec. 245/35r19 would have an overall diameter of 19" wheels at 482.6mm then the 35% of the 245mm wide tire is 85.75 you have to times this by two as the sidewalls are top and bottom so you end up with 482.6 add 171.5 = 654.1mm overall diameter stock.
Apply this same equation to the new setup and you get 508 add 171.5 = 679.5mm overall diameter with new setup.
This means that you will travel with every rotation almost 4% further so when the speedo says 60mph you will actually be doing 62.4mph hence why you are getting hit for speeding.
It will also affect your gear ratio which will mean slower acceleration but a higher top speed, it could also effect the speed at which the car will change gears.
None of this will cause much of a problem apart from the speeding. You can buy a unit to intercept the signal from the wheel speed sensor and the speedo to correct the deviation.
Hope this helps.
Apply this same equation to the new setup and you get 508 add 171.5 = 679.5mm overall diameter with new setup.
This means that you will travel with every rotation almost 4% further so when the speedo says 60mph you will actually be doing 62.4mph hence why you are getting hit for speeding.
It will also affect your gear ratio which will mean slower acceleration but a higher top speed, it could also effect the speed at which the car will change gears.
None of this will cause much of a problem apart from the speeding. You can buy a unit to intercept the signal from the wheel speed sensor and the speedo to correct the deviation.
Hope this helps.
Super Member
Joined: Jun 2014
Posts: 884
Likes: 23
From: U.K.
sl600
One more thing if you want to correct with new tires you will need 285/25r20 and 245/30r20 however i would suggest going with 255/30r20 front as the the factory build a lot of understeer in from the factory for safety and the slightly wider front tire will help.
TR Moderator & Tire God
Joined: Nov 2001
Posts: 3,545
Likes: 5
From: SouthBend, IN USA
depends on the weather
Installing tires with a different overall diameter than that of your vehicle's Original Equipment tires may affect the accuracy of your vehicle’s speedometer and odometer.
If you install taller tires on your vehicle, you should verify your speedometer accuracy to avoid unnecessary meetings with the local constable.
If you install taller tires on your vehicle, you should verify your speedometer accuracy to avoid unnecessary meetings with the local constable.
