Tire size and Speedo calibration
Senior Member
Joined: Jan 2019
Posts: 335
Likes: 103
From: Saddlebrooke, AZ 85739
2013 CLS550 4Matic OE Tuned
With 4Matic, the front and rear tires need to have the same (or very close) circumference. That means you'd need to change the front tires too. You could damage your AWD system if you don't.
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Senior Member
Joined: Jan 2019
Posts: 335
Likes: 103
From: Saddlebrooke, AZ 85739
2013 CLS550 4Matic OE Tuned
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Member
Joined: Jun 2014
Posts: 169
Likes: 22
2014 CLS550
Not a big difference.
The 305/30 R20 will result in 5.2% increase, from factory 285/35 R18's, in vehicle speed vs speedo reading.
So, at 40 mph speedo reading, your actually going 42 mph.
120 mph = 126.2 mph.
285/35 R18 are 25.85" in Dia, 780.1 revs/mile.
305/30 R20 are 27.2" Dia,, 741.3 revs/mile
The 305/30 R20 will result in 5.2% increase, from factory 285/35 R18's, in vehicle speed vs speedo reading.
So, at 40 mph speedo reading, your actually going 42 mph.
120 mph = 126.2 mph.
285/35 R18 are 25.85" in Dia, 780.1 revs/mile.
305/30 R20 are 27.2" Dia,, 741.3 revs/mile
Thread Starter
Newbie
Joined: Apr 2021
Posts: 11
Likes: 0
Cls550
Not a big difference.
The 305/30 R20 will result in 5.2% increase, from factory 285/35 R18's, in vehicle speed vs speedo reading.
So, at 40 mph speedo reading, your actually going 42 mph.
120 mph = 126.2 mph.
285/35 R18 are 25.85" in Dia, 780.1 revs/mile.
305/30 R20 are 27.2" Dia,, 741.3 revs/mile
The 305/30 R20 will result in 5.2% increase, from factory 285/35 R18's, in vehicle speed vs speedo reading.
So, at 40 mph speedo reading, your actually going 42 mph.
120 mph = 126.2 mph.
285/35 R18 are 25.85" in Dia, 780.1 revs/mile.
305/30 R20 are 27.2" Dia,, 741.3 revs/mile
Member
Joined: Jun 2014
Posts: 169
Likes: 22
2014 CLS550
Good.
Also important to consider the effects on the 4Matic dynamics when changing wheel/tire size.
The factory, front tire to rear tire, ratio is important.
What brand tires did you end up going with? 295/25 R21 are unusual size.
Your thought about GPS being able to determine vehicle engine and speed is also good. But, GPS isn't that smart.
It sees the vehicle location and movements. Not what is in the vehicle.
Also important to consider the effects on the 4Matic dynamics when changing wheel/tire size.
The factory, front tire to rear tire, ratio is important.
What brand tires did you end up going with? 295/25 R21 are unusual size.
Your thought about GPS being able to determine vehicle engine and speed is also good. But, GPS isn't that smart.
It sees the vehicle location and movements. Not what is in the vehicle.
Thread Starter
Newbie
Joined: Apr 2021
Posts: 11
Likes: 0
Cls550
Good.
Also important to consider the effects on the 4Matic dynamics when changing wheel/tire size.
The factory, front tire to rear tire, ratio is important.
What brand tires did you end up going with? 295/25 R21 are unusual size.
Your thought about GPS being able to determine vehicle engine and speed is also good. But, GPS isn't that smart.
It sees the vehicle location and movements. Not what is in the vehicle.
Also important to consider the effects on the 4Matic dynamics when changing wheel/tire size.
The factory, front tire to rear tire, ratio is important.
What brand tires did you end up going with? 295/25 R21 are unusual size.
Your thought about GPS being able to determine vehicle engine and speed is also good. But, GPS isn't that smart.
It sees the vehicle location and movements. Not what is in the vehicle.
