Tyre aspect ratio (aka Profile) and wheel speed

[DavidLibGen5]

Hi there!

 

I am curious about the relationship - if any - between changing the tyre profile and the wheel speed sensor. The very little I understand about wheel speed sensors, I get that they work in unity with ABS and traction control, but that's the extent of my knowledge when it comes to this area of the wonderful Subaru.

Swapping the 50 profile to say, a 60 or 65 - at what degree would it confuse the wheel speed sensor and therefore the speedo accuracy?

 

Many thanks

David.

[Scooby Fan]

If your tire circumference and or wheel+tire diameter is anything other than the same as stock then your speedo will be out a certain amount.

Take a look at this and it will show you the amount for various applications.

http://www.willtheyfit.com/

[Humble Rumble]

It depends specifically on all the specifications of your tire and wheel combinations, not just the sidewall alone, but the site that Scooby Fan shared is a great way to get a close idea to how the size will affect your speed reading.

 

For what it's worth, USDM 5th gen speedometers read about 3% faster than GPS actual speed from the factory. This is not at all uncommon in modern passenger cars, and is a bit of an extra safety buffer against drivers to more or less make them drive slower than they think they are, but imperceptibly so. For example, using the 3.6R stock wheel/tire size of 225/50/17 on 17x7.5, when I up sized to a 245/40/19 on 19x8.5, it changed my wheel-to-speedometer relationship much, much closer to GPS actual speed, with a variance closer to about 0.2% rather than 3%. That is about a 1mph per 30mph DECREASE on the speedometer readout compared to stock size, which translates to roughly 1mph per 30 mph INCREASE in actual travel speed compared to speedometer readout. In other words, on stock size, with the speedometer reading 60mph, I may have only been going 58mph. On the 245/40/19 tire, 60mph on the speed would be much closer to actual 60mph. Hope that made sense and wasn't too confusing.

[Aphex28]

https://www.tacomaworld.com/tirecalc

 

I use this site when shopping for used tires to see what sizes won't change the speedo by more than 1%

Edited March 21, 2021 by Aphex28

[Elxiddic]

If you are curious you can try doing the calculation yourself on paper or in your head. Tire width (mm) × aspect ratio ÷ 100 = Sidewall height. Multiply your sidewall height difference by 6.28 (2 × pi) and you've got your difference in tire circumference (difference in distance of one wheel revolution). That difference divided by your original circumference shouldn't exceed 3% supposedly.

 

Envoyé de mon LG-M703 en utilisant Tapatalk

[DavidLibGen5]

Many thanks for the links and info people!

@Elxiddic I am now off to brush up on my high school maths!