How will using a set of 285/35/20 tires affect speed meter reading???

[Zhaoyu Chen]

Hi guys,

I am thinking about getting 1LE goodyear tires for my 13 L99 camaro ss next summer for street and track use. I have 9 inch wheels front and 10 inch wheels rear and clearance should not be a problem. However, I am wondering how these tires will affect the speed meter reading. The height of the goodyears is 285*35=9975mm vs the pirellis 275*40=11000mm. There is almost a 10% difference. Any one running 285/35/20s on their cars? Does the speed meter read inaccurate numbers?

Thanks a lot!

[bannonm]

The difference is really around 2.5%. 1.00-(27.9" / 28.6") = .024475. Diameter of 285 divided by diameter of 275. The 285's make the speedo think you are going faster.

[Zhaoyu Chen]

Originally Posted by bannonm

The difference is really around 2.5%. 1.00-(27.9" / 28.6") = .024475. Diameter of 285 divided by diameter of 275. The 285's make the speedo think you are going faster.

I am kind of confused. What exactly does the size 285/35/20 mean? Does it mean that the diameter of the 285/35 zr20 tire is 285*35=9970 mm?

[Jenner]

Originally Posted by Zhaoyu Chen

I am kind of confused. What exactly does the size 285/35/20 mean? Does it mean that the diameter of the 285/35 zr20 tire is 285*35=9970 mm?

285 is the width of the tire. 35 means that the sidewall is 35% of 285. So the tire is 285mm wide and the sidewall is 99.75mm tall. Simple math 285X.35=99.75.

[1KillerSS]

You will want to compare the rolling diameter of the tire. the factory Pzeros are 27.94, any number different from that will change your speedo. You can correct that in a tune.

[1KillerSS]

27.94 or 27.97.... depending on the year of the car.

[SS PANTHER]

Originally Posted by Zhaoyu Chen

I am kind of confused. What exactly does the size 285/35/20 mean? Does it mean that the diameter of the 285/35 zr20 tire is 285*35=9970 mm?

Tire sizes are a combination of millimeters, percentage and inches. As stated before 285 is the width of the tire in millimeters and if you convert this to inches you divide 285mm by 25.4, as there are 25.4 mm's in one inch. This equates to an 11.22" wide tire. Multiply 11.22 by .35 (35%, the tires sidewall percentage on one side of the tire only) to get your sidewall profile height of 3.927". Double this number 3.927" to get both sidewall heights of 7.854". Then add the 20, this is the wheel diameter, which is in inches, to the total sidewall height of 7.854" for a total diameter of 27.854", ~27.9". The factory 275-40-20 works out to ~28.7".
Divide 28.7 by 27.9 = 1.0287, ~1.03. This is ~3% difference in your speedometer and rear end gearing also. 70mph showing on your speedometer is a true car speed of 68mph. Your OEM gear ratio of 3.27 is now ~3.35's.

[SS PANTHER]

Originally Posted by Zhaoyu Chen

Hi guys,

I am thinking about getting 1LE goodyear tires for my 13 L99 camaro ss next summer for street and track use. I have 9 inch wheels front and 10 inch wheels rear and clearance should not be a problem. However, I am wondering how these tires will affect the speed meter reading. The height of the goodyears is 285*35=9975mm vs the pirellis 275*40=11000mm. There is almost a 10% difference. Any one running 285/35/20s on their cars? Does the speed meter read inaccurate numbers?

Thanks a lot!

What are your current offsets? The factory 20 x 9" wheel has a +40 offset and the 285-35-20 will rub on the front and a 9" wheel is not recommended for that tire. Try to go with 20 x 10's all around to run a square setup for better handling and being able to rotate your tires from front to rear. The offset is important for clearance issues.

[ZWA SS]

I went with 275/35/20 all around on OE 20x9s.
I believe my speedo is off 4% and my gears act as 3.40

[Jenner]

Does playing around with tire sizes affect the odometer at all?

[Destructo09]

Originally Posted by Jenner

Does playing around with tire sizes affect the odometer at all?

Sure does, to the same effect as the speedo. So it would be 3-4% more miles as well as mph. Over the course of 100k miles you're talking something like 3-4k miles, it's pretty minor.

[Jenner]

Oh ok. You're right, that's not bad

[Zhaoyu Chen]

Originally Posted by Jenner

285 is the width of the tire. 35 means that the sidewall is 35% of 285. So the tire is 285mm wide and the sidewall is 99.75mm tall. Simple math 285X.35=99.75.

Oh that makes perfect sense! I got it! Thank you so much!!

[Zhaoyu Chen]

Originally Posted by SS PANTHER

Tire sizes are a combination of millimeters, percentage and inches. As stated before 285 is the width of the tire in millimeters and if you convert this to inches you divide 285mm by 25.4, as there are 25.4 mm's in one inch. This equates to an 11.22" wide tire. Multiply 11.22 by .35 (35%, the tires sidewall percentage on one side of the tire only) to get your sidewall profile height of 3.927". Double this number 3.927" to get both sidewall heights of 7.854". Then add the 20, this is the wheel diameter, which is in inches, to the total sidewall height of 7.854" for a total diameter of 27.854", ~27.9". The factory 275-40-20 works out to ~28.7".
Divide 28.7 by 27.9 = 1.0287, ~1.03. This is ~3% difference in your speedometer and rear end gearing also. 70mph showing on your speedometer is a true car speed of 68mph. Your OEM gear ratio of 3.27 is now ~3.35's.

Excellent explanation!! I think I will go with the 285s since the speed measurement is still very accurate. Thanks a lot bud!!