OEM tire sizes- not possible to keep speedometer correct?
#1
Driver
Thread Starter
Join Date: May 2005
Location: WA
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
OEM tire sizes- not possible to keep speedometer correct?
So I was doing a little research for my GS400. On the decal in the side of the driver's door, the two stock sizes are listed:
225/55/16
235/45/17
But those aren't equivalent. Mathematically, the 17" stock tire/wheel setup is 1.6% smaller, which would (although very slightly) throw your speedometer off.
I guess I'm surprised because Lexus is all about precision. I guess my question is which is the 'baseline' tire size? When your speedometer shows 100mph, which tire size must you be using in order for your speedo to be 100% accurate? And, wouldn't a closer size to the 16" tire be a 245/45/17?
225/55/16
235/45/17
But those aren't equivalent. Mathematically, the 17" stock tire/wheel setup is 1.6% smaller, which would (although very slightly) throw your speedometer off.
I guess I'm surprised because Lexus is all about precision. I guess my question is which is the 'baseline' tire size? When your speedometer shows 100mph, which tire size must you be using in order for your speedo to be 100% accurate? And, wouldn't a closer size to the 16" tire be a 245/45/17?
#2
You are correct the math is off, but depending on the tire you use the math is not always correct. My GS300 stock tire size is 225/55-16 (25.7" dia.) and I will be puting 235/45-17 (25.6" dia.) tires on new rims this spring. I will be using Michelin Pilot Sport A/S and according to the Michelin web site there are close in the outside diameter. See attched file.
#3
Pole Position
Great question. I've been told that the speedometer is calibrated depending on whether your car originally came with 16s or 17s. I find this hard to believe, but that's what I've been told.
By the way, here's a good calculator to use in determining how far off your speedo will be with various rim/tire combos (I can't take credit for this -- chuckb sent me the link):
http://www.miata.net/garage/tirecalcold.html
By the way, here's a good calculator to use in determining how far off your speedo will be with various rim/tire combos (I can't take credit for this -- chuckb sent me the link):
http://www.miata.net/garage/tirecalcold.html
#4
Driver
Thread Starter
Join Date: May 2005
Location: WA
Posts: 107
Likes: 0
Received 0 Likes
on
0 Posts
That's the website I was using! I didn't think anyone else used it but me...
Anyway, so does anyone have an idea why the 17"s aren't 245/45/17? If I'm looking to upgrade wheels/tires, should I be looking for that size instead?
Anyway, so does anyone have an idea why the 17"s aren't 245/45/17? If I'm looking to upgrade wheels/tires, should I be looking for that size instead?
#5
Moderator
hmmmm
My understanding was that the radius of the rubber is almost the same fro 16 or 17 inch. The rim size changes and the height of the side-wall is reduced. [You see less rubber from the side].
As the tires wear out, surely the rotations (measured) --> distance covered gets off.
Salim.
My understanding was that the radius of the rubber is almost the same fro 16 or 17 inch. The rim size changes and the height of the side-wall is reduced. [You see less rubber from the side].
As the tires wear out, surely the rotations (measured) --> distance covered gets off.
Salim.
#6
Registered User
iTrader: (1)
Originally Posted by EyesOnly
That's the website I was using! I didn't think anyone else used it but me...
Anyway, so does anyone have an idea why the 17"s aren't 245/45/17? If I'm looking to upgrade wheels/tires, should I be looking for that size instead?
Anyway, so does anyone have an idea why the 17"s aren't 245/45/17? If I'm looking to upgrade wheels/tires, should I be looking for that size instead?
Thread
Thread Starter
Forum
Replies
Last Post