Speed Change
Senior Member
Joined: Jul 2007
Posts: 3,805
Likes: 480
From: Cape Girardeau MO
Since the aspect ratio (70) and the rim size (17) remain the same, the theoretical difference will be the ratio of the new treadwidth to the old width => 285 / 255 = 1.118.
If this were a perfect world and actual tire sizes really did match the stated tire sizes, then you will be going 11.8% faster on down the road at the same speedo reading. In other words, if you were doing a true 60mph with the old tires, you will now be going just over 67mph for the same speedo reading of 60.
*******
Editted - had a Homer Simpson D'Oh moment.
All seriousness aside, the formula used to approximate tire height is (rim diameter) + 2x(tire sidewall height).
The tire sidewall height is calculated by the width x (aspect ratio).
For the 255x70 - would expect a sidewall height of 255x0.70 = 178.5 millimeters (1inch=25.4mm) or 7.03 inches.
For the 285x70 - would expect a sidewall height of 199.5 millimeters, or 7.85 inches.
Actual heights will differ from theoretical due to manufacturer tolerances, air pressure, and vehicle weight, but this calc should be close enough for making comparisons.
The theoretical height of the 255x70R17 should be about (17) + 2x(7.03), or 31.06 inches.
The theoretical height of the 285x70R17 should be about (17) + 2x(7.85), or 32.7 inches.
The expected speedo error should be 32.7 / 31.06 = 1.053, or a 5.3% error, making for an actual speed of 63.2mph for a presumed accurate speedo reading at 60.
If this were a perfect world and actual tire sizes really did match the stated tire sizes, then you will be going 11.8% faster on down the road at the same speedo reading. In other words, if you were doing a true 60mph with the old tires, you will now be going just over 67mph for the same speedo reading of 60.
*******
Editted - had a Homer Simpson D'Oh moment.
All seriousness aside, the formula used to approximate tire height is (rim diameter) + 2x(tire sidewall height).
The tire sidewall height is calculated by the width x (aspect ratio).
For the 255x70 - would expect a sidewall height of 255x0.70 = 178.5 millimeters (1inch=25.4mm) or 7.03 inches.
For the 285x70 - would expect a sidewall height of 199.5 millimeters, or 7.85 inches.
Actual heights will differ from theoretical due to manufacturer tolerances, air pressure, and vehicle weight, but this calc should be close enough for making comparisons.
The theoretical height of the 255x70R17 should be about (17) + 2x(7.03), or 31.06 inches.
The theoretical height of the 285x70R17 should be about (17) + 2x(7.85), or 32.7 inches.
The expected speedo error should be 32.7 / 31.06 = 1.053, or a 5.3% error, making for an actual speed of 63.2mph for a presumed accurate speedo reading at 60.
Last edited by wde3477; Aug 19, 2007 at 07:16 PM. Reason: Calculator was out to lunch
Senior Member
Joined: Oct 2006
Posts: 6,292
Likes: 123
From: Fort Worth
Since the aspect ratio (70) and the rim size (17) remain the same, the theoretical difference will be the ratio of the new treadwidth to the old width => 285 / 255 = 1.118.
If this were a perfect world and actual tire sizes really did match the stated tire sizes, then you will be going 11.8% faster on down the road at the same speedo reading. In other words, if you were doing a true 60mph with the old tires, you will now be going just over 67mph for the same speedo reading of 60.
*******
Editted - had a Homer Simpson D'Oh moment.
All seriousness aside, the formula used to approximate tire height is (rim diameter) + 2x(tire sidewall height).
The tire sidewall height is calculated by the width x (aspect ratio).
For the 255x70 - would expect a sidewall height of 255x0.70 = 178.5 millimeters (1inch=25.4mm) or 7.03 inches.
For the 285x70 - would expect a sidewall height of 199.5 millimeters, or 7.85 inches.
Actual heights will differ from theoretical due to manufacturer tolerances, air pressure, and vehicle weight, but this calc should be close enough for making comparisons.
The theoretical height of the 255x70R17 should be about (17) + 2x(7.03), or 31.06 inches.
The theoretical height of the 285x70R17 should be about (17) + 2x(7.85), or 32.7 inches.
The expected speedo error should be 32.7 / 31.06 = 1.053, or a 5.3% error, making for an actual speed of 63.2mph for a presumed accurate speedo reading at 60.
If this were a perfect world and actual tire sizes really did match the stated tire sizes, then you will be going 11.8% faster on down the road at the same speedo reading. In other words, if you were doing a true 60mph with the old tires, you will now be going just over 67mph for the same speedo reading of 60.
*******
Editted - had a Homer Simpson D'Oh moment.
All seriousness aside, the formula used to approximate tire height is (rim diameter) + 2x(tire sidewall height).
The tire sidewall height is calculated by the width x (aspect ratio).
For the 255x70 - would expect a sidewall height of 255x0.70 = 178.5 millimeters (1inch=25.4mm) or 7.03 inches.
For the 285x70 - would expect a sidewall height of 199.5 millimeters, or 7.85 inches.
Actual heights will differ from theoretical due to manufacturer tolerances, air pressure, and vehicle weight, but this calc should be close enough for making comparisons.
The theoretical height of the 255x70R17 should be about (17) + 2x(7.03), or 31.06 inches.
The theoretical height of the 285x70R17 should be about (17) + 2x(7.85), or 32.7 inches.
The expected speedo error should be 32.7 / 31.06 = 1.053, or a 5.3% error, making for an actual speed of 63.2mph for a presumed accurate speedo reading at 60.
Senior Member
Joined: Apr 2007
Posts: 395
Likes: 0
From: Fort Wayne Indiana
screw all of that math, i got the same answer using this website http://www.miata.net/garage/tirecalc.html
and from what i hear about using the spare that is a different size is, that you shouldnt with 4wd, but i may be mistaken.
and from what i hear about using the spare that is a different size is, that you shouldnt with 4wd, but i may be mistaken.
Resident A-hole
Joined: Feb 2007
Posts: 4,098
Likes: 33
From: Tallahassee
On the front it won't matter if you use the spare as long as you are not in 4wd. On the rear I am sure you have a locker of some sort and diffrent sized tires can burn up the clutches in a locker in under a mile.
