Speedometer/Odometer and Tire Size
09-19-11, 07:38 AM
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Speedometer/Odometer and Tire Size
I recognize this has been covered somewhat, but it's in bits and pieces of threads and incomplete somewhat. Here are my questions.
I'm changing my wheels/tires package. I have OEM 18" RWD setup now.
Moving to:
F: 18x8 225/40/18
R: 18x9.5 (Not sure what size)
I realize that changing the overall diameter of the rear tire will affect speedometer, and that the speedometer from the factory is ~3% faster to begin with. What about the odometer? I read this:
Does this mean its more/less accurate than the speedometer?
When changing my tire size and according to the logic above, I'm less concerned with speedo changes, and more concerned with odometer changes.
I'd like to go with a 275/35/18 which would cause the speedo to read 1.8% faster than it already reads, but I'm not willing to be racking up 4.8% more total miles since I drive 15K miles annually.
My next closest option to OEM diameter would be a 265/40/18 which would read 1.19% too slow. (Combating the OEM 3% over, but I'd like the wider tire)
I also realize this about the calculator assuming all 4 wheels are taken into account for the speedo/odometer signal:
Can anyone provide more specific info on how the speedometer and odometer use separate formulas to calculate speed and mileage? Which one is more accurate?
Any suggestions for a tire size other than what I've listed?
I'm changing my wheels/tires package. I have OEM 18" RWD setup now.
Moving to:
F: 18x8 225/40/18
R: 18x9.5 (Not sure what size)
I realize that changing the overall diameter of the rear tire will affect speedometer, and that the speedometer from the factory is ~3% faster to begin with. What about the odometer? I read this:
The odometer and the speedometer do not use the same calculation. They both receive the same signal, but each does it's own arithmetic.
When changing my tire size and according to the logic above, I'm less concerned with speedo changes, and more concerned with odometer changes.
I'd like to go with a 275/35/18 which would cause the speedo to read 1.8% faster than it already reads, but I'm not willing to be racking up 4.8% more total miles since I drive 15K miles annually.
My next closest option to OEM diameter would be a 265/40/18 which would read 1.19% too slow. (Combating the OEM 3% over, but I'd like the wider tire)
I also realize this about the calculator assuming all 4 wheels are taken into account for the speedo/odometer signal:
1010tires is assuming only the rear wheels are used for measuring speed. That's not the case with any 2IS. All four wheels are used, so the error will likely be smaller than what you see here.
Any suggestions for a tire size other than what I've listed?
09-27-11, 09:09 AM
#5
I recognize this has been covered somewhat, but it's in bits and pieces of threads and incomplete somewhat. Here are my questions.
I'm changing my wheels/tires package. I have OEM 18" RWD setup now.
Moving to:
F: 18x8 225/40/18
R: 18x9.5 (Not sure what size)
I realize that changing the overall diameter of the rear tire will affect speedometer, and that the speedometer from the factory is ~3% faster to begin with. What about the odometer? I read this:
Does this mean its more/less accurate than the speedometer?
When changing my tire size and according to the logic above, I'm less concerned with speedo changes, and more concerned with odometer changes.
I'd like to go with a 275/35/18 which would cause the speedo to read 1.8% faster than it already reads, but I'm not willing to be racking up 4.8% more total miles since I drive 15K miles annually.
My next closest option to OEM diameter would be a 265/40/18 which would read 1.19% too slow. (Combating the OEM 3% over, but I'd like the wider tire)
I also realize this about the calculator assuming all 4 wheels are taken into account for the speedo/odometer signal:
Can anyone provide more specific info on how the speedometer and odometer use separate formulas to calculate speed and mileage? Which one is more accurate?
Any suggestions for a tire size other than what I've listed?
I'm changing my wheels/tires package. I have OEM 18" RWD setup now.
Moving to:
F: 18x8 225/40/18
R: 18x9.5 (Not sure what size)
I realize that changing the overall diameter of the rear tire will affect speedometer, and that the speedometer from the factory is ~3% faster to begin with. What about the odometer? I read this:
Does this mean its more/less accurate than the speedometer?
When changing my tire size and according to the logic above, I'm less concerned with speedo changes, and more concerned with odometer changes.
I'd like to go with a 275/35/18 which would cause the speedo to read 1.8% faster than it already reads, but I'm not willing to be racking up 4.8% more total miles since I drive 15K miles annually.
My next closest option to OEM diameter would be a 265/40/18 which would read 1.19% too slow. (Combating the OEM 3% over, but I'd like the wider tire)
I also realize this about the calculator assuming all 4 wheels are taken into account for the speedo/odometer signal:
Can anyone provide more specific info on how the speedometer and odometer use separate formulas to calculate speed and mileage? Which one is more accurate?
Any suggestions for a tire size other than what I've listed?
09-27-11, 10:54 AM
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Where did you hear that about the speedo? If that was the case, I could run a 275/35/18 (1.8% fast) and be close to accurate on the speedo.
Last edited by TheSenator; 09-27-11 at 10:57 AM.
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09-27-11, 11:05 AM
#8
Interesting, I've always heard Toyota/Lexus integrate a 3% too fast to save you from speeding. Thank you for the calc though that kind of answers the odometer question. I think it would be even less that the 2IS takes into account all 4 tires, and I'm keeping stock size up front.
Where did you hear that about the speedo? If that was the case, I could run a 275/35/18 (1.8% fast) and be close to accurate on the speedo.
Where did you hear that about the speedo? If that was the case, I could run a 275/35/18 (1.8% fast) and be close to accurate on the speedo.
http://www.miata.net/garage/tirecalc.html
I pretty sure only the rear wheels are counted for the odometer.
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