Yeah, that 10-fold difference looked suspicious to me as well, especially that it wasn't explained where that figure came from.
Should you never "upsize" tires?
- Thread starter crw
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Originally Posted By: eljefino
Plug your prospective size in here: http://www.miata.net/garage/tirecalc.html
For example going from a 195/60/15 to a 195/65/15 will fix your speed by 3.2%.
As said above you'll need a GPS to track mileage, as it will appear worse more than it gets better.
The taller sidewall will have more load capacity, you'll have a greater diameter (fewer revs per mile), so the tire will last longer, and probably be cheaper to boot!
Tires are built in "size ranges". Actual diameters/circumferences vary by manufacturer and even within manufacturer lines. One must actually go by published data on the specs for the tire.
So, while a 65 aspect ratio tire should be taller than a 60 aspect ratio tire, unless one actually checks the data, it can be smaller, the same, or taller than what one currently has.
Plug your prospective size in here: http://www.miata.net/garage/tirecalc.html
For example going from a 195/60/15 to a 195/65/15 will fix your speed by 3.2%.
As said above you'll need a GPS to track mileage, as it will appear worse more than it gets better.
The taller sidewall will have more load capacity, you'll have a greater diameter (fewer revs per mile), so the tire will last longer, and probably be cheaper to boot!
Tires are built in "size ranges". Actual diameters/circumferences vary by manufacturer and even within manufacturer lines. One must actually go by published data on the specs for the tire.
So, while a 65 aspect ratio tire should be taller than a 60 aspect ratio tire, unless one actually checks the data, it can be smaller, the same, or taller than what one currently has.
I have up sized tires on my GM trucks and taken the truck to a dealer and had the ECM re-flashed for that tire size. Usually there are settings in the ECM firmware for all the tire sizes offered on that vehicle platform so all the tech does is plug into the ECM bus and select the size you go changed to and then save that size in the system. Takes about 30 min and cost me $75. This recalibates all the speed sensors in the vehicle as the critical ones are the trans and tcase on AWD vehicles. Check with your dealer to see if this feature is built in. It is on GM vehicles but you can only select tire sizes spec'ed by the manufacturer.
Note: Changing tires sizes will change wheel speed sensor data and can adversely effect transmission shift points and some abs data. SO be careful what you are doing. I would never use a tire not spec'ed for the vehicle platform.
Note: Changing tires sizes will change wheel speed sensor data and can adversely effect transmission shift points and some abs data. SO be careful what you are doing. I would never use a tire not spec'ed for the vehicle platform.
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Originally Posted By: crw
>>> So, what the manufacturers do is take the actual reading from the speed sensor and then add a correction factor (a few mph or a few %) and then display that value on your speedo. The odo doesn't get fed the same khrap.
Wow. That is dumb. I'll have to use my GPS to see if they match or not!
Maybe my mpg isn't as bad as I thought it was. I was assuming it was actually 7% less.
on a trip across australia my GPS read about 1.7% more than my car Odo.
Gps points were only taken every 500m so the actual difference may have been some 2% between real and car Odo. (real distance being greater than the Odo)
>>> So, what the manufacturers do is take the actual reading from the speed sensor and then add a correction factor (a few mph or a few %) and then display that value on your speedo. The odo doesn't get fed the same khrap.
Wow. That is dumb. I'll have to use my GPS to see if they match or not!
Maybe my mpg isn't as bad as I thought it was. I was assuming it was actually 7% less.
on a trip across australia my GPS read about 1.7% more than my car Odo.
Gps points were only taken every 500m so the actual difference may have been some 2% between real and car Odo. (real distance being greater than the Odo)
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