tire size and speedometer accuracy

When I was hot rodding in the late '60s, I spent a lot of time researching how tire size and different differential gear ratios affected speedometer accuracy. I bought a lot of different speedo drive gears for the tranny (this was on a GM TH400) to compensate for the changes I made. All this just to keep my speedo as close accurate as possible as I didn't want to speed and collect tickets. Now someone tells me that after putting in larger diameter tires on his car (Honda Element), the tire dealer said that his speedo would still be accurate. I find this hard to believe. Are today's electronics intelligent enough to automatically adjust for different diameter tires? I would think at least something needs to be manually re-calibrated? This is a good group to get information even though this is not a Toyota specific issues. Thank you.

Reply to
Dennis Leong
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If the circumference of the replacement tire's tread is the same as the circumference of the original tires, then the speedometer and odometer will still be accurate. 2 tires can have different diameters and have the same circumference if the aspect ratio is proportionally different. A tire's diameter is measured from the center of the "hole" to the bead, not the tread.

Reply to
Ray O

I'm with you on this one. I can't see how the car would know the tires are a different size, and alter the speedo calibration by itself.

If the original size and the new size are compliments of each other, then it is easily possible that the speedo is not affected enough to warrant changing gears.

I don't know the tires that the Element comes with, but assuming a size, there are other sizes that offer more tread contact area, but are the same diameter -- within a half-inch -- which would not adversely alter the speedo accuracy (and the new tires would be larger, making the speedo a bit slower and probably improve its accuracy anyhow).

For example, a 235/75x15 is 28.89 inches in diameter, wheras a 255/70x15 is

29.05 inches. The difference in DIAMETER is less than 1/4 inch. But, the contact patch would be wider by 20 mm, or about 3/4 inch. More on the ground, but hardly any change in speedo accuracy, and any change would probably make it more accurate than it already is.

Reply to
Jeff Strickland

What I can see on the Element are significantly larger (diameter) tires than original. So I would say that his speedometer accuracy is off but the owner really is too dense to know. Thank you.

Reply to
Dennis Leong

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