Another tire question / suggestion request

[wayoflife]

Now after 45k miles, my speedo is off about 5 mph above about 50, but when my tires were new (4 years ago), I entered 36.85" into the appropriate spot on my AEV ProCal chart and my speedo at that time was spot on, as measured by GPS. I still don't see how even the compressed value of the tire would be twice the measurement from the ground to the center of the wheel. The tire, rolling or otherwise, will never be compressed on the top of the tread. So the distance from tne cener of the wheel to the top of the tread seems like it would be a more constant measurement, as so many things can affect how much the bottom of the tread is compressed. Heat and ambient temperature, weight transfer under acceleration/braking, road surface (hard vs. soft), fuel....and that's not even accounting for the driver, passengers, payload, etc. The distance from the center of the wheel to the top of the tire; however, will never change aside from tire pressure affected by temperature.

I have never seen a speedo get off 5 mph just because the tires had 45,000 miles on them. I have however seen a speedo be off about 5 mph because it was calibrated by measuring to the top of the tread like you had done. Fact of the matter is, your speed sensors are only interested in the RADIUS of your tire, the point from the center of your wheel to the ground and NOT the diameter. The only reason why you need to know the diameter is because that's what your programmer wants to know. As far as gauging your actual speed goes, try using a radar speed trap or a friend following you on the highway who has a car with a known accurate speedo as opposed to a GPS.

And Eddie....just to clarify, you said in your post that people in the know refer to "advertised size", which is always smaller and I think you actually meant "actual size" is always smaller. :beer:

Advertised size as in what is written on the sidewall. In other words, if the tire says 35x12.50. Yes, the "actual" size of this would be closer to 33" tall and that would be the same as a 305 or maybe even a 285 metric depending on the make.

 

[Sh0rtBus]

I have never seen a speedo get off 5 mph just because the tires had 45,000 miles on them. I have however seen a speedo be off about 5 mph because it was calibrated by measuring to the top of the tread like you had done. Fact of the matter is, your speed sensors are only interested in the RADIUS of your tire, the point from the center of your wheel to the ground and NOT the diameter. The only reason why you need to know the diameter is because that's what your programmer wants to know. As far as gauging your actual speed goes, try using a radar speed trap or a friend following you on the highway who has a car with a known accurate speedo as opposed to a GPS.

Obviously with wear, even the radius of the tire gets smaller as the tread diminishes. So if the speed is calibrated at one measurement and then the measurement changes, it's reasonable to expect that the original calibration would no longer be correct because the size of the tire has changed. Does that make sense at all?

And despite my usage of the actual diameter of the tire during my original calbibration, my speedo was dead to rights, as measured by GPS and several Police radar trailers. Now I haven't driven by too many of those recently in my Jeep since I don't drive it every day, but as measured with GPS, the speedo now reads about 5 mph faster than I'm actually traveling. I'd almost be willing to bet that if I take a measurement of the diameter of my tire and reprogram my ProCal using that number, my speedo will go back to being accurate again. I'll try to do that (with pics) in the next few days and report back.

 

[wayoflife]

Obviously with wear, even the radius of the tire gets smaller as the tread diminishes. So if the speed is calibrated at one measurement and then the measurement changes, it's reasonable to expect that the original calibration would no longer be correct because the size of the tire has changed. Does that make sense at all?

Of course it makes sense and I'm not arguing that there wouldn't be a "slight" change - just not as much as 5 MPH and FASTER. That doesn't make any sense at all. Maybe if by top of the you mean right where the sidewall becomes the tread.

And despite my usage of the actual diameter of the tire during my original calbibration, my speedo was dead to rights, as measured by GPS and several Police radar trailers. Now I haven't driven by too many of those recently in my Jeep since I don't drive it every day, but as measured with GPS, the speedo now reads about 5 mph faster than I'm actually traveling. I'd almost be willing to bet that if I take a measurement of the diameter of my tire and reprogram my ProCal using that number, my speedo will go back to being accurate again. I'll try to do that (with pics) in the next few days and report back.

Please do.

 

[wayoflife]

So, I just went outside and measured my brand new 37x12.50 Nitto Trail Grappler from the floor to the top edge of the tire where the sidewall turns 90° to become the tread and got exactly 35.5" This is about what I get when I measure from the center of the wheel and x2. My speedo is exactly on based on a local speed trap radar as well as pacing my Jeep with my new Renegade which is still stock. I have found this method of measuring tires to be 100% accurate on all 4 JK's I've owned. Back in 2007 when I didn't know any better, I too used to measure to the top of the tread and ALWAYS got a speedo that was off. Now, that's just me and what I have experienced time and time again with all my JK's, all the different tires I have run on them and on all the JK's I've helped to calibrate over the years. But then, that's just me.