The speedometer / odometer tend to read high.

So a slightly larger diameter than mathematically determined may make them read more accurately...

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Not true. The odometer is simply a mechanical counter for keeping track of tire revolutions. Its accuracy is only as good the gear and tire combination used. An odometer will be most accurate when the tire size used is closest in diameter to the one specified by the calculations.

The speedometer is a different matter as its accuracy can be calibrated. Having spoken with a speedometer shop already, most speedometers are out of calibration.

An odometer will read high even if you've mathematically determined the proper diameter.............

The calculation doesn't take into consideration rolling radius due to tire compression from the weight of the vehicle..................

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

You make a very good point about the effect of loading and speed on the dynamic rolling circumference of tires. My expectation was that the combined effect of load and tire growth due to speed would have a negligible effect on the actual rolling circumference.

I started off by trying to determine the correct diameter of the old 6.50x13 tires but did not have any success. The best I could find was that those old tires had an aspect ratio of 80-90%. Using 85%, I came up with tire diameter of 24.02" (24.32" was required by 2.93 & 17). Often, I have seen 175/80R13 (24.02") and 185/80R13 (24.65") as being the recommended metric replacements so the mathematical calculations seemed to be correct.

Your point is confirmed by BTT's Tire Dimensions. Barry suggests that the rolling circumference is closer to 97% of the static circumference. With this correction factor, the calculations show that 2.93:1 & 17 require a 25.07" tire while 2.76:1 & 16 require a 25.15" tire for better odometer accuracy.

This makes my tire selection a bit easier as larger tires seem to be easier to find. In my case, a 215/60R15 seems to be the ideal tire with an accuracy of +0.03%.

Ed, as I was searching through the old topics, I came across one of your old postings:
http://www.slantsix.org/forum/viewtopic ... 1757#81757

If the "really old bias-ply snow tires" you refer to were actually 6.50-13s, then it looks like the rolling circumference is about 97% of the circumference calculated from the tire size. That means that the odometer should read correctly when tires are 1.031 times larger than those required by the axle and speedometer pinion calculation.

Yep, they were 6.50 13s.......

BTW, I added a bicycle speedometer to my motorcycle and the calibration was 98% of measured circumference..........

So 97% sounds about right.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Found this chart:
http://www.yearone.com/yodnn/tech/TechF ... fault.aspx

_________________
<img src="http://farm4.static.flickr.com/3173/2992308402_5481afb26e_t.jpg">

That chart is for the newer transmissions and is not applicable to the 62-65 versions. Thanks anyway.

Regarding my earlier question about the appearance of 15" wheels on 65 Barracudas, see the 15x7 wheel discussion on FABO.