Smarty S-06 tire diameter changes

[LRutigliano]

Smarty set to the stock tire diameter says 30. 65" diameter.



When I go to the Bridgestone website for their Dueller Revo's LT265/70R17, their spec is 31. 6" diameter.



When I go the BFG website (my stock original equipment tires), their Rugged Trail TA spec for LT265/70R17 is 31. 5" diameter.



Will changing the diameter with Smarty improve my mpg's and show more miles travelled per rev? I say yes.



Does anyone know why the discrepancy?



When I try to measure the actual diameter, I come up approx 31" to 31. 25" but my measurement techniques are not very accurate...



Thanks,

Louis

 

[twmcneilly]

I measured my BFGs diameter, and input that value into Smarty. Didn't get me the accuracy I was looking for. Then I used a GPS to home in on perfection, altering about 0. 10 - 0. 20 inches at a time. My tires resulted in over 10% error. In other works 70 miles per hour indicated was really 77 miles per hour.



You don't usually win with a "Stater" when you try to argue that your speedo is off.



If you don't have accurate distance measuring (odometer) your MPG calculation will not be accurate either.



If you have a larger than stock tire then your speedo is reading slower than actual and you odometer is recording less than actual. Great if you want to squek a bit more out of warranty I suppose, but speeding ticket likelyhood goes up.

 

[p-Bar]

I also used the GPS to calibrate speedo with the Smarty got dead nuts

 

[JVolpe]

I have found that the manufacturer specs are not real world mounted diameter. Probably are with tire mounted, full aired up before mounting. As stated above a GPS is the best way to tune the tire diameter setting to be as acurate as possible. My BFG 285 A/Ts are listed at 32. 7", but with 50% wear and mounted setting my Smarty size at 32. 1" is about as close as I can get to the GPS speed and distance readings.

 

[JR2]

This is a little off the point, but my Ford Excursion speedo was off 2. 5 mph with stock tires. The dodge was about 1 mph off with the stock tires. When I got my BFG 285/75/16's on stock rims, 33 seemed to get the speedo spot on with the GPS.

 

[brods]

Forget the advertised diameter or loaded radius as neither one work out right. Use the tire manufacturer’s chart to find the revolutions per mile. Take the number 20,168 and divide by the revs per mile. That will give you the tire diameter to punch into the Smarty and will get you very close. Adjust as needed for different inflation pressures or tread wear.



A more complicated and confusing explanation can be found here: https://www.turbodieselregister.com/forums/1967819-post5.html