16% OD, +3" on tires = 12% speedo error ??

[Stolen76]

I took my Wag for a long drive this week, 200 miles. I used the mile markers on the interstate to calculate my speedo error and got 11.8% !! This wouldn't be bad, except I have the OD kit in my t-case and my tires are at least 3.5" taller then the originals. Shouldn't it be 16% plus tire difference ?

This makes no sense to me. Can anyone tell me where I screwed up my math ? (3.55 gears)

[Cecil14]

I could be wrong but I don't think the OD should come into play as far as the speedo goes. The speedo gear is the last thing in the t-case and doesn't depend on the gearing up to that point. The only things that affect the speedo are the ring and pinion and tire size. Could this be where your error is coming from?

Good luck,
Anthony

[Stuka]

Cecil is correct. The QT OD does not effect the speedo.

[Don S]

Stolen76... I don't think you screwed up your math...

Most tires are NOT usually the actual (call out) size they claim to be. It is equally or more important to think about ‘static radius’ and this is the dimension from the ground footprint to the axle (center) which will be LESS than half the diameter of the tire. As the static radius decreases the distance traveled per revolution decreases. As the static radius of the tire lessens the effective over all gear ratio becomes better for off roading.. ;) ;) .. ds..


AIRING DOWN PROs & CONs
http://groups.msn.com/ATexasOklahomaJeepClub/ouraycotopic.msnw

[ August 09, 2002, 08:19 PM: Message edited by: Don S ]

[joe]

Sounds normal to me. As mentioned above I don't think the speedo is affected by the OD in the x-case and your tires are about 10-11% larger and figuring polly a couple % slop in a 27 year old speedo/drive train etc I'd say you're right in the ballpark if it reads 12% slow...unless it's reading faster than your actually going then you've got a prob.

[PhilSine]

What was your original set-up?

Have you changed your gearing and what tires were OEM and what do you have now?

[Stolen76]

ok, this makes more sense now. I thought the OD kit in the Qtrac was going to make a difference.

My gears are the same as originals, 3.54. I can't remember the original tire size, but the measured height while on the truck was 27.75", measured hieght of the new tires installed on the truck is 31.5. Since I'm only dealing with the difference in tire size, the 11.8% seems to be about right as the difference in the tires is 11.90%

Thanks for the help, I shoulda thought of that the first time around !