Against everyones good advice, I upsized my tires

Against very good advice I went up one tire size for my 1998 GMC Z71. I went to a 285/75/16. I have had to buy new rims to accomodate the slightly
wider tire and have a different offset in the front for proper tire clearence. My concern is that the speedo and stuff like that will be slightly out of wack and I was hoping someone here could tell me waht gears or such I would have to change to run these new tires and get the correct reading on the speedo.
Thanks in advance,
Blair
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Correcting the error would get mathematically involved - and I doubt you could find a set of standard gears for an exact correction of it anyway. Know what we used to do in the old days ? Have a friend drive in front of you at a set speed, and check what the error on the speedometer is - then you will know ! Then you simply correct for the error when driving. In your case, the speedo will actually read a bit slower than you are actually going, since the tires will make less revolutions per mile....

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same idea- brought into the 21st century, and thereby avoiding the inherent inaccuracy in your friend's speedo:
use a handheld GPS to find your TRUE speed-- and proceed as above-- correct as you drive!
<my 89 S-10 with 3" larger tires is actually within .7 MPH at 70MPH. >
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yep.. did the same thing, accidently..lol We got a GPS package for the notebook and I thought it was out of whack because it showed us going faster than the speedo indicated... Then, I remembered that the new tires were one size bigger than stock... checked with the guys in the Dodge group and the new tire size would cause a 4% error, just what the GPS said... When in doubt, (translate to: when we see a CHP) we just do the speed limit, knowing that at 65 indicated, we're doing 68 and not worth pulling over.. YM (and speed)MV
mac
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"burntkat" wrote:

That might report the speed but it will not make up for lost power from taller effective ratio now, make OD work better or fix reduced brake effectiveness because of increase leverage on brakes fron bigger diameter tires. The best reall fix if you like big rubber it to regear the axles and it will fix everything except the brake issue.
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umm-- Duh?
Of course I was addressing the accuracy of his speedo, only.
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"SnoMan" wrote:

I was referring to SnoMans reply,not Burntcats......
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Blair Chesterton wrote:

Yup, Just borrow a Garmin GPS and get your true speeds at 25,
35, 45, 55, 65, & 75 MPH speedometer readings.
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Try this site.It will give you the percentage the different size tires will be on your speed???
http://www.1010tires.com/TireSizeCalculator.asp?action=submit
Doug

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Thanks to everyone for the advice, it is greatly appreciated.
Blair
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"" wrote: > Thanks to everyone for the advice, it is greatly appreciated. > > Blair
No way does he have to "RE-Gear" the rear end if he only went up one size.Not even 2 sizes. No braking improvements needed,either;but better pads/braided hoses are always nice.
All you really need to do is >Go to a Speedometer Calibration Shop<. They set some tiny switches behind the Speedo,and youre done.
Cheaper than a GPS !
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On 20 Aug 2005 22:36:29 -0400, guitronics

Or leave it and consider it an extended warranty :)
--- Rich http://richlockyer.tripod.com /
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