BIGGER TIRES & SPEEDOMETER
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Freshman User
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BIGGER TIRES & SPEEDOMETER
I just put 33x12.50 17" on my 04 FX4 in place of my factory 275/65r18 and I thought w/ bigger tires that you are actually going faster than what your speedo says. Well I checked this w/ my GPS and I am actually going slower. Example, my speedo says 74mph and my gps says 70.
It is off about 4mph at higher speeds. Does this maybe mean it was never set right from Ford?
Thanks,
JRFX4
It is off about 4mph at higher speeds. Does this maybe mean it was never set right from Ford?Thanks,
JRFX4
Senior User
Joined: Jun 2004
Posts: 313
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Picture this...a larger tire sitting next to your stock tire next to a small tire. Roll them at the same time! The larger will take longer to make a complete full turn than your stock and the smaller tire will make a complete turn faster than your stock tire. Meaning...your larger tire you have will make your speedo say you are going slower than you really are! And a smaller tire...faster!
Senior User
Joined: May 2005
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From: CSofA
get your self a tunner like superchips and fix that speedo<a href="http://img100.imageshack.us/my.php?image=p31400045hz.jpg" target="_blank"><img src="http://img100.imageshack.us/img100/1778/p31400045hz.th.jpg" border="0" alt="Free Image Hosting at www.ImageShack.us" /></a>
Freshman User
Joined: May 2006
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I picked up and edge tuner. Factory tire calibration was at 2900+mm circumference. True circumference for the 275/70/18's I have is 2645. Way off. I just picked up the truck used, so I don't know if the speedo had been recalculted or not. If not, the factory calibration was off.
Fleet Mechanic
Joined: Mar 2005
Posts: 1,462
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From: Dedham, MA
JRFX,
It would seem you have a quandry. The speedo should be reading slower, should be turning miles slower, and your highway RPM's should be lower than with the smaller diameter stock tires (Equals better MPG HWY, less CITY) How accurate is your GPS? I wouldn't base too much on satellite positioning systems. You'd be better off finding one of those roadside radar trap thingies and experiment with that. Or pace a friend on the highway at a pre-set speed and see what happens.
It would seem you have a quandry. The speedo should be reading slower, should be turning miles slower, and your highway RPM's should be lower than with the smaller diameter stock tires (Equals better MPG HWY, less CITY) How accurate is your GPS? I wouldn't base too much on satellite positioning systems. You'd be better off finding one of those roadside radar trap thingies and experiment with that. Or pace a friend on the highway at a pre-set speed and see what happens.
