messed up computer
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messed up computer
I put LT 315/75 R16 tires on my new F-250. It looks and runs great, but now the MPG info on my trucks computer readout is incorrect. Before the new tires it reported 18-19 MPG, now it reports 11-12 MPG. I am quite sure that this is false info. Is there any way to straighten the computer out to work with the new tires? Thanks, Kingman
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Joined: Apr 2000
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From: Mills River
messed up computer
The computer is fine -- it's the data doing to it that's messed up.
The speedometer, odometer and trip computer are calibrated at the factory for the OEM tires (235/85R16 or 265/75R16), which are about 31" in diameter. The signal for all of thse functions, as well as for the rear ABS, is supplied by the vehicle speed sensor (VSS), which reads a 120-tooth tone ring located inside the differential housing.
The 315's you installed are about 34" in diameter -- about 10% larger (which also effectively lowers your final drive ratio). Each revolution of the tire still generates 120 pulses, but you've traveled ~10% further than the truck "thinks" you have.
As a result, your speedo, odo and trip computer will all read about 10% low. To put this in perspective, at an indicated 70 MPH (legal in may jurisdictions), you'll actually be traveling at ~77 MPH (illegal in most jurisdictions).
The solution is to have your computer recalibrated by a Ford dealer. Due to ABS range limits, the 315/75R16 is about the largest tire that can be accommodated.
The speedometer, odometer and trip computer are calibrated at the factory for the OEM tires (235/85R16 or 265/75R16), which are about 31" in diameter. The signal for all of thse functions, as well as for the rear ABS, is supplied by the vehicle speed sensor (VSS), which reads a 120-tooth tone ring located inside the differential housing.
The 315's you installed are about 34" in diameter -- about 10% larger (which also effectively lowers your final drive ratio). Each revolution of the tire still generates 120 pulses, but you've traveled ~10% further than the truck "thinks" you have.
As a result, your speedo, odo and trip computer will all read about 10% low. To put this in perspective, at an indicated 70 MPH (legal in may jurisdictions), you'll actually be traveling at ~77 MPH (illegal in most jurisdictions).
The solution is to have your computer recalibrated by a Ford dealer. Due to ABS range limits, the 315/75R16 is about the largest tire that can be accommodated.
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messed up computer
To add one thing the computer uses tire revolutions-per-mile as the input number. From the factory it's probably close to 654 rpm. You need about +-599.
1999 F-250 SD CC V-10 LWB SRW 4X4 Lariat Red clear coat 33x12.5 on 16.5x10.5" wheels K&N
1999 F-250 SD CC V-10 LWB SRW 4X4 Lariat Red clear coat 33x12.5 on 16.5x10.5" wheels K&N
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Joined: Apr 2000
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From: Mills River
messed up computer
You're welcome.
Rob brings up a good point about the recalibration. Something around 600 revs per mile is the right number for the 315's, but rumor has that entering a number lower than 600 (or is it 601?) revs per mile when calibrating the computer will disable the ABS function.
Good luck!
Rob brings up a good point about the recalibration. Something around 600 revs per mile is the right number for the 315's, but rumor has that entering a number lower than 600 (or is it 601?) revs per mile when calibrating the computer will disable the ABS function.
Good luck!
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