Tire size vs speedometer reading.

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Jun 27, 2014 | 07:09 PM

LostKnyt

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Tire size vs speedometer reading.

I am in need of some technical input to a question I have. I purchased a 2010 F-150 XLT extended cab. I noticed after driving it a while that everyone around me was moving faster than I was traveling at the posted speed according to my speedometer. I installed my GPS and noticed that I was moving 5mph slower than what my speedometer was reading. I flagged down a local police officer and we went to a parking lot and he shot me at different speeds, all reading 5mph slower than what my speedometer was reading. OEM tire size is 235/70R-17, but the tires on the truck are 255/65R-17. Will the taller aspect ratio come out to the speed difference or do I need to go to the Ford dealer for reprogramming. Please advise.

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Jun 27, 2014 | 07:28 PM

QwkTrip

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Changing tire size would produce an error that is a percentage of true speed, converging at zero and being further off the faster you go. Being off by the same 5 mph at various speeds is an offset of speedo reading, not a tire size issue. That is assuming your test had enough differentiation in speeds to actually measure a real change.

In the old days you would just bend the needle in the gauge until it read correct. Not sure how that is handled today.

Jun 28, 2014 | 04:01 AM

D8chumley

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From: Oaks,PA

If you can't live with it a trip to the dealer is in order. They can program for several factory sizes. I have an SCT programmer that compensated for my taller tires. While its' not dead on, it is close enough for me. Welcome to FTE !!

Jun 28, 2014 | 09:54 AM

Truckpilot1329

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Those tire sizes are listed as the original and the optional tire sizes at Tire Rack for
a 2010 2WD F-150. And both tires are 30" diameter.

Running 5mph low at 50 mph would be a 10% error, requiring a tire 10% bigger in
diameter, or 33" to compensate. But you say you're running 5 mph off at all speeds.

I'm wondering about your comment that you had a cop radar check you in a parking lot.
What speeds were you able to get to, and what speed range were you traveling on the
road with your GPS? In other words, were you "about" 5 mph off just going
25-35 in the parking lot and around town? Or were you exactly 5 mph off whether going 35 or 80?

Jun 28, 2014 | 02:43 PM

gDMJoe

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Joined: Dec 2010

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From: Timbuk3, MI

Quote:

LostKnyt ... OEM tire size is 235/70R-17, but the tires on the truck are 255/65R-17.

255/65x17 (30.1" diameter / 694 FoMoCo IDS rev's per mile) -IS- the OEM, stock size for the XLT 4x2.

The other OEM, stock size is 235/75x17 (30.9" diameter / 675 FoMoCo IDS rev's per mile).

The difference in the 2 sizes is 2.7% MEANING: At 60MPH you're off (slow) by 1.6MPH

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Jun 28, 2014 | 06:24 PM

tseekins

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From: Maine, Virginia

This is kind of odd. I asked the service advisor at my dealer about a reprogram as I intend to change from the stock 235-75-17's to the optional stock 265-70-17's that many XLT 4x4's come with. He said that there would be such a small difference that a reprogram isn't needed.