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Old 06-20-2018, 10:54 AM   #1
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Speedometer recalibration

On the way home from the purchase of our new rig I realized the speedo was about 5 mi/hr off. checked the new tires the dealer put on and they are the wrong size, 255/70 instead of 255/80!! Have made calls to Ford customer help and local Ford fleet service and they can't seem to help me. Any of a fix without spending $4000 for new tires?Thanks
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Old 06-20-2018, 01:52 PM   #2
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We have had both the ScanGauge and ScanGauge D and only rely upon them for accuracy.
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Old 06-20-2018, 02:11 PM   #3
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The Ford dealer can recalibrate your speedo.
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Old 06-20-2018, 02:19 PM   #4
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I use our GPS for accurate speeds.
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Old 06-20-2018, 03:18 PM   #5
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Unless I have it wrong, 255/70 and 255/80 are the same diameter from a rim standpoint but the 70 and 80 represent profile, width/sidewall height ratio or some such. Is it possible this represents such a change in rolling diameter as to affect the speedo that much? Curious minds want to know.
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Old 06-20-2018, 04:05 PM   #6
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Quote:
Originally Posted by Zigzag03 View Post
Unless I have it wrong, 255/70 and 255/80 are the same diameter from a rim standpoint but the 70 and 80 represent profile, width/sidewall height ratio or some such. Is it possible this represents such a change in rolling diameter as to affect the speedo that much? Curious minds want to know.
As I understand it, the first number is width, and the second number is the height as a percentage of width. If that's correct, a 255/80 would be a bit taller and larger diameter.

I'm curious at what speed it's 5 mph fast. If it's tire diameter it wouldn't be the same at all speeds.
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Old 06-20-2018, 05:10 PM   #7
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Ea I'm sure you have it right.
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Old 06-20-2018, 05:21 PM   #8
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Your acceleration will be very slightly better but your fuel mileage will be lower, more rpms and more wear on the power train. If the tire dealer screwed up have them replace them or get a large credit. If you told them the size your screwed.
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Old 06-20-2018, 10:30 PM   #9
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Was so excited after 6 months to find a clean unit in our budget with new tires that I didn't think about tire size. So flew to Florida , checked it out, signed the papers, gave them a check. Two days later loaded a speedo app and found out the speed was off. I'm pretty sure the dealer will say you got new tires what more do you want. some where in this forum a tire pro has a thread that shows the tires are 1" different in height and that is what causes the speedo to be off. It's one of those things that is not a BIG deal but it just bugs me that the speed is off. I am constantly looking at the speedo when driving
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Old 06-21-2018, 11:12 PM   #10
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I learned the hard way that tire manufactures are allowed shrinkage in their sizes. What this means is some will lie and tell you it's 35" tall and the actual size is 33.5" tall. All tire dealers have a chart with the tire size (what they call it) and the actual size, Height, width, tread width. The same size tire can be very different between manufactures.
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