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Old 12-07-2017, 11:11 AM   #1
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Default Speedo descrepency

My '13 F150 had 275/65R18 tires from the factory. I just replaced them with new Goodyear Armortrak tires of the same size. I've noticed that since new, my trucks speedo is just a little off from my Garmin GPS. For instance, when my Garmin shows that I'm doing 60 mph...my speedo shows around 62-63.

Is the speedo and odometer both ran off of the same source? I'm just wondering if the odometer is putting more miles on the truck than I'm actually going. I know that 2-3 mph is not much...but over thousands of miles it would add up.

I was also curious if ForScan has the capability to adjust the speedo setting (tire size). I have ForScan installed on my laptop and have the USB ELM327. Although I have not even hooked it up to my truck yet.
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Old 12-07-2017, 12:07 PM   #2
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I believe the ''digital'' speedo that's available in your cluster screen options is not ran off the same source as your ''physical'' speedometer operated by a needle.

It is known that there's a difference between actual speed & shown speed, mainly (I think!) for legal purposes, for instance, in case there would be a difference between a police officer radar & your vehicle's speed reading.

When I got it new, my speedometer read 122km/h while I was actually going 120km/h according to a GPS (1.63% difference). I fixed this by installing slightly bigger tires (275/60r20 instead of 275/55r20), and it's now dead on accurate.

All this to say that it's normal AND that I might get a speeding ticket now. lol

Last edited by Eduskator; 12-07-2017 at 12:12 PM.
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Old 12-07-2017, 12:37 PM   #3
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According to Ford, the speedo has an accuracy of +- 7mph so at 60 gps you could be going 56.5-63.5 and be within specs. I dialed mine in to exact speed using my SCTx4 programmer. Forscan has options available in the menu but it's no better than Ford's own IDS and I have never had it work using either.

With that said you would have to have a police interceptor model in order to have a calibrated speedo and cops won't hassle you for being 5mph over the limit as they are aware most speedo's aren't as precise. Worse for motorcycle speedo's.
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Old 12-07-2017, 01:01 PM   #4
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Default Speedo errors

I think Ford has done this all along for a couple reasons.
First, it’s about the lie-o-meter. Too many of us use this tool to brag our trucks fuel mileage up. There are plenty who do hand calculations as well and always, always find with stock tire diameters that they are off several MPG to the lower end.
Many of us who later put larger diameter tires on our trucks (I typically go up 1-2 inches in overall diameter) find that our Speedo’s and our dash mpg gauge now are more accurate - after doing this.
Love my Ford and realize the mileage is great for a 4x4 and that the gauge is only as good as all the pieces that make it work.
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Old 12-07-2017, 01:34 PM   #5
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To echo what others have said, my stock size was 265/70-17. My speedo read too high and my MPG meter was optimistic. After going 285/70-17 my speedo was spot-on and my MPG meter was within 0.2 MPG of being correct with each fill-up.
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Old 12-07-2017, 02:26 PM   #6
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Out of curiosity, now accurate is the odometer? That needs to be accurate because things like the warranty and maintenance intervals are based off of miles driven. Certain liabilities apply. I would check it a couple of times over 10 mile measured stretches of road, and/or against a GPS-based distance measurement app on the phone to make sure that is correct. If not, I think the dealer might be able to re-calibrate it for you. It's all electronics these days.

EDIT: The reason I bring up the odometer is that in the past, I've had vehicles where the speedometer reads high while the odometer is right on the money. My last couple of cars (a Honda and an Escape) have been very accurate on both though.

Last edited by Tom_with_a_Truck; 12-07-2017 at 02:29 PM. Reason: additional info.
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Old 12-07-2017, 02:48 PM   #7
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The odometer/speedmeter isn't 100% accurate, they never have been. You're off by 3% at most and that is probably as close as it gets. There are just too many factors

The diameter of the tires are the biggest difference. Even 2 tires in the same size from different manufacturers will have slightly different circumferences which will throw the odo/speedo off.

And tires change with wear. A set of All Terrain tires with 40,000 miles on them will be about 1" shorter than when new. My Tacoma was right on the money with my GPS when the tires were new but read 2-3 mph faster than the GPS right before I replaced them.

My son is an electrician and travels to the same construction site daily for months until that project is finished then changes. A couple of years ago he told me that for the last 4 months it had been exactly a 100 mile round trip to work and back. But the last few weeks his car was showing 95 miles round trip. I pointed out that he had recently bought new tires for his car. Same size,but the diameter of the new tires with full tread was enough bigger to account for 5% fewer miles.
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Old 12-07-2017, 03:05 PM   #8
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Good point on the tire wear, although I only saw about a 1 mph difference at 75 mph compared to my gps during the life of the tires on my Escape.

Now, on my Explorer (many years back), I had quite a difference going from the 30" diameter stock tires to the 32" tires that I put on. As expected, it read low. As also expected, I took a hit in low-end power even though I had 4.10 gears in it. That old 4.0 SOHC engine didn't have the torque of these ecoboost and 5.0 engines, that's for sure.
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Old 12-07-2017, 05:58 PM   #9
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Thanks guys. I was also curious about worn tires not having quite the diameter of new tires. My factory 4 year old tires only had 34K miles...but being in NW Arkansas I'm constantly driving up, down, or around corners . So we can expect our tires to wear much quicker than highway driving. But I could definitely tell that I had at least 3/8" less tread...which would equal 3/4" difference in diameter. Kinda wish now that I had went with a slightly larger diameter tire this go around. But I got a good deal on the Armortrac tires, so I'm OK with them.
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Old 12-07-2017, 07:14 PM   #10
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Quote:
Originally Posted by Rnlcomp View Post
According to Ford, the speedo has an accuracy of +- 7mph so at 60 gps you could be going 56.5-63.5 and be within specs.....
.
Thats hard to believe, a +/-7 mph speedometer tolerance?
Is there an official Ford document showing this?
Could come in handy in court for a Barney Fife type speeding ticket.

My digital, cruise and analog mph's are all dead nuts exactly the same.
Haven't though compared them to a GPS speedometer yet.
Thats on my low priority to do list.
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