'11-Up General QuestionsGeneral questions related to 2011-Up Super Duty trucks. If it doesn't fit the other categories, post it here. Gas engine discussion that pertains to all models is allowed. Specific gas engine questions should use the Gas Engines forum.
On a recent road trip I used my standalone GPS unit on the dash because, with my camper on, the stock XM radio antenna signal sucks and I use an auxiliary antenna which the stock GPS doesn't see. (another thread coming on that one someday).
Anyway, I noticed the speedometer on the dash seems to track the truck's speed at about 4 MPH less than the GPS speed indication. I'm running the stock 18" Michelin tires & wheels.
Does anyone know why the discrepancy?
Which speed is the actual truck's velocity? I tend to believe the GPS indication vs. the truck's speedometer. Is there any way to adjust and correct the truck's speed indication?
Thanks,
JoeKl
Last edited by JoeKL; 01-21-2013 at 10:31 AM.
Reason: bad spelling
Anyway, I noticed the speedometer on the dash seems to track the truck's speed at about 4 MPH less than the GPS speed indication. I'm running the stock 18" Michelin tires & wheels.
Does anyone know why the discrepancy?
Which speed is the actual truck's velocity? I tend to believe the GPS indication vs. the truck's speedometer. Is there any way to adjust and correct the truck's speed indication?
Thanks,
JoeKl
The renowned automotive journal the Wall Street Journal addressed this a while back - see here. Also, a good article explaining GPS vs. Speedo here. In my experience, my speedo is usually 2-3 OVER my 'actual' speed as measured by GPS, roadside radar signs, police officers claims, etc.
As far as calibrating, I'll let one of the Ford techs weigh in here, but I'd imagine it can be done since you need to adjust it for tire size, etc...
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2011 F350 KR 6.7 CC SRW SB FX4 20" with 3.55 e-locker, chrome package, Nav, Rapid heat, Dual climate control, 5th wheel and snowplow prep, dual alternators, Ranchhand Sport Front Bumper with Warn 16.5ti winch, Line-X Xtra, Duraflaps, KC Rally 800 long range and driving lights
Ordered 5/2011, Delivered 7/14/2011
2006 F250 XLT 6.0 CC SB FX4, Stealthbox, traded in for the '11
You will always see some variation in actual speed depending on how worn you tires are. The stock Michelins come with 16/32 of tread. So new tires are 34.4" in diameter. Worn out tires will lose that 1/2" of tread all around the tire so the worn out tire will be 33.4" in diameter.
34.4" tire = 606 Rev per mile
33.4" tire = 622 rev per mile.
That is a little more that 2.5% variation in speed readings just depending on the tire wear.
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11 F350 CC Lariat SRW LWB Tuxedo Black
B&W turnover ball
On a recent 2,000 mile round trip from our home in southeast Texas to Beaufort, SC & back I checked the truck's odometer with the freeway mile markers and, to my surprise, they agreed completely! I've never seen that before. I then checked the speedometer by setting the cruise control on 60 mph and timing between mile markers. The mile markers came by at 60 second intervals. One mile per minute = 60 mph. I've never seen a vehicle with a speedometer/odometer this accurate. Now, I realize that this may not be true for every truck, but I witnessed it on this trip.
I guess I could check the speedometer & odometer against my hand held garmin, but I imagine the results would be the same.
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2013 F-350 Lariat 6.7 L PSD Crew Cab Dually, FX4, white platinum metalic tri-coat with black interior, N-Fab step bars, factory spray-in bedliner and 5th wheel hitch, trailer tow package, integrated brake controller, upfitter switches, Sync with My Ford Touch, memory group, remote start, reverse sensing with rear view camera, nav, satillite radio, Weather Tech floor mats, limited slip diff, moon roof, climate controlled seats, engine block heater and tailgate step.
2008 Big Horn 5th wheel RV by Heartland model 3670RL. 38' long with 4 slides & all the goodies.
SOLD:2005 F-350 Lariat, 6.0 PSD, Torqueshift auto trans, crew cab, SRW, short box, FX4, ESOF, limited slip dif., tow command, upfitter switches, 6 CD changer, Lariat Lux. package, camper package, engine block heater, triple A-pillar gauge set (Autometer) with pyrometer, trans temp & voltmeter, MBRP 4" cat back exhaust w/5" dual wall chrome tip, dark shadow gray/ med. flint interior.
my speedo reads 60 and gps says 58 , speedo reads 70 gps says 68 same at 50 gps says 48
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2011 KR Dually CC 4X4 Royal Red
2006 KR Dually CC 4X4 ,True Blue
2005 KR Dually CC 4X4 ,Dark Copper,sold 12/30/10
2005 Eddie Bauer Excursion 4X4 ,White
All purchased from Bobby Williams At Sunrise in Fontana
My brand new tires had about 160 miles on them at the start of the trip where I began comparing the speed indications. I don't think they would have much of an affect on speed readings out of the box but, who knows. I'm not sure its worth calibrating now that I know the delta. I also checked my gps against a few speed indicating radar signs and they generally agree - unlike the speedo on the truck.
JoeKl
You will always see some variation in actual speed depending on how worn you tires are. The stock Michelins come with 16/32 of tread. So new tires are 34.4" in diameter. Worn out tires will lose that 1/2" of tread all around the tire so the worn out tire will be 33.4" in diameter.
34.4" tire = 606 Rev per mile
33.4" tire = 622 rev per mile.
That is a little more that 2.5% variation in speed readings just depending on the tire wear.
Great info Painted Horse... thanks!
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2011 F350 KR 6.7 CC SRW SB FX4 20" with 3.55 e-locker, chrome package, Nav, Rapid heat, Dual climate control, 5th wheel and snowplow prep, dual alternators, Ranchhand Sport Front Bumper with Warn 16.5ti winch, Line-X Xtra, Duraflaps, KC Rally 800 long range and driving lights
Ordered 5/2011, Delivered 7/14/2011
2006 F250 XLT 6.0 CC SB FX4, Stealthbox, traded in for the '11
Here is a foot note for mile markers. In PA we have the nations oldest super highway, the PA Turnpike. A friend of mine does engineering work for the Turnpike. There are several old tunnels that have been bypassed by going over the mountains instead of thru them. Since they didn't want to renumber the entire highway, there are sections he calls 'long miles' and others where the road has been straightened he calls 'short miles'. So in those areas the mile markers aren't accurate and cannot be relied upon for checking a speedo or odometer.
Food for thought as I'm sure this condition exists on other roads in other parts of the country.
2011 F250 CC Lariat Ultimate, Royal Red/Pale Adobe, 18" wheels, 3.31 locker, 172"WB factory Nav/Sync,etc.
Also have a 1997 F450 7.3 XLT with 14' box, Hypertech programmer
1987 F350 6.9 Banks Sidewinder, 12' box
Still going at 200K+
Traded:08 F250 CC 4x4 Lariat, 172"WB
Traded:01 F350 CC 4x4 Lariat, 172"WB
That's good info. I recall a few sections of highway in the Golden state years back that were marked with "speedometer test section" signs. I haven't seen any lately. Wonder if that program still exists with CALTRANS.
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addressed this a while back - 
F350 KR 6.7 CC SRW SB FX4 20" with 3.55 e-locker, chrome package, Nav, Rapid heat, Dual climate control, 5th wheel and snowplow prep, dual alternators, Ranchhand Sport Front Bumper with Warn 16.5ti winch, Line-X Xtra, Duraflaps, KC Rally 800 long range and driving lights
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