Speedo varies - Diesel Forum - TheDieselStop.com
'99 & up 7.3L Power Stroke Engine and Drivetrain Discussion of the '99 & up 7.3L Power Stroke diesel engine and drivetrain in the 1999-Up Super Duty trucks and Excursions. No gas engine discussion allowed except on transmissions and drivetrain that pertain to all models. Please confine discussion of topics in this forum to those items that are specific to the 7.3L Power Stroke engine.

 
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post #1 of 6 (permalink) Old 04-14-2011, 09:32 PM Thread Starter
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Speedo varies

The speedo on my truck (99 F250 SD, supercrew shortbed, 7.3, 4x4) is inaccurate due to the larger tires that came on it. I purchased it used about 2 months ago, and the previous owner put 325 75 16s on it. I know the stock tires were 265 75 16s. Knowing that the speedo is off there, it seems to be off more at higher speeds than lower ones. My GPS puts me at 2 miles faster than speedo reads at 25, but then says 6 miles faster when I reach highway speeds, about 70. Has anyone else had this issue? Is it possibly a GPS error? I haven't had an opportunity to check the speeds against anything other than the police radar trailers, and it seemed that the GPS was dead on at about 35 mph...

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post #2 of 6 (permalink) Old 04-14-2011, 09:54 PM
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I have a friend of mine who has an 03 F350. A tall lift and huge tires. I was driving my F350 with 285/75/16 at, what my speedo said was, 65 MPH. He passed me like I was standing still. He later told me he was doing 65 and wondered why I was going so slow. So, yes, I would guess that you could be 6 or 7 MPH off at freeway speeds.

'03 Excursion, 7.3L, 4X4, Auto tranny of course (they all did), 4" MBRP exhaust, TS performance chip, K&N FIPK w/ an Amsoil EA filter, Hellwig anti-sway bar, drilled/slotted front & rear rotors. Rancho XL shocks, Rancho steering damper, Airlift 5000, S&S Diversified headlight mod. This baby surprises a lot of, so called, performance cars.
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'04 F 350, extended cab, 6.0L, FX4, manual tranny, 4" MBRP exhaust, Bully Dog Triple Dog downloader w/ Outlook monitor (Very disappointed in the Bully Dog), 2m-70cm Yaesu ft-8500, S&S Diversified headlight mod.
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'96 Mustang Cobra convertible, B&M short throw shifter, Koni adjustable shocks, viper chip, drilled/slotted rotors front and rear, rear sequential turn signals.
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'11 Harley-Davidson Road King Classic 103 ci PowerPak w/ anti lock brakes & Smart Security system. It even has cruise control!!! Candy Dark Root Beer over Candy Light Root Beer
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post #3 of 6 (permalink) Old 04-14-2011, 10:43 PM
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Recalibrate your speedometer. It is not that hard to do with Autoenginuity or you can find a speedometer shop and they should be able to do it too.

Your speedo is off by a percentage, that means the faster you travel the more it will be off.

Ken
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post #4 of 6 (permalink) Old 04-14-2011, 11:46 PM Thread Starter
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Wow. Hadn't thought about it like that. I'll take care of that, then move on to the next upgrade.

Thanks!
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post #5 of 6 (permalink) Old 04-15-2011, 12:10 AM
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I have 315s and I will confirm that the gps is correct as confirmed by the police radar gun. FYI, if you are ever on I-20 in Louisiana they will pull you over if you go faster than 82....I was goin 83. Tru-speed is another option. I think the part # is 10001. I called the Ford dealer a couple of years ago and they said they couldn't adjust for the tire size. Re-gearing is also another expensive option...but may be better on the transmission depending on how much you tow, mods, and current gearing.

On edit: the gps actually was saying 82mph and the cop said he clocked me at 83. So I guess the radar gun pulls rank on the gps.

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post #6 of 6 (permalink) Old 04-15-2011, 07:07 AM
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Re-gearing will not change the speedometer. The speed is determined bythe VSS which is located on the rear pumpkin, it reads the the targets on the ABS plate that is bolted to the ring gear. When you re-gear you do not change that plate, so your speedo will still read the same. Re-gearing only works if the speed is taken off of the driveshaft rpm.

Ken
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