Anyone notice if their speedometer is off by a few MPH?

[GTP]

The gtpp is a slightly unique case given the larger sidewall on the rears. All total tire diameters are practically identical but those rears do add an extra 0.4" approximately which makes your speed off by 1.5-2 mph at 65 mph. Not much of an impact but I run at 79 on the highway to avoid tickets but that 1-2 could put me at 81 on their radar which = flashing blue lights.

Isn't the speedo connected to the front wheel? The front tire diameter is constant no matter which factory configuration of the car.

If this is the case, then different rear tire diameters affect the overall gear ratio slightly, but the speedo remains the same.

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[cyandhappiness]

https://en.m.wikipedia.org/wiki/Parallax

 

[Ilovemyv8]

I got a speeding ticket for 81.....thought I was doing 80....:headbonk:

 

[Mattneedforgreen]

Digital vs analog the struggle is real and off in every car i have ever had with both

 

[MikeHTally]

My speedo registers 2mph faster than actual (as measured by my Garmin).

Same here, even with 245s instead of 235s. My Ram reads 4.3% low due to larger tires.

 

[JKL1031]

Yea mine does this as well, I'm not sure how far off it is but even my cruise control reads about 1-2 mph low compared to my speedo at highway speeds. Not really sure why


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because the speedo isnt directly infront of you,,the angle your sitting to the speedo can influence this and what some r seeing here..

 

[Zooks527]

FWIW, my car is within 1 MPH of both my GPS and roadside indicator signs that I see.

 

[frank s]

May I revive this thread, please?

The 2018 GT PP1 digital matched speedometer-to-flasher-warning 25mph pretty close: car said "26", sign said "25". Extra, external Headup speedo display started out about 2mph higher reading than the sign. but was adjusted to dead-on.

2019 car's digital hard-to-ignore display says"27-28" when the sign says "25". Is there a possibility of adjusting-out that discrepancy, or should I go back to the the adjustable Head-up speedo?

 

[SAL-E]

I would not trust the accuracy of those speed limit signs. Verify your speedo with a gps unit. You will get much more accurate readings. Mine is accurate all the way to 70. Above that, the car is 1mph fast.

 

[rocky5517]

When I plug Waze into Apple play, it ( Waze) often shows me going faster than the car's speedo. And by 5-9 mph.

 

[TunedN]

A. Fed rules allow a 5% error in speedometer accuracy from manufacturing variables.

B. Check your tire pressure.

C. Use a gear ratio calculator [after you confirm tire pressure] to see what speed you should be at a given gear and RPM. Confirm while driving.

D. The odometer is based on # of tire rotations, which more accurately represents how much wear is on the rotating parts aside from the engine. (city vs hwy wear and all). Speedometer is based on a calculation from final drive ratio and tire rotations.

 

[Armageddon Turbo]

Those things have to be calibrated...I would not be surprised it it were off.

 

[MikeHTally]

I would not trust the accuracy of those speed limit signs. Verify your speedo with a gps unit. You will get much more accurate readings. Mine is accurate all the way to 70. Above that, the car is 1mph fast.

^^^^^^^^^^^^^^^^^^
What he said. Larger tires will slow the speedometer down a pinch. One mph off is not a bad error, though. The 245/55s on mine brought the speedometer down. It and the odometer are as accurate as can be expected. It's the computer's "guess" about mileage that makes me crazy 5%-8% optimistic. The EB gets about 1.2 mpg less than my Speed 3 on the same driving cycle. Not a major concern, just aggravating. I've not seen a computer's "guesstimate" close enough to be worthwhile anyway.

 

[Vicr]

I don't look at the speedo. Too busy lookin' for cops.

 

[drive_55_not]

^^^^^^^^^^^^^^^^^^
It's the computer's "guess" about mileage that makes me crazy 5%-8% optimistic. The EB gets about 1.2 mpg less than my Speed 3 on the same driving cycle. Not a major concern, just aggravating. I've not seen a computer's "guesstimate" close enough to be worthwhile anyway.

Have you changed the AFE bias setting, the default is 1000 ... My 2018 GT CPU calculation was close to 2mpg higher than actual, I changed the AFE bias to 950 and the actual and CPU calculated mpg's is now 1/2 mpg off ..

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