are these speedometers acurate or not. i get up to highway speeds and hit cruise and almost every single car passes me. my girlfriend followed me in her car and said that it was about 5mph slower. i have a tune on my car and it says stock tires but im just not sure if the stock tires on an 08 gt are 17s or 18s. either way it drives great im just wondering if anyone else has noticed this.
Speed-o-meter?
- Thread starter nmb15831
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Ken04
New Member
have you tried the speedometer check things along side the highway ? Other than taking it to a speedo shop that's your only other option. All factory speedos vary, some more, some less.
bradleyem
Member
My speedometer, at rest (sitting still with the car running) reads almost 5MPH. According to my SCT Livewire and my GPS the speedometer is off between 2MPH and 5MPH. The faster I am going, the more it is off. 
nmb15831, What size are your tires ? Use this calculator - Tire Dimensions Made Simple - Discount Tire Just put in your tire size in both sections. This will show your overall diameter (like 27.1 or 28.2, etc) and you will also see revs per mile, put in the actual revs per mile in your tuner (or dia, whichever it asks for) then load it in the car and you should be dead on.
The faster I am going, the more it is off.
They are off by a percentage, so at low speeds it may not be noticable, mid speeds 1 or 2 mpg, highway speeds 3-5mph, and high even more so.
When the speedo reads 85mph my Stock 06gt 5speed with 3.55 and 235/55-17's is going 81/82mph on my GPS. My wife's Flex shows about the same variance.
NastyStang113
New Member
Late model Ford vehicles with stock height tires will read 2 mph faster on the speedometer than you're actually going..
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