# speedometer accuracy



## eq_altima (Jun 16, 2005)

Hi folks, Is there anyway to determine the accuracy of my speedometer, i.e. if it reads 55 mph, am I really doing 55 mph, or is there any leeway in the reading vs the actual speed. I was considering having a cop clock my speed while on the highway. I have an 05 nissan altima. Thanx


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## RBI*04 (Sep 10, 2004)

you have a brand new car. i highly doubt that theres is that much of a difference in gauge reading and the actual speed youre driving. and the only way to know is to have someone check you with a recently recalibrated radar gun. or drive past one of those "your speed is ## MPH" signs


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## eq_altima (Jun 16, 2005)

Thanx RBI. Maybe on a dead Sunday I'll track down a cop and ask him/her to clock me. Thanx again.


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## Marvin (Oct 16, 2002)

My Altima was 3+ mph off when I took it from the dealer. I gauged the speedo against 2 of my friends cars, and gauged the mileage off of the mile markers on the side of the road. Needless to say, I think my actual mileage is much less than what my odo reads at 67k.


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## eq_altima (Jun 16, 2005)

OK one question. does the speedo decrease/increase in accuracy with age/mileage?
Maybe it's just me. cause it seems like when my speedo says 55, it seems like i'm going slower, and when it feels like i'm going about 60, my speedo reads 75.


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## blitzboi (Apr 13, 2004)

eq_altima said:


> OK one question. does the speedo decrease/increase in accuracy with age/mileage?


Sort of.... The speedo reading is based on how fast the tires are spinning (although the sensor is usually in the transmission). Since the overall diameter of a tire changes as it slowly wears down over time, this gradually affects the accuracy of the speedo. This is also the reason why it's always recommended to use a similar diameter tire when plus-sizing (i.e. going from 16" rims to 17's).

Most carmakers build in a certain amount of compensation for the changing tire diameter and other factors, generally erring on the side of "safety" (i.e. making the speedo read faster than actual speed).

Car and driver (or was it Road and Track?) did some testing back in the late '90s on this exact topic, with the aim of finding out which cars have the most accurate speedos (the euros were worst, if I remember correctly).

Anyway, hope this useless bit of information helps somebody....


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## eq_altima (Jun 16, 2005)

It helped alot. thanx blitzboi. I'm just to assume mine speedo is accurate.


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## blitzboi (Apr 13, 2004)

Most cars today have electronic speedos, and I'm pretty sure the Altima is no different, even though they display the speed using an analog face (don't you just hate those LED speedos from the late 80's -- 55, 56, 55, 56, 55....but I digress). The electronic factor pretty much eliminates any outright errors or need for calibration, except for the built-in allowances for tire wear, etc., as mentioned in my previous post above.

Just FYI.. older cars mostly used a cable with one end connected into the tranny, with the other end spinning a bunch of little magnets which moved the shaft connected to the speedo needle (which was spring loaded, of course).


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## eq_altima (Jun 16, 2005)

I was able to get a state trooper to clock me and my speedo is accurate within 1 mph.
Thanx guys for ur help.


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