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Been away for awhile. I've noticed that when using my Tom Tom GPS, that it shows speed as being above what is shown on the speedometer. Speedo 70, GPS 72, Speedo 80, GPS 84. Has anyone else noted this. BTW, it appears on level ground that 80 is about where the turbo starts working. Any comments? BTW, I haven't updated my profile to show the 1.0 replacing my '07 SES Hatch -I'll take care of that.
 

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Do you have smaller sized wheels/tires that aren't OEM-sized? My speedo reads ~0.6mph lower than my actual speed when on factory wheels (according to those portable/light post radar stations). I'd have to go 5mm higher on the sidewalls to make the speedo difference 2MPH.
 

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No, standard wheels and tires. I'll have to find one of those radar signs to check the speed. Thanks for the suggestion.
 

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I've noticed the same thing on my car (2018 1.0 6MT) from brand new. Set the cruise at 71 and the GPS shows I'm doing 73.

Warranty will last a bit longer, I guess? [cheers]
 

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If it is not a consistent percentage out (70/72=97.2% but 80/84=95.2%) then it's almost certainly the instrument itself, and not easily changed. Your GPS is almost certainly more correct than the instrument. The car's speedometer knows what RPM the wheel hub is spinning at, and makes assumptions based on the nominal radius of the wheel, so even tire wear will affect it over the course of tire life.

Most speedometers are a little behind "real-time" so they actually show what speed the car was at a second or so before. Your best time to check the speedometer is on a straight, level stretch of road with the cruise control engaged and after it's had time to stabilize speed. Most of us nowadays have a GPS built into our phones, too. There are several decent apps which will give you a speed read-out on the phone as well. Have a passenger (or two) check their GPS against your speedometer.

I live in a city infamous for speed traps and photo-enforcement, so I check my speedo on a fairly regular basis.

In general, an error of less than 3% is considered acceptable, and most manufacturers will engineer their speedometers to read higher than actual speed where there's an error (Limits liability for tickets on their part.) though they're getting to be more reliably accurate in general.

It actually amazes me that old-school cable-driven speedometers were anywhere near as accurate as they were. Modern ones (Like yours) are far more consistent, and less prone to wear and physical differences.
 

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Most speedometers are a little behind "real-time" so they actually show what speed the car was at a second or so before. Your best time to check the speedometer is on a straight, level stretch of road with the cruise control engaged and after it's had time to stabilize speed. Most of us nowadays have a GPS built into our phones, too. There are several decent apps which will give you a speed read-out on the phone as well. Have a passenger (or two) check their GPS against your speedometer.

In general, an error of less than 3% is considered acceptable, and most manufacturers will engineer their speedometers to read higher than actual speed where there's an error (Limits liability for tickets on their part.) though they're getting to be more reliably accurate in general.
I've noticed the slight discrepancies between my speedometer and my Garmin GPS. Knowing that my GPS is faster by about 2km/h (3 mph) I usually just work the mental math and adjust accordingly, even if I'm just using the speedometer only. I did not know the speedometer lags behind the "real time". Thanks for the tip. I did suspect that manufacturers "program" their speedometers to under-read somehow.
 

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Your mileage may vary.

You can observe the delay if you're observant.

This is probably a bad idea in traffic, but if you accelerate quickly, and let off the gas abruptly, you'll feel the car stop accelerating immediately, but the speedometer will still climb that last bit afterwards. The effect is more pronounced in mechanically driven speedometers, but is still present. Similarly when you are cruising along, and hit the brakes, it will be a moment before the speedometer needle drops.

This happens because the driver circuit is counting pulses over time. It can be a relatively short time, but the shorter the time, the less accurate the speedometer will be, particularly at lower speeds. This is one reason why most speedometers are useless at parking lot speeds. The driver circuit counts the pulses and adjusts the gauge while it starts counting pulses for the next update. There's a smoothing algorithm in there too so the needle doesn't jump around, and all of this means that the needle is a little behind.
 

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I’ve noticed this as well. Speedometer undercounts actual speed by just a bit - 1-2mph at highway speeds. Stock-sized tires.

Fwiw, smaller tires would cause the opposite to happen.
 

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Federally, There is a 5% tolerance for error in vehicles speedometer ( +/- 2.5% )
This mostly allows for mechanical error and tire variations

Most cops won't give you a ticket if you are within 10% of the speed limit
 

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I’ve noticed this as well. Speedometer undercounts actual speed by just a bit - 1-2mph at highway speeds. Stock-sized tires.

Fwiw, smaller tires would cause the opposite to happen.
Incorrect. Smaller tires will exaggerate the effect

Smaller tires will increase the effect, since for each tire revolution your actual distance traveled would be less than that which the system expects. The speedometer reading will be related to the axle RPM based on the expected tire diameter.

My tires are 235/40R18, which have an overall diameter of 25.4" so each revolution of the tires moves the car 79.8" or about 794 revolutions per mile. Based on that, an axle speed of about 926 RPM means 70 mph. That's what your speedometer will measure.

Change the tire size to something 5% smaller, and that same RPM will read the exact same on your speedometer, but you'll be going 5% slower.

My car was sold to me with oversize tires (245/45R18 IIRC), resulting in the speedometer actually underreporting speed. At an indicated 100km/h I was actually going about 106km/h according to my GPS.
 

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Incorrect. Smaller tires will exaggerate the effect

Smaller tires will increase the effect, since for each tire revolution your actual distance traveled would be less than that which the system expects. The speedometer reading will be related to the axle RPM based on the expected tire diameter.
My friend, we’re both saying the same thing - read my post again.

As I said, my speedometer reports a slower speed than actual.

As you decrease tire size, you’re increasing reported speed relative to actual.
 

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I've noticed the slight discrepancies between my speedometer and my Garmin GPS. Knowing that my GPS is faster by about 2km/h (3 mph) I usually just work the mental math and adjust accordingly, even if I'm just using the speedometer only. I did not know the speedometer lags behind the "real time". Thanks for the tip. I did suspect that manufacturers "program" their speedometers to under-read somehow.
I just bought 4 new tires were as the originals were 235/45R/18 now 135/50R/18 yes my tires are taller as I felt the height would soften the ride more then as I call it a low profile tire. I find Garmin vs Speedo, the GPS is 4 MPH faster then Speedo. Shy of asking a LEO to check me Radar who knows. I am not worried
 
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