Car Talk

[Ollie]

Ollie, I'm going to say that if you're moving, the GPS digital speedometer on your phone is going to be far more accurate than the speedometer on your car. Your speedometer on your car may be precise ( meaning consistently accurate to the number of rotations of the wheel and therefore the distance traveled based on the diameter of the tire), and accurate to the diameter of your tire and the pressures that you have it set to. Unfortunately if the tire pressure is off there will be an error in the calculation that will be consistent across all speeds.

Also if the tire is not the proper circumference and diameter as specified by the manufacturer there will again be a consistent error rate across the entire speed.

Finally as your tire wears down over time the rate will be different from when the tire was installed brand new to when the tire is removed and replaced because the diameter of the tire will have changed from new to replace by approximately a centimeter, of not more.

On the other hand with the digital speedometer using GPS, even though GPS has a significant error factor when stationary, conversely when moving the error factor diminishes to near 0 because with multiple satellites the calculations that are being done realtime have the ability to iteratively solve for the error factor and make the corrections to determine your accurate speed.

We could also throw in a possible difference in the accuracy of the clock as it pertains to the speedometer, and therefore the indication of miles per hour or kilometers per hour could be affected by that as well. I would venture to guess that the atomic clocks to which are cell phones are set is going to be more accurate than the clock in your car which the speedometer uses in the calculation for miles per hour or kilometers per hour accordingly.

I know for a fact that my car speedometer indicates I'm traveling at approximately two and a half miles per hour faster than I am. I have also seen that ratio or difference between the two get narrower and wider right before and right after the replacement of a tire.

Sent from my Droid Turbo on Tapatalk.

Some cars' speedos are calibrated between 5 and 10 percent low for insurance reasons.

Ok so I just drove 120 miles.

Nokia Here gps speed: 68 mph
Digital speedometer: 69 mph
Analog speedometer: 71 mph

 

[furbearingmammal]

Mayhap I misspoke with the calibrated low. They're calibrated high -- they show higher than they actually are. I've seen way too many documented pieces of evidence claiming this to discount it.

Plus there's a difference in revolutions per mile (RPM or Revs) on new tires versus worn tires. 8/32" over a few hundred Revs adds up.

I've tried radar, GPS, speedo, and none of them seem to agree on speed. I haven't tried laser, but that's because I don't want to ask a cop for anything. Whenever I see one I get given a hard time.

 

[Ollie]

Mayhap I misspoke with the calibrated low. They're calibrated high -- they show higher than they actually are. I've seen way too many documented pieces of evidence claiming this to discount it.

Plus there's a difference in revolutions per mile (RPM or Revs) on new tires versus worn tires. 8/32" over a few hundred Revs adds up.

I've tried radar, GPS, speedo, and none of them seem to agree on speed. I haven't tried laser, but that's because I don't want to ask a cop for anything. Whenever I see one I get given a hard time.

I understood your original reply and have been suspecting the same thing. Apparently they have been doing this since the 70's on Toyotas.

 

[furbearingmammal]

I just figured I'd clarify. Enough disinformation gets spread about cars (never going back from synthetic oil, can't break an engine in with synthetic oil, wider tires are better in the snow, etc) as it is. Don't want to make the problem worse.

 

[94lt1]

I just figured I'd clarify. Enough disinformation gets spread about cars (never going back from synthetic oil, can't break an engine in with synthetic oil, wider tires are better in the snow, etc) as it is. Don't want to make the problem worse.

That's right... Synthetic oil on wider tires is better for in the snow..

 

[thunderbolt_nick]

Wow. How did I miss this thread!? I drive a 2011 GTI and it's my baby despite her luxurious lifestyle (gas, oil, etc.)

 

[dezymond]

Wow. How did I miss this thread!? I drive a 2011 GTI and it's my baby despite her luxurious lifestyle (gas, oil, etc.)

Good friend of mine has one. I'm still surprised at how much that car can push me back into the seat when you really get on the pedal. Definitely a fun DD

 

[eddiekeyton]

I used to have a navigation app on my phone that showed MPH and it was dead on with the regular speedometer in my last car. I can't remember which one it is though. I could always compare that way.

My tires/tire pressures are stock and inflated to the proper manufacturer's suggested pressures.

Either both the digital speedometer and the regular speedometer are both wrong in my car or I am the only one who drives the speed limit.

My wife swears this is the case too since she says everyone else passes her as well when she occasionally drives my car.

Waze app does it. I use it all of the time

 

[jstafford1]

I'm wishing I could afford the new Focus RS. It's gonna be a beast!

 

[cerisecons]

I'm wishing I could afford the new Focus RS. It's gonna be a beast!

Those little cars have come a long way. Good looking too.

 

[jstafford1]

Those little cars have come a long way. Good looking too.

Yeah... 320hp, torque vectoring awd. This is THE hot hatch.

 

[cerisecons]

Yeah... 320hp, torque vectoring awd. This is THE hot hatch.

Are they using the 2.0 Eco-Boost in it, or something else?

 

[jstafford1]

2.3 ecoboost.

 

[cerisecons]

2.3 ecoboost.

Same one from the MuTang, nice...

 

[cr6]

I didn't think the US carried the models that came with that much HP, only the European models? Has that changed for 2015?
Lot of power for such a small car!

S5 tap'n