205-55-16's and speedometer

[Skarbro]

According to tire size calculators on the internet, my speedometer should only be 0.444% faster than normal with my winter 16's on. That means when my speedometer says I'm going 100 km/hr, I'm actually going 99.56 km/hr.

But according to my GPS, when my speedometer says 100 km/hr, I'm actually going 97 km/hr. That's a huge 3% difference.

My winter tires have plenty of tread - only used for one season. It makes me wonder if the speedometer calibration is off? Or maybe the GPS is wrong?

I'll have to check it again with my stock 17's on.

Anyone else notice anything like this?

[Wild Weasel]

What size wheels are your winter tires on? This can have an effect, as a wider wheel will produce a slightly shorter tire.

I would have more faith in the GPS than I would in the speedometer.

[Skarbro]

What size wheels are your winter tires on?
Um... that's in my original post. :)

[3GFX]

Tire inflation??

If I'm not mistaken the whole reason cops don't charge for tickets within 10kmh over is because you can argue speedometer calibration. I believe the leaneance is 7 or 8%, so going by that 3% is nothing ;)

[Wild Weasel]

Um... that's in my original post. :)

No it isn't. I'm talking about wheel width. For example, let's say both tire measurements are based on a 7" wide wheel. If the stock wheels are 6.5" wide (I have no idea what they are) and the ones with your winters are 7.5" wide, this will accentuate the difference in tire heights.

[Xenon]

Skarbro: Honestly, i wouldn't really rely on the GPS being as accurate as you think. the fact that the GPS is 3 km/h off is rather reasonable for a GPS unit, especially a handheld or car windshield-mounted unit. Because of the distance that the GPS signal has to travel, and most GPS units are accurate to a certain distance, you don't have anything to fear.

I think, considering the accuracy which each of the devices (online calculator, speedo and GPS) are capable of, you don't have anything to worry about.

[S.F.W.]

hmm, I am going to check this out later today. My GPS unit is integrated into my car, and has a VSS wire connection as well. I expect it is pretty accurate.

[Fuman]

My GPS and my speedometer matches up.
I can't see any difference with my naked eye
I am using 205-50-17 (stock), winter (w/ GPS) 205-55-16 (same as you skarbro).

But what model GPS do you have?

[Skarbro]

Tire inflation??
I just checked the pressure before a long trip and all were exactly 32 psi - and the temperature didn't change.


No it isn't. I'm talking about wheel width. For example, let's say both tire measurements are based on a 7" wide wheel. If the stock wheels are 6.5" wide (I have no idea what they are) and the ones with your winters are 7.5" wide, this will accentuate the difference in tire heights.
Hmm... you may be on to something there. The winters buldge out more than the stock tires. Maybe I should put more than recommended air pressure to compensate. I'll have to keep an eye on the treadwear to determine if this is needed.


Skarbro: Honestly, i wouldn't really rely on the GPS being as accurate as you think. the fact that the GPS is 3 km/h off is rather reasonable for a GPS unit, especially a handheld or car windshield-mounted unit. Because of the distance that the GPS signal has to travel, and most GPS units are accurate to a certain distance, you don't have anything to fear.

I think, considering the accuracy which each of the devices (online calculator, speedo and GPS) are capable of, you don't have anything to worry about.
Hmm.. I would have thought that GPS speed tracking would be pretty accurate. I have no real basis to base that on though.


My GPS and my speedometer matches up.
I can't see any difference with my naked eye
I am using 205-50-17 (stock), winter (w/ GPS) 205-55-16 (same as you skarbro).

But what model GPS do you have?
I have a low-end Garmin C330.

[Wild Weasel]

Hmm... you may be on to something there. The winters buldge out more than the stock tires. Maybe I should put more than recommended air pressure to compensate. I'll have to keep an eye on the treadwear to determine if this is needed.


If it's due to the wheel width, the pressure isn't going to make much of a difference.

I did a quick look earlier this morning. I can't remember which site I was on, but it was a tire company and mentioned that their 205/50/17's are measured on a 7" wide wheel while their 205/55/16's are measured on a 6.5" wide wheel.

Thus, if you had the 16's on a 7" wide wheel, they'd be a tad shorter.

I would think this was pretty trivial though, but could be enough to make up your difference when coupled with your season's worth of wear on them.

Remember... the diameter difference between a 205/50/16 and a 205/55/16 is only 0.8". That's more than the amount that you're saying your speedo is off, but only amounts to less than half an inch off the tread. So if you're a quarter inch down in tread and a little more down due to the wheel width, or some such thing, then it will all make sense.

Basically... I think they're fine. :)

[Skarbro]

Ah I thought you meant the tire. The wheel I guess can be measured when off the car.

[Xenon]

In reference to my previous comment,

I know GPS works in pulses or cycles. GPS Unit requests location from satellite, satellite replies back to GPS Unit... each cycle takes about 1 second.

I also know that in the time of that one second, the car would move a certain distance within that time and that distance wouldn't be updated until the next cycle is complete.

Most GPS units are accurate within 10 metres or so. Better ones will be more accurate.

All I am saying is that accuracy variable and the time delay could in fact be the reason for your 3 km/h variance. Which, as far as I know, is an acceptable variance for a GPS system.

I will tell you right now, i have 215/45/17s for summer tires, and 205/55/16s for winters. In both cases my speedo and my GPS vary by about +4% (104 on the speedo is 100 on GPS). I use a Garmin Nuvi 250.

[S.F.W.]

I checked mine out on the drive home tonight. My speed on the Nav system, and speedo were right on(as I could tell). I am running 205/55/16's on Rav 4 rims.

[Fuman]

it also depends how many sats you have a signal from.

I used to use a Global-sat BT hooked up to a blackberry.
It was pretty accurate. But Xenon is right, there is a delay in information update.

[southpawboston]

according to my calculations, switching from a 205/50-17 tire to a 205/55-16 results in a decrease in tread circumference of 0.8%. therefore 100kph actual speed would correspond to 100kph readout with the 205/50-17s and 100.8kph readout with the 205/55-16s.

it's the difference in tread circumference that corresponds linearly with speedometer error. remember from high school math, circ=2 x pi x radius. radius = rim diameter + 2 x sidewall height.

[McGuyver_3]

I have the stock recommended tire size on my car for my winters (195/65r15) and my nav tells me that when i am doing 100 on the speedo the nav says 97.8 when i have my summers on 225/45r17 i get the same readings. I am 100% sure on this i first thaught this was because i modified my cluster but reading this i am starting to think this is normal

[saintjames7]

My speedo has always been different from my Garmin c330 and recently I upgraded to the Garmin Nuvi 760 and it's still different from my speedo.

But both my gps match up perfectly to my dad's car.

I always set my cruise control to the gps speedo and not to the car.

[Skarbro]

I'm starting to think that our speedo's are lower than they should be.

[little_wheelz]

I have the garmin 650 and the GPS read ~ 3% slower then the speedo in the car.

from the link below "Garmin® GPS receivers are accurate to within 15 meters on average."

http://www8.garmin.com/aboutGPS/

[Skarbro]

I tried it out in our Corolla yesterday and it is just about exactly in sync with the Corolla's speedo. I'm getting even more convinced that our Mazda3 speedos are too fast from the factory.

[Wild Weasel]

FWIW, mine's off by quite a bit as well, but that's expected with my tire size. I'm running 225/45/17's on the MS3. I believe when the speedo says 120 I'm only doing around 115 or so.

I'll be interested in how it measures when the stock tires are back on.

[tweak_s]

I thought 205 55 16s are the same as the stock tire.... unless I'm mistaken those aer waht I'm running for both summer and winter (I have a GS hatch stock with 16")

[Wild Weasel]

I thought 205 55 16s are the same as the stock tire.... unless I'm mistaken those aer waht I'm running for both summer and winter (I have a GS hatch stock with 16")

You're right, they are the correct size. As mentioned above though, there are other factors involved that affect the tire height such as the wheel width and even a year or two of wear on them.

[OnkelHans]

I got a Garmin 650 for Christmas and installed it (stuck it to the windshield) today. I noticed it was about 4 km/h per hour. Made me wonder why but now reading this I know I'm not alone. Gonna try it against the speedo in my Corolla and see it there's a difference.

[Skarbro]

Interesting tidbit from Garmin's website on the c330:



GPS Accuracy:
Position: <15 meters (49 feet), 95% typical

Velocity: 0.05 meter/sec steady state

[RX9]

will that effect the mileage as well, should be more than what u should get, right? i am using smaller radian tire than stock as well.

[starwind]

Xenon:

That is not how GPS works.

GPS receivers are receivers -- they do not transmit or send signals to the satellites. They just passively receive the signals from multiple satellites (whose signals are synched and transmitted on a regular basis) and then do some internal calculations to determine where they are in space.

Some good "how GPS works" primers:

http://www.howstuffworks.com/gps.htm

http://www.tomtom.com/howdoesitwork/

http://www8.garmin.com/aboutGPS/

Also, FYI, according to the second link "Garmin® GPS receivers are accurate to within 15 meters on average."




In reference to my previous comment,

I know GPS works in pulses or cycles. GPS Unit requests location from satellite, satellite replies back to GPS Unit... each cycle takes about 1 second.

I also know that in the time of that one second, the car would move a certain distance within that time and that distance wouldn't be updated until the next cycle is complete.

[Elder_MMHS]

FWIW, the stock wheel dimensions for 16 - 18 are:

16"
Rim: 16x6.5
Tire: 205/55/R16

17"
Rim: 17x6.5
Tire: 205/50/R17

18" (MS3)
Rim: 18x7
Tire: 215/45/R18

Yeah the MS3 stock rims are narrow. :P

[Wild Weasel]

will that effect the mileage as well, should be more than what u should get, right? i am using smaller radian tire than stock as well.

That's correct.