Gps Miles Per Hour Accuracy

[tjmack05]

I've always had an (unproven) suspicion that car manufacturers set the speedometers on new cars a few MPH lower than "true". I mean, if we all think we're speeding, but in reality we're going 55, what's the harm in that? Anyway, I was consulting my Vista on the ride home today and it showed my speed to be between 3-4 MPH slower that what the car's speedo read. I have no idea how accurate (or not) these GPS devices are, so I thought I'd ask the class.

Thanks

[+Jamie Z]

Here is a good discussion of that topic from a couple months ago.

 

Jamie

[+Renegade Knight]

Very accurate. My Pontiac Montana was spot on.

My Toyota Corolla is 10% too slow.

My Bronco was 10% too slow but bigger tires put it 5% faster than the speedo.

[peter]

They're very good (within a few tenths of a mph) when you're going a steady speed and have a reasonably clear view of the sky. But they do some averaging so if you change speed quickly you can notice a little lag in the GPS reading. And they're subject to rare glitches from reflected signals, poor reception, etc. that can lead to absurdly high speed readings, like thousands of miles per hour while walking around.

 

You can easily check your car speedometer with a watch by driving a steady speed on the freeway where there are mileage markers on the side of the road. An inaccuracy on the high side of a few mph isn't uncommon. Many countries specify that speedometers should never read low but can err by a few percent too high.

[+robert]

Very accurate.  My Pontiac Montana was spot on.

My Toyota Corolla is 10% too slow.

My Bronco was 10% too slow but bigger tires put it 5% faster than the speedo.

I always get stuck behind slow moving Corollas!!! nobody believed me until I started pointing it out, now people come in to work talking about getting Corolla'd.

 

I was actually wondering about this same topic about a week ago, thanks for asking it and thanks to Jamie Z for the link to past discussion.

Edited January 22, 2005 by robert

[tjmack05]

Thanks for the correction. I meant to say that the speedo might be set too high, so that you are actually moving slower that the speedo indicates. I'm wondering if it's intentional, to keep everyone driving just a little bit slower. Thanks for the link to that thread.

[PyrateintheCity]

I have to agree...My GPS states that I am going about 10-12% slower than the speedo.

 

Oh BTW...I drive a Ford Thunderbird...That might be why

 

LongJack

Pyrate in the City

[+Team_T&P]

Also depends on the GPS and software involved. My Garmin is spot on, software I wrote that polls the garmin and computes speed based on 3d vectors is spot. My Palm running Pathaway3 is way off. Approx, 10-15%. The overall accuracy of the navman is similar to my Garmin, so I can only speculate that it is the software.

[+NightPilot]

A car speedometer isn't a precision device. It only measures the number of times the wheels turn (actually not the wheels, but the drivetrain) and converts that to a pointer position. Depending on your tire size, inflation pressure, tread wear, the temperature, etc. the speedometer will almost always be incorrect. Just to make sure you don't get caught speeding and sue the manufacturer, it seems to be set to read higher than actual speed, but it certainly reads higher as the tires wear and the inflation pressure decreases.

[+Couparangus]

Very accurate. My Pontiac Montana was spot on.

My Toyota Corolla is 10% too slow.

 

I noticed this too on my Montana. It was good to know as every car I've had up to this has a higher indicated speed and I usually run at the max speed. I also noticed the Montana has an aluminum hood so my GPS mag-mount antenna doesn't stick to it.

 

C-A

[Overland1]

My Jeep's speedometer is within one MPH at 60 MPH, probably because it has the larger factory-supplied tires on it (example "C" below). They offered three tire choices on the Grand Cherokee and there is quite a difference between them.

 

A.) P235/65R17 Goodyear Wrangler HP black sidewall all-season

(721 Revs per mile)

 

B.) P235/65R17 Goodyear Eagle LS black sidewall all-season performance

(719 Revs per mile) N/A P

 

C.) P245/65R17 Goodyear Wrangler SR-A outline white letter all-terrain

(710 Revs per mile)

 

If choice "B" had been on the vehicle, I suspect the speedometer error would be greater.

 

By the way, I confirmed accuracy by measured mile/stopwatch, GPS, and radar.

 

On a previous vehicle (1998 Grand Cherokee), I upgraded to one size larger (P235/75-15) when the OEM tires (225/75's) wore out, and speedometer accuracy improved markedly.

[this_is_nascar]

One the new cars, the auto manufactures set this purposely by 2 or 3 MPH higher than what youi're actually going. They were getting tired of getting suide by people that were showing they were doing 65 MPH and they were actually doing 61/62. The decided to alway err on the side of reporting a higher speed than the actual speed.

[+park2]

I rent a lot of cars (day job - also gets in the way of caching)

I always have my GPS with me.

In a sample size of 50 cars in the last 3 years, US, Canada, all brands including a few imports, I'd say

40 of them would read 2-3 mph/3-4 kmh higher on the speedo then I was really going.

The others would be right on (within the resolution of the speedo ticks).

Never seen one read low.

[vagabond]

If you can find out for sure which way your speedo is off, you can go to your dealer and you should be able to pick up the proper speedo gear. Although you do need to know how many teeth there are on your present gear.

I dont think it would be worthwhile to take the vechile in and have it done as you'd probably be looking at an hours labor or so.

 

PS at one time one tooth would equal 5 miles an hour, so between tire size and speedo gears you could get them very close.

[+IV_Warrior]

I'm wondering if it's intentional, to keep everyone driving just a little bit slower.

Yes, it's intentional. Keep in mind that the speedometer AND odometer get their feed from the same place. Laws dictate that the odometer has to show AT LEAST the actual number of miles on the vehicle, and be within x% of actual. Most manufacturers err on the side of over-reporting speed/miles slightly, so they don't get in trouble for under-reporting mileage....

[DriverFound]

My Jetta is about 1-2 Km/h off comparing to the reading on my Vista with accuracy of 6m.

[kscaddo]

Wife's 2004 Nissan Xterra reads 3 mph over.

Daughter's 2004 Nissan Sentra reads 3 mph under.

My old GMC pickup is dead nuts.

[+RAYAR]

My '99 Contour is right on.

[Sarjor]

I've calculated the error to be 5KPH on My Honda Accord

[+NightPilot]

Most manufacturers err on the side of over-reporting speed/miles slightly, so they don't get in trouble for under-reporting mileage....

 

And this also helps out the manufacturers by causing your warranty to expire more quickly. The faster the miles roll over on the odometer, the quicker the warranty is over. 5% error means that on a 50,000 mile warranty, it will expire at 47,500 actual miles, meaning you lose 2500 miles of warranty. 5 mph low at 50 mph is a 10% difference, so your warranty would expire after you have driven only 40,000 miles.

Edited January 24, 2005 by NightPilot