GPS Speed vs. Speedometer

[Roger]

I've noticed when using my TomTom that the speed I'm traveling at is not the same speed as my speedometer on the Camry (or my Sienna van for that matter too). The GPS is always higher. For example, when the car says I'm going 70 mph, the GPS reads 74-75. Lately I went past a radar gun in a construction zone and it read low too, i.e.: my speed was 42 when the speedometer was set at 45. When I asked my dealer if they could check/adjust the speedometer, I was told no. They are sealed.

How accurate is a GPS speed reading? I'm beginning to suspect it is correct. Kinda curious before I go past a trooper.

 

[cadfaeltex]

I had the same experience and slowed down a mile or two just to play it safe. If my garmin is wrong I've probably been really annoying people for the last few months.

 

[spackard]

You probably have different than OEM size tires.
The GPS speed is more accurate than the speedometer.

 

[gathermewool]

I've confirmed with multiple GPS's that my speedo reads low by around 1 mph with my summer tires/rims and high by 1-2 with my winter tires/rims. I guess it's a good thing, since most of the year I have my summer rims/tires installed.

 

[Quattro Pete]

Originally Posted By: Roger
When I asked my dealer if they could check/adjust the speedometer, I was told no. They are sealed.

I wonder if you'd get any response if you contacted Toyota corporate about this.

By design, speedos are allowed to read high, but they are not allowed to read low. This is a potential safety issue. If corporate does not listen, I would contact NHTSA.

 

[Lead Shoes]

The speedometer in my Honda is off by 3mph at 30mph, 4 at 45mph, 5 at 55mph, and 7 at 75mph.

All because I used my gps as my speedo for a week, it has been verified by me getting pulled over for going 60 which is 65 on mine, and those dumb slow down signs that clock your speed.

 

[Kestas]

SAE requirements allow speedomoters to read up to 3% fast and 1% slow.

When I buy a new car, I sometimes calibrate the speedometer by driving a steady speed on the open highway, and using the mile markers and a stopwatch.

I don't know if I'd admit to a cop that my speedometer is off or not working. He may additionally write you up for defective equipment, which will force you to spend money to fix the problem, and prove it's fixed.

 

[Quattro Pete]

Originally Posted By: Kestas
SAE requirements allow speedomoters to read up to 3% fast and 1% slow.

Would you happen to have a link to this publication? I've been looking for some US spec on this...

 

[needsducktape]

I take my gps everywhere- I have noticed most cars to be off (+/-) very few were spot on.

Its not a big deal to me, just kind of interesting to see which cars are more accurate.

I also live exactly 300 miles from home in PA vs home in NY. -I go the exact same route every time, yet many cars odometer reads 307 some of my other cars would be 297 etc etc...

If memory serves correctly, Honda had a class action lawsuit regarding the odometer inaccuracy.

 

[HM12460]

My Magellan is off from my speedo also. About 1.5 mph@60. I don't really concern myself with it. My truck has oem sized tires on it.

 

[Kestas]

Originally Posted By: Quattro Pete
Originally Posted By: Kestas
SAE requirements allow speedomoters to read up to 3% fast and 1% slow.

Would you happen to have a link to this publication? I've been looking for some US spec on this...

Not at my fingertips. This is something I conmmited to memory 30 years ago. At the time I bought a car where the speedometer was clearly off, and I was working for one of the auto manufacturers, which had an excellent resource of information.

 

[ethangsmith]

My 86 Capri is dead on accurate when it comes to the speedometer at all speeds. My 11 Ranger and 99 Escort are both off by 1-3mph at 55. There is something to be said for old cable driven speedometers that are working properly......

 

[08_SHELBY]

Mine is dead on too. New cars are calculated and ran through the computer so it should be accurate unless the tires are off size. I can change the tire size on my handheld SCT and it will adjust the speed accordingly. Years ago they were way off though, they usually read faster as I recall

 

[cadfaeltex]

Originally Posted By: spackard
You probably have different than OEM size tires.
The GPS speed is more accurate than the speedometer.


Definitely OEM in my case but the Speedo is 18 years old.

 

[NHGUY]

2002 Caravan,speedo MPH 65
GPS MPH 67-68
roadside portable automated radar unit= matches exact speedo reading.
GPS used to be rather accurate,seems to read 2-3 mph faster now.Same tires,same everything.

 

[Garak]

Originally Posted By: ethangsmith
My 86 Capri is dead on accurate when it comes to the speedometer at all speeds.


Oddly enough, Capris/Mustangs and Town Cars/Crown Vics/Grand Marquis often had the most accurate speedometers. The police versions (i.e. the Mustang the the CVPI) had certified calibration speedometers. For whatever reason, that type of speedometer occasionally carried over to other models. My 1991 Town Car had certified calibration, too.

 

[Chris142]

My Jeep is within 1mph. My Ford truck reads 2 mph fast and my International isn't even close. The faster I go the farther off (slow) it reads.

 

[Smokescreen]

I can have the car set cruise 55mph on the speedo, on my dash display it reads 1-2mph slower. The dash display matches gps & OBDII driven ScanguageII.

 

[Lyman]

Most speedometers are off by a little. I got a ticket in my old Explorer with bigger tires on it. Im a truck driver and can't be getting tickets for obvious reasons. I went out and bought a Garmin. I had my Explorer calibrated at the local dealer as it was way off. Most of the other vehicles I drive were pretty close but none were right on.

 

[cchase]

Of all the American cars I've owned or driven in, none was off more than about 1 or 2 mph at 70.

Germans love to have "optimistic" speedometers. BMW specs call for 10% +/- 5 mph accuracy or some such nonsense. And every BMW of the past 15 years that I've driven has been off by close to 10 mph at 70.