speedometer accuracy

We have a 2009 Tacoma. We use a GPS on long trips to new areas. The speed recorded by the GPS in on average 10% less than what the truck is showing. We have used the GPS in several other vehicles and the speed is dead accurate. The dealer says the GPS is wrong...they say the tires on the Tacoma are worn. Seems that it is never the Tacoma that is the problem. I have talked to the manufacturer of the GPS and they say that the tracking of the satellite is accurate within 3 meters and is real time. Also they stated that the size of the tires have no effect. Normally this wouldn't be a problem but the truck is a lease and we will be charged for the extra miles accumulated over the 4 year period of the lease. What do I do to get the dealer to take action on this. I have over 110,000 km on it now and another year on my lease which will incur at least another 30,000Km.

 

GPS is the correct one.

buy bigger tires?

 

Mine seems accurate (based on comparison with my GPS) up until around 50-55 MPH...at "75 MPH" I am doing more like 71.

That is with 265/75/16R.

 

What size tires are you running. When I had my 265/70/16 my speedo read faster than my gps. Now that I have 265/75/16 they are dead on till about 65/70 ish.

 

My GPS is is dead on with my odometer, speedo reads wrong, on any trip I use the GPS.

 

With a 1" taller tire (265/75-16) over stock (265/70-16) your speedometer will become accurate with the GPS (satellites don't lie)... Before the taller tires... the speedometer read about 3 mph more than I was really going (does help you from getting a ticket)... at Hwy. speeds.

 

I went from the stock 265/70/16s to 255/80/17s and have noticed as my speed increases the GPS and my speedo differ more and more. At slow speeds it's not much but if my speedo says I'm doing 55mph my GPS tells me I'm doing 59 or 60. I've learned to live with it.

 

My 2011 is spot on my 2002 VW is off by at least 3 MPH my old 97 Taco was off 3 MPH both of them read more then the actual speed but the mileage was correct. I'm going with the GPS. I think a good part of it was to keep the manufactures out of the middle of disputes over speeding tickets.

 

Speedo reads high so that you don't get tickets and sue Toyota. (You think you're going 60 MPH, but you're actually traveling at 57 MPH).

And The size of the tires absolutely affects the speedo.

 

3mph less than GPS at any speed above 20mph.
Also shows 3mph less than those mobile "slow down" units that the village sets up.

 

My 2012 tacoma with the stock 265/65/17 also has an inaccurate speedometer.
Speedo reads 55, GPS reads 53.

 

I have used the same gps in two toyotas a subaru and two fords and they all showd 1 mph faster than the gps.I tend to think the gps is wrong as it is unlikely five different vehicles would be wrong.On one ford it showd 1 mph faster but if you compared your miles on the interstate mile posts over 100 miles the truck only recorded 94 miles.The mph and the miles driven did not match.Gps can be wrong also.

 

Speedometers read fast. The manufacturer does that on purpose, they don't want a statistical distribution around the actual speed, because that means that about half of the people driving an indicated speed limit will be speeding.

Every car I've ever tested with GPS or milepost timing has read slightly faster than the actual speed. My Tacoma reads about 5% fast. When it says 30, it really means about 28-29; when it says 60, it really means 56-57.

Interestingly, the speed indicated on the scangauge is always within about one mph of the GPS indicated speed.

 

Speedometer for mine is about 10% high, this means total mileage will also be 10% more. 60,000 mile on odometer is really about 54,000, this can reduce resale value. I started getting 26mpg, but with the 10% error that will be about 23mpg ( I was bragging until i figured the mpg did not change). A 32.6 diameter wheel will fix this, vs stock worn tire.

 

False.

 

Just got an OBD2 reader and it links to the GPS in my cell phone (Same one used by navigation program used in Garmon GPS)
So heres the deal, theres a radar sign on my commute home from work on the highway..

Speeds from:
Highway radar
My speedometer
Theres the speed my OBD2 reader says (Matches gauge perfectly)
Speed my GPS says.

The radar sign and the GPS matched both saying I was doing 130, but the OBD2 speed reader and speedo said I was going 134 faster, they were wrong.

 

Even with pure stock tires your speedometer will start to show faster than actual speed over time. This is because as the tread wears on the tires the diameter decreases and means it won't travel as far per rotation. A GPS will show the correct speed because it measures distance instead of counting tire rotations.

 

Seems most of us are getting the same results.

I am still waiting for an explanation on how the odometer is true when speedometer is turning faster than we are really driving? How does it know when we put a taller tire on to read the same as with the stock tire???