gps says 55mph speedometer says 60mph?

i was wondering which is right. gps or speedometer. seems like the speedometer is 5mph over everytime.

 

gps

 

GPS should be right. Now do you have any mods done to your truck lift or wheel wise?

 

GPS is going to be more accurate than your speedo... search here and you'll see how much other peoples speedometers are off as well...

 

Yeah like Delmarva said. Mine is off a couple of MPH because I went with bigger wheels/tires.

 

Hell, right off the lot your speedo could be off by as much as 10% - the trend is that bigger tires gets your speedo closer to actual speed... lol

 

With the GPS, the scan gage and the speedo, I end up with a three different speeds occasionally. Normally the scan gage and the gps are dead on... the gps will say 57 and the speedo with say 60.

According to my stealership my odometer is dead on so I won't run out of warranty sooner than I should.

 

GPS and radar (buddy is a cop) had me at 57mph with 245/75/16 tires and speedo was 60mph. With 265/75/16, they both have me correct at 60mph.

 

Since I don't have a scan gage or gps I ran a 5 kilometer test range and found my ododmeter reported 5.3 kilometers @ 100 klms/hr. I used a tire size calulator to find the right sized tire to correct the error. Since the speedo calculates the speed through a gear in the tranny (I believe), it uses X revolutions to determine the distance and speed (ie: 411 revolutions equals 1 klm). I found a slightly smaller tire (235/70R16 instead of the factory 245/75R16) gave me a near perfect distance correction but am not sure if the speed would be corrected as well. The only problem is that it also give me a warning for greater than 3% tire size difference. I know you can get (aftermaket) gears for the speedo/odometer to correct this and it may be better to get this done instead. Food for thought!

 

Have lift with bigger tires, GPS is right on, speedo if now off.

 

Well I know my speedo is off because my GPS matches my cops friends radar! Haha!!! So I'm going to go with my GPS.

 

First post, hello everyone!

So I can install slightly larger tires and have a slightly longer warranty?

 

These trucks speed isn't measured in the tranny with gears, etc. like my old mustang was. Each truck has an electronic speed sensor which goes directly to the ECU. From the ECU it goes to the dash to display speed.

 

No, speedo and odo are different.

 

basically, yes. I say that because my when my gps says i've driven 100 miles my odo says 92 miles. and welcome to Tacoma World.

 

Thanks Rick, any ideas where I can pick up 50" tires?

 

I have a Passport 9500ix with GPS lockout. It will also display the speed of the vehicle since it has satellite connection for the GPS lockout functionality. I also put oversized tires on my truck. At highway speeds, my speedo reads about 3 MPH slower than I'm actually going.

 

If you're odometer reports you went farther than you actually did, you need a larger tire not a smaller one. A smaller tire would complete one revolution in a shorter distance, meaning it would take more revolutions of the smaller tire to cover the same distance. If it revolves more, the odometer at the end of the test would be higher. You need one that revolves less.

Think about it in terms of more extreme case (e.g. 20" bmx wheel and a 27" racing bicycle wheel). Instead of the transmission, imagine two equally geared bikes with odometer "distance indicated" being determined by the turning of the gear. One turn of the cranks will produce one revolution on the wheel. Which bike will go farther with one turn of the crank? Of course, the 27" (circumference of the 20" wheel is 62.8" whereas the circumference of the 27" wheel is 84.8"). If you go to a smaller wheel, you'll have to turn the crank more to cover the same distance (1.35 times more). If the odometer only measured gear revolutions (measured at the tranny), the 20" bicycle would show a higher odometer reading at the end because the gear has to turn more times to cover the distance.

For example, there are 63360" in one mile. The 27" tire would revolve 747.2 times whereas the 20" wheel would revolve 1008.9 times. If the odometer of the 27" wheel indicated too high (too many revolutions) going to a smaller wheel will make that worse.

 

I can see your logic but I was using absolutes, ie: 400 revs times a 30" circ equals 1200", whereas 400 revs times a 25" circ equals 1000". Since the larger wheel travels further in the same number of revs and shows it to be more than the distance actually traveled (my issue) and the smaller wheel in the same number of turns shows the correct distance then it stands that the smaller wheel is the right choice...doesn't it?

I may misunderstand about how the odo functions but I was told that it uses a fixed number of revs to determine distance. The speedo, as pointed out by Delmarva, uses a speed sensor to determine the speed. I thought that the odo and speedo where linked by revs in that one determines distance by revs and the other how many revs per second, or other factor: ie magnetic pulse or strength. Therefore, if the ECU uses this info to determine speed, then couldn't this be altered via re-programing?

But back to my tire size calculation, when I put the info into the calculator and use a fixed number of revs (determined by my stock tire) then the smaller tire shows the correct distance. I used this absolute number because the truck doesn't know what size of tire I'm using and has to have someway of measuring the distance, ie: revolutions. Correct me if I'm wrong but this is what I've read and been told as to how it works.

http://auto.howstuffworks.com/speedometer.htm

http://auto.howstuffworks.com/odometer.htm

 

No, it's just the opposite. The rate at which the odometer turns is inversely related to the tire size (the larger the tire is, the slower the odometer will click off distance). A smaller tire will turn more times in one mile than a larger tire meaning the odometer will turn faster for a given distance. If your odometer is indicating too high (5.3 miles vs 5.0 miles) then that means the wheel is turning too many times in one mile because the counter is turning too fast. You need to slow down the rate at which the odometer clicks off mileage. To make a smaller number of revolutions per mile, you need to go to a larger tire.