For some reason Ifeel I can get a more direct answer to this post down here. Mainly because down here we're more "family" and because most of us are certified technical nuts.
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Everytime I go by one of those radar units that sit on the side of road telling you your speed, it shows that my speedo is 5mph off. Tonight I was going 35 and it showed 30. It is the same everytime. This unit has been in place due to a construction zone for about 3 months.

Now...the radar could be off but it is the same on both units I have "tested". There was one on the other side of the island last week. Again...3-5 MPH off!

Let's ASSume that they are perfectly calibrated... (Oh and there were no other cars to interfere with the signal, BTW. I waited for the raod to be clear.)

Would an optomistic speedometer translate into an optomistic odometer?

Is the speedometer and odometer inexplicably linked?

I know the computer counts revs-per-mile as its method for keeping the speedo "accurate". Is the odometer's displayed information gathered from the same source?

If so, I have some warranty mileage questions that need to be brought up.

 

IF your speedo is fast, so is you're odometer. I know mine did the same thing when I go by the radar units around town. This is assuming that the speedo is correctly marked on the dash and not just incorrectly spaced.

I broke out the GPS to verify my speed and, sure enough, the radar was correct and the speedo was 3-5 MPH fast. This was before I did all the lift, tire, gear stuff and the truck was still basically stock.

After I did the lift and tires, I had to play with the SCMT to get my speedo to read correctly. I have 36" tires, but I have the SCMT set to 38" to get the correct speed. Again, I put up the GPS to verify the speed and now it's dead on.

 

ken
i had the same problem. read fast by 5. i even tryed with my mac scanner hooked up that said real speed was 5 slower then the speedo head. sounds like you need to recal the head unit.
around where i live i left it. as the cops like to write for 2 over. yes they are guys! real ones with out saying it.

 

Monsta do your own revs per mile determination and then depending on what tire design you always use adjust it slightly and have the Ford tech do the 5 min update to the PCM.

What do I mean by adjust it?

Manually calculate YOUR loaded rolling diameter after you research what the tire mfg says it should be. Add some modest weight, chalk marks one on tire and one on ground. Rool forward amrking 10 marks on the ground. Do this three times with some mild weaving within the lane. measure ever chalk to chalk mark very accuratly then average all 30 marks to get loaded rolling diameter. Divide thid average number in inches in a mile and you have YOUR revs per mile.

I always run the All Terrain or heavy lugged mud design tire but spend a majority of time on highway.

My experience over last 10 years it that most of these tire design LOOSE 1/2 their tread "depth" in the first 15,000 miles then the last 1/4 over a much longer mileage. I typically get 45-55K out of a set. (this causes revs per mile to INCREASE)

I don't have my calculator here so these numbers are just for illustration and NOT the actual numbers.

Lets say the new tire is called a 32" but in reality the MFG and calculations set it as 31.6 inches.

The revs per mile = 600 (Note: loaded rolling diameter is NOT the same as static dimension)

Half tread revs per mile is 610 (this is where tire will see most of it's life)

1/4 tread revs 612

So over life of the tire Fords system will gradually indicate faster speed then actual.

I have my speedos calibrated to read high initially and during the three years it takes to wear out the tire it is almost always within 1-2 mpg indicated of actual.

For what it is worth every new Ford I have with the speed sensor in the rear end has NOT been set properly by the factory. I think they set a best guess in the middle based on the three or four different tires a customer could option into.

I am not sure this argument holds water because I have never got one that indicated 65mph when reality was 70mph...they always are off the other way (thankfully 'cuz I always run over the limit out on the highway and the ticket would be real bad if the cop got to ding me for reckless driving level when I though I was just speeding)

 

Spoze Ford does this to make the MPG look better????

Monsta, if it's a rev per mile issue, your 5 mph @ 30 would be 10 mph @ 60.
If it's always about 5 mph off, then I'd say it's the pointer of the speedo thats off. In that case the odo is prolly OK.

 

I remember seeing an article in Motertrend and Car and Driver a few years ago the tested 7 or 8 mfg's cars and all were off (read too slow) by at least 5mph at 70.

With my 35's, my 70 is same as 77 in my g/f's tahoe, but my truck reads exactly the same as those road side speed things.

 

I have also heard that manufacturers under shoot the actual speed so as to avoid the possibility of unintentionally making people speed. I have personally found all cars or trucks I have ever been in to be slow according to a gps.

Blindlightguy

 

I've had my truck checked with GPS and radar. Stock it was 3 mph fast at 60 indicated om the speedo. Since replacing the stock tires with 285/70/17s the speedo no reads properly. The odo however now reads slightly slow based on the super accurate (yeah right) roadside mile marker meathod.
8000 mi on the 285s now and last night I passed a roadside radar and the speedo is still dead on.

 

Those green 6" high numbered markers on U.S. Interstate highways are exactly 1 mile apart. With a stopwatch determine the number of seconds at constant speed required to get from one to the next, then divide into 3600. I.e., if it takes you 60 seconds, then you are doing 3600/60 = 60 mph. If it takes you 120 seconds, you are travelling 30 mph. No muss, no fuss, no argument.

 

Thta'd be cool if I had interstate highways here.

It is not the speed at which I'm travelling that bugs me. It's the fact that if my speedo is optomistic then my odometer is going to say I've reached my warranty expiration sooner than what reality says. I either want it fixed or my factory warranty extended.

 

I know I have read articles that auto manus/ purposely calibrate
the speedo to read faster than actual--I think for the abvious speeding issue
and maybe more importantly to cover their butts should an accident or such occur involvling speed
wish I could remember where I have read this b4

Josh