I have a 04 fx4 Screw with 18" wheels and with the 275's. I noticed this with my GPS and also verifying with the radar post that are up and down my street. Also had the updated clutch pack and shim kit installed. Now I have the truck back in the shop for somewhat of the same rear end problems. Also have major hesitation after driving for more than 100 or so miles in the same day, it almost seams like someone is hitting me in the rear. Brakes now are starting to squeal which is common, but seem to not stop after brake is depressed. Last time I had it in the shop to get the updated clutch pack it took 4 days to get back! I can't imagine how long this time?

 

My odo is off too - 4 MPH. I was a little disappointed when I was told that it was within specs (7 MPH) and therefore nothing could be done. Doesnt that seem terrible?

 

The odometer may be off 3 MILES, 33 MILES, or whatever, but it is not off by any miles per HOUR.

 

DOT spec is +/- 7%. Not 7 mph as Wolfman stated.

So...

@ 30 mph: +/- 2.1 mph

@55 mph: +/- 3.85 mph

@ 70 mph: +/- 4.9 mph

@ 80 mph: +/- 5.6 mph

These numbers are acceptable. Anything higher or lower and you should contact your dealer, or at least be aware of it if you receieve a speeding ticket and wish to contest it.

 

I have a GPS on the dash of mine and it's pretty close, off by maybe 1 or 2 mph on the optimistic side. I wish the GPS was that close on addresses. On some of the streets here it's off by 1000 or more. I guess it's time to update the database

 

I have a LOBO (the exported to Mexico) 04 fx4 SCab with 18" wheels and with the 275's. I also noticed this with my GPS some - 4mph @ 70 MPH.

But the distance is very consistency with my gps and the marks on the road.

 

my 2003 FX4 5.6 says 73 on speedometer 71 on GPS

 

Hmmmmmm....

I just went from 30" STX wheels/tires to 32" FX4 wheels/tires. That is 6.6% difference.


So 70mph is now 74.6mph according to the math (not actual testing).

Next week I am meeting a buddy at the dealership to put it on a computer and see if we can reprogram it to read 70mph (not 74.6). Now you got me thinking that maybe I'm already closer that I think...

 

I put the instrument panel of my '04 FX4 SCrew into test mode (do a search here on the site for (instrument AND "test mode") minus the parens to find out how to do that) to check this out. Digital speed readout on the dash is 3 MPH slower than the analog, needle pointer on the gauge.

 

At 65MPH ours is within 1MPH of the GPS reading. At lower speeds, it's even closer.

 

Got a call back from the dealer today and he said it's within tolerance. It's sitting at .5 % and he said the tolerance is @ + or - 3%. Still find that hard to believe. They use the spds test I think, so they actually don't drive the truck to test it out. Only get a reading from a tester... They will still have my truck for a week for the rear end noise. I'm thinking that this will be my second clutch pack in less than a month.

 

Manufacters all provide a little room on the low side. Can you imagine all the %$*^?? they would get for tickets issued by the the "good old boys" in small towns for being 3 MPH over the speed limit.

Since this is not a perfect world they have to error on the conservative side.....

Suck it up!