02 F150 gages in "test" mode
 

I'm not sure if this is the correct forum or not, but here it goes. OK I'm just wondering if I put the gauges in the Test mode, does the electronic speedometer still read accurate? I've got the FX4 package on my truck and have since upgraded from 265/70-17 to 285/70-17. Since I've done that my mileage has dropped from 16 to 13.5mpg. I've got a Superchips tuner, but when I put in the tire height (32.7in), my speedo (needle) is off by quite a few mph. I just want to verify that I'm going the correct speed. I also checked with my GPS, and found that my setting at 31.25 matches my GPS. Now when I put the GPS in my wife's car, it's off by 5 mph, it reads 75mph and here speedo reads 70? So I'm totally confused.....Any suggestions on how to check it, besides going to a speed shop? Now when I ride next to a car with big electronic speedo, mine is right with what there's say....

The digital is the same info that the analog speedo is using for reading.

The info is still from the VSS, which is set for the tire size from the factory.

sscully im not sure that that statement is true, ive had mine in test mode and the digital seems fast compared to the analog which i have tuned slower since my tire diameters are undersized