I had been trying to flash my speedo on my 2005 Ram and can't seem to get it right. I went from p245 to p265. The Michelin website said my LTX MS2's are 31.7" diameter. I set it at 31.75". That wasn't close so I entered 31.5". It is accurate at 40mph but is GPS'd roughly 2mph under the speedo at 60 to 90mpg.
Is calibrating it to the GPS accurate?
I am taking a road trip and want an accurate speedo.

 

I'd say they are as accurate as you or I could ever get. When you can have 3+ units all showing a gps speed of the same result, that's closer than any cop will ever be able to prove.

 

So should I ignore the tire diameter from the manufacture and adjust the tire size on the programmer until the speedo matches the gps unit?

 

Absolutely. The Garmin "phones home" every six seconds and is accurate to within ten feet. Presumably, they all are similar. That is how I learned my Ram is 4.3% off (low) with the 285/70s. Making that adjustment will also correct your odometer.

 

GPS is correct. Just play with the size until it reads right @ 60mph.

 

I thought this was a family forum!

 

That would be distracted driving! In all seriousness, thanks for the posts. I will adjust the diameter until it reads correctly. Ialso have a Garmin.

 

my garmin is deadone in all 3 vehicles so i would say they are very accurate.

 

My truck is like 15ish off at 70, so I have to keep a garmin in the truck just to know my speed. I guess I need to order a programmer one of these days

 

I went from 265s to 285s and read here and other places that this would make my speedo 2 miles or so slower then my actual speed. I had to see for myself as every vehicle/situation is different. I checked first with the GPS on my phone and at 60 it said I was going 62. I checked next with a Tom Tom and it read the same, so I adjusted accordingly. After passing a mobile radar unit that the police put up it said that at 60 I was going 60, so I repeated both previous checks with GPS as well as borrowing a co workers Garmin and they all read the same, 2 mph over. I'm not going to go out and spend money on either a programmer or the dealer to calibrate it for 2 mph so I just adjust my speed. If I needed/ wanted to see actual sppeed all the time I would buy another GPS and leave it up all the time.