Tire Size/Odometer/Speedo Correction...

[Winder]

Do not use the radar systems you see on the side of the road to check your speed those things are never very accurate.

My 08 Titan shows @3mph faster on the speed than it is actually going on stock tires.

If you go to a larger tire size your speed-o will be off by a %. So if the tire you put on is 10% larger then you will always be going 10% faster than what your speed-o reads. SO at 10mph you are traveling 1mph faster than what your speed-o shows and at 100mph you are traveling 10mph faster than what it shows.

There is also @ +/-3mph margin of error for the factory speed-o. Same with most GPS systems.

One of the reasons that police usually do not bother people traveling 5-10mph over the speed limit is that the margin of error of all the equipment involved falls within this range. Laser however is said to be very accurate.

[ETx08Titan]

I purchased the Hypertech In-Line Speedometer Calibrator from RealTruck.com for $206.99 shipped. (Use the coupon code SJ921) This was on my 08. It offeres tire/wheel size correction and gear ration correction. It was a pain in the arse to install. I had pull out the instrument panel to get the plug it connects to, but it worked. The tires I put on were wider but an 1 1/2" shorter so I was putting more miles on my truck than I was actually driving. But this fixed it and everything is reading correct according to my GPS.

[Cajunguy]

Handy dandy calculator for changing your tire sizes:

http://www.dakota-truck.net/tirecalc/tirecalc.html