For a long time I had 14's on my S10, but a couple of months ago for my birthday we had some 16's put on. The thing is, whenever I hit 50 or 55, it seems like I'm going faster compared to when I drive other vehicles. And when someone follows me, and my meter reads 55 mph, they tell me I was doing 60. The engine rpm at given speeds hasn't changed (I don't have a tachometer, but I can tell my listening to the engine), and it still switches back into 3rd when it reads 35, etc. I've heard changing tire sizes will cause it to read differently. Is this true? Thanks. :)

yes

You can closely match the outside diameter of your tires and keeop the speedo reading pretty well the same.
5mph difference when jumping up 2 wheel sizes really isn't too bad.

go and get it flashed

http://www.miata.net/garage/tirecalc.html

never tried using if for big truck tires. But should tell you the speed difference.

its $30 to get your speedo adjusted at most auto shops.