i have a 77 f-250 crewcab 4x4 (lowboy style truck) that i am lifting and puting 38 inch tires on 16.5 wheels the rims that i have they are 16.5 rims and 9.750 wide i purchesed them from summit on the website they say the offset is -16mm when i picked them up on the box the only difference on the box was it said that the offset is -21mm if it is -21mm will it cause any problems or how can it be corected or what do i have to watch for?

 

-21mm offset is no big thing. It means your wheel mounting surface is 21mm off center, closer to the inside of the wheel. That equates to less than 1" off center giving you a backspacing of roughly 4" which is about right for those wheels. 4-4.5" is what's usually recommended. -21mm vs the -16mm you originally thought you were getting moves your wheel out about 1/4" . . . not enough to make any kind of significant difference. And with those fat tires you wouldn't be able to tell anyhow. Hope you have fun with them!

 

or how can i find out the actual offset where would i mesure and how?

 

measure from the edge of the rim to where the bolt pattern is on the inner (truck) side.

 

If you want to measure the offset yourself you can lay the rim on its side (outside to the ground), lay a straight edge across the back side of the rim and then measure in mm the distance from the mounting surface to the straight edge. Then measure the entire width of the rim in mm (you need to be exact here) and divide it by 2. The difference between the two numbers will be your offset.

Have a look at the picture below (if it appears properly). This may give you a better understanding as to offset. Basically though, if you draw a line straight through the center of the wheel (in terms of width) that's your baseline. If the mounting surface is dead even with that line the offset is 0. If it's farther toward the outside of the wheel that's + offset or if it's closer to the inside of that wheel like yours is then it's - offset. Your wheel mounting surface is 21mm in from center. Now just as a comparison to give you an idea here: My wheels are 12" wide. My offset is -63.5mm, meaning I have a lot of wheel sticking out past the mounting surface and a backspacing of only 3.5" on a wide rim. I did this for a wider footprint and better clearance BUT it's very hard on wheel bearings because there's so much leverage. Your wheel/offset combo only moves it out slightly so the impact on wheel bearings won't be an issue for you.