I'm sure if I dug deep enough, I'd find a thread like this in the archives, but I'm ready to cut. I ordered a 3' X 9' piece of Tyvek from Ebay. I had planned on using my scrap for some MYOG stuff sacks and the like. I'm just feeling kind of insecure about how much to cut off. Is 6 inches beyond my pad enough to leave on? Or too much. Any help would be appreciated.

I use a 3' x 8' groundsheet when car camping.

If you're using a tarp, make sure it's at least 6" away from the edge, so rain doesn't drip off the edge of the tarp, onto your groundsheet, which then runs under your sleeping bag.

So, it depends on the size of your tarp if you're using one.

It's a Golite Shangri-La 1. I'd be well within if I went 3 X 8. I guess I'll do that and just cut off more as I determine what is excessive. thanks.

I assume you'll be using it under the included floor?

Then make it maybe 2 inches narrower on each side of the floor. When you pitch the tent, make sure the ground cover doesn't extend beyond the floor, or water will land on it and go between ground cover and floor and tend to soak through the floor.

I've done that before and the "waterproof" floor leaked through a bit.

If you're using it without floor, cut it off 6 inces from each edge. Maybe more or less depending on how high you pitch it and how windy and rainy it is.

The Shangrila isn't rectangular so the ground cover shouldn't be either.

Make it bigger initially, and then make it smaller if there's a problem when you're using it.

At least there's several ideas to consider.

Actually, I mean to use it in lieu of the floor in most cases. Thanks for your help.