When choosing tires it is best to first determine the primary use of your van. Will it be used mostly on road getting from one destination to the other? Will it see more than usual off-road use such as a prerunner or safari vehicle. Will you need a tire to handle icy road conditions or are you more concerned with soft sand and rocks? Is road noise an issue or do you want them to hear your beast coming? First determine what type tire you need and then choose one to fit your needs and van.
There are not unlimited choices in choosing the size tire you can fit on these vans. GM did not have huge tires in mind when they designed the Astro/Safari. A 30 x 9.50 all-terrain tire on a 15 x 7.5 rim is a very nice all around choice for these vans. We have found a 31" tire up to 11.5" wide to be as large as you can fit on a lifted IFS van. But, care must be taken in choosing a wheel with the right spacing. Converting over to a high lift solid axle system allows larger tires however, the wheel wells would need modification and they may restrict opening the sliding door.
Choosing tires is all about compromise. What works good in one environment doesn't work quite so well in another. An all purpose tire are ok if you seldom do any off roading. An all terrain tire is the most versatile providing good off road traction while maintaining decent road manners. Aggressive mud tires work great on rocks, snow and mud but usually wear faster and make more noise on the highway.
Keep in mind the amount of lift you have before making a choice. If you are running a stock van then 225/75x15 tires are about as big as you should go. With coil-over shocks and adjusted torsions you can run 235/75x15 tires will work but they may scuff the inner fenders on big bumps. Our 2" lift will allow for the use of 30x9.50 tires. Going with our 5" lift package permits the use of 31x10.50 tires. Use of 31" tires may require some inner fender trimming.
Just because 31x10.50 tires will fit doesn't mean thats what you need. If you are a dune runner you may want to go with a shorter wider tire. Perhaps a 29x11.50 would work better for you. Remember the larger diameter your tire is the lower your gearing should be and/or the more horsepower you should have. If you drive in deep snows an agressive taller narrow tire seems to work best. Consider an all-terrain tire with good siping for icy roads. For all-round off-road vanning a 30x9.50 all-terrain just seems right.
If you like the larger 16" rims you can go with a 245/70x16 in place of the 30x9.50. The 265/70x16 is similiar in size to the 31x10.50. A 235/70x16 is similar to a 235/75x15. A tire shop can help you choose a comparable tire. Just don't say it is for an Astro or Safari of they will try to talk you into OEM size tires. No imagination.
Care must be taken in selecting wheels. Spacing is critical. Don't forget you do need to open your sliding door. We have found that if you are only going to a 235/75x15 or a 30x 9.50 you can use the stock 15x6.5 GM wheels. Sandblasting and painting the older styled steel wheel black or white give it the look of some popular aftermarket off-road wheels. The aluminum wheels can also be used. (RPO PF3)
If you are purchasing wheels and you plan on running 30x9.50 tires then go for a 15x7 or 8" wheel as it will accomidate it better and alow use of larger tires if you decide to go bigger. A 16x7.5" wheel should handle the 16" tires just fine. The bolt pattern is 5 on 5".
Stock wheels are 15x6.5" with 4.25" backspacing and fit 225/75R15, 235/75R15 tires.
Wider wheels like 15x7.0" with 3.75-4.5" backspacing will fit 29x9.50 - 31x10.50 tires.
Oversized wheels 15x8.0" with 4.5-4.75" backspacing will fit 29x11.50 - 31x11.50 tires. There are only a few 15x8" wheels made with this amount of backspacing though.
The 16x7.5" wheels with 4.5" backspacing fit 235/70R16, 245/70R16 & 265/70R16.