Im looking into adjusting speed with tire size instead of changing gearing, going from a 26" tire to under 30 inch i have a 57 ranchero with a 4" drop blocks (too much), 15x8 4.5bs welds with a 215/65/15 (26") with only contact coming from driveshaft to body when i was picking up a 292 and fmx. mearuing it looks like backspacing of 5.5" is safe and can use a fairly tall tire without any close contact areas.  the issue i come across is the fender rolling in, limiting me to sections width around 9 and tread width under 8.

A 30-inch tire is a tall tire to put on a 57 Ranchero/w 4-inch drop.

Drops is getting cut at least in half, but a skinny 30 would fit even that low until a bug lands in bed then your rubbing. Most likely going to be 2 inch drop and around a 28 inch tire.

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A little update, 57 ranchero 3 in lowering blocks, air shocks @ 0 psi, weld 15x8 4.5 back spacing i put a set of 255/70/15 tires fit nicely. 29.1" tall, 10" sectional width 8.5" tread. would like a little more backspacing because its closer to outer panels by a fair amount. haven't had a chance to drive but plenty of clearance for suspension compression. I'm not sure about body roll yet or how air shocks will effect it.

Why not 65 or 60-series tires, not available? What are you putting on the front?