Rear tire 20inch fitment ?
 

Will a 275/40/20 tire fit the rear of my fleetside, it has been lowered 4 inches

4.75 inch backspace

28.7 inch diameter,

If not, what is a good size for a 20 inch wheel in the rear.

Re: Rear tire 20inch fitment ?
 

Stock 12 bolt or slightly wider axle from a later year truck?

Re: Rear tire 20inch fitment ?
 

Stock 12 bolt rear

Currently using 275/60/15 on the rear, they are tight, but don rub.

Re: Rear tire 20inch fitment ?
 

There's never an exact answer for these questions. What fits on 5 different trucks might have issues on 5 other trucks because of subtle variances. What's the BS on the 15's?

If the wheels are the same width & BS, only the height is slightly different. At only 4" of rear drop, that change shouldn't be an issue.

There can be minor size variances between similar section width tires (275mm), especially when using different diameter combos w/different aspect ratios (40 vs 60).
It comes down to where the new combo will sit relative to where the old was & the amount of clearance the old combo has.

Re: Rear tire 20inch fitment ?
 

Will a 29 inch tall tire be too much?

Re: Rear tire 20inch fitment ?
 

It shouldn't @ only 4" of drop.

To 100% verify, set the truck up w/some jack stands so the rear end is compressed/has weight on it but off the ground. Tape some 2x4 blocks to the tire/s, spin the wheels/tires & see what happens.

Re: Rear tire 20inch fitment ?
 

I use this tire calculator quite a bit when trying to decide on wheels/tires. It’s nice because you can enter your existing tire size and then compare to the new one. Just an idea to help you out.

https://tiresize.com/comparison/