It seems the prefered baitcast gear ratio for fishing with crankbaits is around 5:1.   Among different manufacturers, the inches-per-turn rate pretty much hovers around 21 inches.  

But this loose rule of thumb doesn't seem to apply to spinning reels.  For the most part, spinning reels in 5:1 speed seems to draw in line as quickly as, say, a 6.3:1 baitcast reel - around 27 inches.  (An approximation, I know there are some exceptions).  Slower reel are usually bigger reels, which actually may have a greater inches-per-turn rate - even though they are geared slower.

What makes a good spinning reel for crankbaits?  Do you just crank slower?   Do some of you just not fish cranks with spinning tackle?  

when fishing cranks with a fast retrieve reel all you have to do is use the rod to move the bait and reel the slack line in with the reel.

That's pretty much what I do.   I don't have any super slow reels.   My slowest baitcaster is a 6.2:1 and I use this for crankbaits in the manner you described.

But I've been using a steady retrieve, usually, on my spinning rigs not really realizing how fast they were drawing in line.  I catch fish and the lures seem to track well.  I just didn't realize I was actually cranking them in as fast as I would if I steady retrieved them on my 6.2:1 baitcaster.  

It seems that line retrieve rate is a more useful guide than gear ratio.   I never gave it much thought.  

What makes a good spinning reel for crankbaits?  Do you just crank slower?   Do some of you just not fish cranks with spinning tackle?

Any spinning reel could do; it's personal preference. I've been cranking with spinning reels my whole fishing life. Whether using spinning or baitcasting, the fisherman is always going to adjust his cranking speed to get the lure movement he wants. There's no one "best" or "right" speed.

I use a Abu garcia cardinal 104 with a 5.1:1 reel for pretty much all my needs. I have notice it takes a slower retrieve on my part but works fine for crank baits.

I really only throw cranks that are too small to throw baitcasters. Otherwise i just stick to baitcasters.

You will miss a lot of fish if changing speed is not part of your game.

Generally largies are a little slower and a more helpless speed/look to cranks and Smallies on the higher end of speed and changing speed.

Most of my Largie cranking with spinning is with a 2000 reel and most Smallie cranking with spinning is 4000 reels.

Garnet

I only throw cranks on baitcasters.

I use a Curado 7:1 ratio reel so I can reel fast or slow.

Also use a Curado 100s and 200s when pond fishing.

My issue is that up until recently I didn't have any ideal crankbait baitcasting setups.  I still don't have the ideal rig, but what I do have works well enough.  So I'm used to fishing crankbaits on spinning rigs.  I've never really taken into consideration the line retrieve rate, just the gear ratio.  What I didn't realize is that a 5.1:1 spinning reel may be a heckuva lot faster than a 5.1:1 baitcaster with respect to line retrieve.  

None of this is a problem since I've still been managing to catch fish.  I was just curious if anyone else used spinning outfit to fish crankbaits and what they thought of it.  

My set-up for cranks is E20PT1A spinning reel(gear ratio-5.17:1) (IPT-24") paired with a St. Croix Pro glass /GS66M. It works well for me.

As somebody else already mentioned, I only use spinning gear to throw cranks that I can't throw with a baitcast outfit.  Baits like the smaller sizes of Shad Raps and Rapala Original Minnows etc.  Admittedly I've paid no attention to the retrieve speed of the reel when doing this, I just crank them and switch up my speed if needed.