I'm curious. Generally, baitcasters come in one size but several gear ratios/retrieve speeds, but Spinning reels generally come in different sizes(which may have different speeds). I know there are 50/100/200 sized baitcasters, but for the most part a specific model of baitcaster will have 5:1, 6:1, 7:1 all in the same size, but the only way to get different retrieve speeds on spinning gear is to move up or down in spool size. Why is this? Is it a mechanical reason?

 

 

My answer probably won't be right, but here's my $.02 anyway. 

 

Baitcast reels have different ratios for different techniques....slower for deep cranking, faster for burning lures and middle of the road speed for all around use.  Why a spinning reel wouldn't benefit from the same options is beyond my comprehension since they can be used for any technique a baitcaster can be used.

 

Spinning reels come in different sizes for 1) holding different amounts of line and 2) for use with larger fish.  You wouldn't fish for Marlin with a 1000 size spinning reel.  First it wouldn't hold enough line.  Second I doubt the reel could withstand the pressure exerted on the reel by such a large fish.

 

Some baitcast reels offer a deeper spool to hold more line without having to go to a bigger reel.  Just like some spinning reels offer an extra spool for braid.  That extra spool may be shallower so you won't have to spool 450 yds of braid when 100 yards will do the trick. 

IPT will vary on spinning reels based on the diameter of the spool (assuming they have the same gear ratio).  Larger spool the more IPT.

 

Also after looking around it looks like some reels do have different gear ratios depending on the spool size and manufacturer

My guess is cuz its a lot easier to make spinning reels different sizes. It's basically the same frame/gearbox just scaled down. A lot of them use the same frame and gearbox, just a different sized spool. For a baitcaster this could require a complete frame design to fit a different sized spool (50,100,etc) and keep the overall size relatively the same. Look at the shimano core, completely different frame for the 50 size reel vs the larger size(s).

In spinning IPT IMO takes precedence over ratio, but the stated number is on a full spool so less IPT with less line on the spool after a cast.  Larger spinning reels generally have a lower ratio and higher IPT, bear in mind these reels are not always used for cast and retrieve of lures.  On the smaller spinners, and this how I fish them, my retrieval speed is not robotic.  I'm always working the lure and vary the speed, not uncommon to work the lure with an arm sweep then just reel up slack, like a jerkbait.  Not much different in inshore fishing as I let the waves do some of the work, with spoons and jigs.  I don't often use crankbaits.

IPT is the best metric to gauge cranking power in either casting or spinning. On a spinning reel the rotor revolves around the spool and wraps the line so the larger the spool/rotor the more line you take up. In a baitcaster the spool is actually driven when you crank so the more revolutions per crank, the more the IPT (given constant spool dia.)

Just because you don´t see them it doesn´t mean they don´t exist, the reason why you only see few gear ratios in spinning reels is because that´s what the manufacturers are offering in this part of the world, Shimano and Daiwa offer other options to other markets, for example many Japanese reels have 3 gear ratios available for the same model, they also offer different spool sizes/depths.

Interesting. Since I mostly throw baits that I move with the rod, I like my spinning reels to be a little faster. I also like it faster for reeling fish in on a drop shot. It seems most of the reels make you move up in size to get a decent IPT. I find that rather annoying. I'd like to give the higher end Pfluegars a try but you have to move up to a 40 size to get the same IPT as a 2500 Shimano or Daiwa.