I've been doing my indoor workouts on a 20-year-old Racermate wind trainer. The bearings are starting to go, so I jumped into the 21st century and bought a fluid trainer.

On my old wind trainer my RPE, HR and "speed" matched up pretty well with what I experience on the road, at least for tempo-like efforts. But I'm finding that the effort level on the fluid trainer is waaaaay off from what I'm used to. It seems like I need to be about three to four teeth larger in back to get my RPE and HR to match, and even then my "speed" is several MPH lower than I would expect. Also, it seems like I'm getting a lot more leg burn on the fluid trainer for a given cadence and speed than I would expect.

Is this common - this need to drop a few gears when you are on a fluid trainer? Or is there something else going on I haven't considered?

They have different types of resistance. Get your RPE, Heartrate, cadence and/or power to where you want and don't worry about the gearing.

Wind trainers have WAY less resistance than fluid trainers, all you have are those tiny little fans and wind resistance.
I had a wind trainer before I got a heart rate monitor, but it never seemed to give a realistic workout, I could spin out
in my top gear, not nearly hitting max effort.

I bought a Kurt Kenetic it matches my real world speed/HR really well. I did a sprint on it when I first got it and was
really pushing it at 34 mph and was still 2 cogs from my smallest in back, so it seems pretty dead on.

34mph on your KK? Wow. The most I've done is 26.4 and that took a tremendous amount of effort on my part. I've never stood up on my trainer yet though.

You can get it up to 34mph+, but you have to spin pretty fast on the largest gear on a standard. Weirdly, the maximum resistance on the KK/Cycleops2 isn't so high that I can readily stand on it - it seems to "give" too easily, so I can't push lots of wattage at a very low cadence to maintain speed. However, it's really hard to spin it up at high rpms at big gearing.

It's still helped my climbing regardless. Definitely a keeper, and will improve your road riding if you keep at it. HIGHLY recommend spinervals as well - I'm not into gimmicky workout fads/dvds, but Spinervals is legit, and will keep you completely focused. You can't get bored - you'll be too busy surviving the workout!

Love the "Mental Toughness" 90 minute workout. That's 90 mins of mixed hi-intensity intervals!