After each indoor trainer road training workout i check my analysis and see I have high avg cadence (90+) and avg power (180+) however my speed is low 20’s kph ??
I checked the “rim” size on my garmin speed unit on my wheel and it seems ok
any other ideas
Put it in a higher gear - it will spin the rear wheel/trainer axle faster.
Although you are still sitting in your pain cave and haven’t gone anywhere - your speed is zero.
On the trainer or road? If on the trainer, I really wouldn’t worry as speed is fairly meaningless.
Also, low 20s…kph/mph/light years? 
thanks for the reply…
on the trainer and kph…
Then your speed is actually Over reading…your true speed is zero! Honestly speed on a trainer is irrelevant and pointless, just ignore it.
In TR speed is determined by the speed of the flywheel. So doing workouts in a lower gear results in a lower speed vs workouts done in a higher gear.