The most important component is your power-to-CdA ratio. CdA is like an aerodynamic mass, and ranges from 0.190 m^2 to 0.240 m^2 for fast track cyclists. The first step is to know where you stand.
To measure your CdA, you can download GoldenCheetah and follow the best practices found in Slowtwitch's Platypus Thread for the Aerolab module.
Or you can get your CdA measured by Missy Erickson of ERO-Pennsylvania at the Milton velodrome.
Either way, knowing your 1-hr average power and your CdA will allow us to figure out what you need to move to about 1600 W/m^2 power-to-CdA ratio.
Full disclosure: ERO-Pennsylvania is a partner of the company I work for.
Speed is useless for trainers with adjustable resistance (you could put resistance all the way down and fly along at 50kph, but do about the same power as 10kph with high resistance).
There is one use: for trainers (mostly the cheap ones) that don't have adjustable resistance, you can combine the speed reading with a known power curve for the trainer and somewhat accurately determine the power you are putting out. TrainerRoad has power curves for many trainers, I think. (of course, the relationship to speed on the road or distance is still tenuous, but who cares if you can measure power)
If you don't want to pay for TrainerRoad, you can try GoldenCheetah, though some setup is required. I usually have GoldenCheetah on one screen in training mode on some workout, connected to HR + the trainer for cadence and power over an ANT+ stick, and some movie/TdF stage on the primary screen.
At first I was all like Golden Cheetah, but then I was like all Garmin ANT. But it may not, ultimately, satisfy your needs. I'm curious what you find.
If you're into more advanced analysis capabilities (in a totally free, open source platform!) check out Golden Cheetah (http://goldencheetah.org/). I've also got a HUGE list of other options here: http://darrencope.com/cycling-software/
Golden Cheetah does it for free
Or here's a doc on the calculation here
GoldenCheetah is free(/open-source) and has this, and available for Mac/Windows/Linux
Hm, actually, this webapp might work for fixing such errors too