Gear inches is a formula, has to do with front tooth, rear tooth count and wheel diameter. So with a 26 inch mtb, vs a 700c FG/SS, the tooth count will have to be different to get exactly the same GI calculation. Weight becomes a factor when it's dead weight, whether it's unfit cyclist body weight or the actual bike itself. And then there's rolling resistance from the tire itself. So same gear inches with one bike is definitely going to feel different on 2 types of bikes. I can go much faster on 700c tires than I can on 26 mtb tires. I have several bikes and each has a setup that pedals & feels very similar, if not identical, but that doesn't mean that with the weight of the bike and every other factor considered, that I can go the same speed, even pedal it for the same duration without burning my legs out. That's the beauty though of having several types of bikes, I can use one to train and get more physically fit pushing myself on one bike and then when you ride the other, a benefit accrues when riding another. Naturally, you may out-fitness a FG/SS GI setup in a couple of weeks of hard riding on a bike that is capable of more.