I ride mainly railtrails. I start on 2.5 mi paved then it turns to packed gravel for 18 mi. When it turns back to pavement I kick it up a gear and go from 13 AVS to 18 AVS My question is am I getting the same basic workout at 18 in a higher gear on pavement as 13 on gravel. My heartrate monitor broke and I am waiting for the replacement to come in. Also wondered if going from 700c38 to 700c28 will make much difference on gravel.