There exist different formulas to calculate how weight loss affect running speed. Running speed not only depens on weight, but also depends on a variety of factors such VO2 max, that represents the maximum amount of oxygen that can be delivered to working muscles in a given amount of time per unit of body mass.
Obviously less weight means also less effort for muscles at the same level of speed, therefore weight is an important factor affecting running speed formulas.
To simplify, we could say that approximately around ten pounds (5 Kg approx.) weight loss can result in a savings of 1 minute each 5 Kilometer (3 miles approx.).
Remember you can use Weight Loss RunnerSquare feature to regularly monitor your weight.