I blame strava! I uploaded some of my old gps traces from about 3 or 4 years ago when I was fit and running a hell of a lot. Surgery and child got in the way of keeping this up and now I am 3 stone heavier.

I am starting trying to get fit again and was wondering how much difference my weight makes to my speed and how much is just being unfit.

I get that dropping weight will help increase speed as the power to weight ratio shifts. But is there an easy way to actually work out how much by?

so for instance if I can do a section currently in 5 minutes on the bike can I roughly work out the time difference if the power output remained static but the weight dropped by 3 stone?

Assuming no aerodynamic drag, your performance will increase by the same proportion you lose weight right down to the point where the loss of muscle mass influences power output.

Riding a bike uphill is effectively filling an “energy bucket”. By reducing your weight you make the bucket smaller in proportion while filling it at the same rate. Slightly reduced drag from tyres probably negligible.

But if any kind of aerodynamic drag then a heavier (ie stronger) rider with no fat (so increased power output but same power to weight) has an unfair advantage.

So not as simple as it might appear.

As flaperon mentioned the tyre drag is neglidgeable but would be worked out and offset by pumping up your tyres a bit more.

In terms of a flat speed (assuming no change in your profile) then the only governing matter is your power you have in your legs, which will increase as you get fitter.

In terms of climbing thats where the weight and power matters, your power will increase quicker the heavier you are (for a given speed up hill) but the quicker you ride the more weight you will lose so will reach an equlibrium point of you are not able to go up the hill any easier at that speed unless you do additional strength training.

TL;DR Lose weight – get faster.

so if my performance increases proportionatly then if I lose 1/4 to a 1/5th of my body weight my speed will increase by the same amount? As long as I am not really losing muscle mass therefore power?

So a minute from a 5 minute hill give or take a bit?? That seems a lot?

Power to weight ratio is THE number. It’s why many pros struggle with eating disorders- despite their bodies craving calories, they have to stay ridiculously skinny. As long as you continue to train whilst losing weight (and what better way) you shouldn’t lose muscle.
I’m four weeks into my training for the Brecon beast and have gone from 15st0 to 13st13 so far without any major dietary changes- I just made sure not to eat more whilst training 😉

I did half heartedly work out I needed a 36000 calorie deficit to hit my target weight but that sounded far too scary 😀