I find coming off the throttle early rather than braking reduces fuel consumption, any form of braking means having to accelerate which wastes fuel. High corner speeds also, much to my wife's annoyance, i usually say well i'm in a 60 zone & didn't go round the corner over 60
I've also heard some interesting reports on some modern cars having better fuel economy when the engine is under load, the technique is a pumping motion of the throttle, which i've tried, but it felt too weird & was annoying to do, so didn't really explore that avenue any further. I may try again if its £2 a litre for diesel
Another technique is to once you are upto a specific speed, gently ease back on the throttle & quite often the car will stay at the required speed with less throttle & hopefully less fuell being injected into the cylinders??
A reduction in overall speed greatly helps with fuel economy, especially on longer runs, it's amazing how a drop from 70 to 60 makes most cars quite a bit more economical. The reduction in speed is phsycologicaly difficult to deal with, i was always ok with it in my TXD as the speedo over read by 10%, so the brain sees the speedo doing 70, when i was infact only doing 63, it never seemed that much slower tbh. I've not done this in the C5 yet as i found its soo quiet in comparision to the TXD, that higher speeds aren't noisy at all, infact slower speeds appear to be worse?? work that one out if you will, haha....
I'm reasonably happy with the economy i'm getting in the C5, yes i wish it was more, but for what i paid & the old technology it now is, if i can get high 40's, thats not bad really. Go back 20 years, & the equivalent granada etc would have struggled to get 25mpg.