I tried getting the highest MPG other day, I drove down the ring road (so going 60 ish) and once I saw the roundabout, I put it in N (Yes, the engine uses fuel to make it idle, BUT, it wouldn’t role to the roundabout if I left it in D due to engine breaking!)
When Im a bit closer to the roundabout I put it into 3 (going about 50mph now), and a few seconds later, select 2... and then when im almost ontop of the roundabout, select 1st... and then at the last few car lengths, apply the brakes (I will have been going about 15-20mph at this point.
Its important that there is no car behind you though, as there is obviously no brake lights when using engine braking!
I have only done this a few times because I don’t know if its bad for the auto gearbox? or should it be fine?
Engine braking is how real drivers drive...
Auto? but certainly with manual, none of this modern brakes are cheaper that a gearbox stuff, ask any advance driver 
Here we go again - sorry. My IAM "observer" and the big red IAM book said "use the the brakes not the gearbox to slow the car", which was one of the reasons I was so disappointed with what they were trying to teach me.
But that apart, on the Omega Auto, you don't actually get any noticeable engine braking effect by changing from D to 3. I have only very rarely experienced circumstances where D to 2 was appropriate, but it does slow the car.
The 5 speed Mondeo diesel auto that I had before the Omega was really special - it was absolutely never in the right gear at the right time, and the "tiptronic" mode had a mind of it's own - it changed when it wanted to, not when you told it to.
And I'm firmly in the one foot camp - you don't use the left foot when driving an automatic.
I'm a committed auto driver - my last 6 cars - about 25 years, I think - have been autos. Manuals are mostly a waste of effort - a nuisance round town and once you're into top gear on the motorway, it doesn't make an awful lot of difference. But I now live in Suffolk, and we have some really narrow winding roads round here. The Omega is OK, but not ideally suited to that kind of driving. I tend to prefer driving a smaller manual car if I'm not going on the dual carriageway.
