Hi!
Been loads of posts on this subject … what I find on my 2.0 Coupe is … I get an average of between 36 - 40 mpg and is dependent on where you buy the fuel from, especially lately!
Supermarket fuel mpg/returns are a lot lower I have found.
Tesco’s seem to have the monopoly around where we live… the fuel is crap, and whenever possible we buy elsewhere, and the performance is a lot better too!
Is there a difference in fuel consumption between having the top up or down?
Hey I think that great, I have only got 280 max on a full tank, how do you do it ?
I have noticed i can get nearly 200 miles out of the first “half” of a tank according to the gauge but only a maximum of 80 miles out of the second “half” of a tank!!
Do i have a spacial vortex in my tank or are all mazda fuel gauges crap?? 
The ‘200’ filled to the brim is about right … Half on gauge to light coming on I get 100!
According to the OBC, mine is doing 40.3mpg on its first-ever tank. It should improve by a couple of mpg as it runs in. On the other hand, I’m not taking it much over 3000rpm yet; I’ll start letting it stretch its legs when it gets to 1,000 miles but I won’t be redlining it until it’s done at least 4,000 miles. This makes things rather dull as the 1.8 doesn’t really pick up until 3,000 rpm.
Those getting significantly less than 40mpg - are you doing a lot of short trips or town driving? IME MX-5s don’t care how hard you drive them (NA engines are more efficient at wider throttle openings anyway) but they hate short trips.
That’s a good result on a tight new engine, but I wouldn’t put too much trust in the OBD - they are often optimistic by 2-3 MPG. Brim to brim litres divide by 4.546 is the only true way of doing it.
I recently had a cross country journey to the coast and back. Re-fueling showed 41.6 MPG which I was well impressed with. On the other hand, the cold weather and some short journeys resulted in another brim to brim of 33.6. I think all cars hate short journeys.
My 2013 1.8 has done just over 4,000 miles now and breaking in nicely 
Most of my driving is long runs on A & B roads with a small amout of town, and it sounds like we have quite similar driving styles.
I consistently get between 38 and 42 mpg.
Doing the maths manually I’ve found the Mazda fuel consumption computer to be the most accurate of any car I’ve had (BMW’s, Mini and a VW). It seems to be 1 to 2 MPG out.
I’ve found the last quarter of a tank goes down very quickly, and I’m rather nevrous about the indicated reserve (needle and light) even though I too worked it out to be about 10 litres. I have found myself in the very uncomfortable position of the gauage not even reading zero (how I learnt about the disproportionate last quarter), and wouldn’t want to repeat it!
As previously said I would avoid running to too low on purpose - as you’re going to have to fill it at some point so why risk sucking up the cr&p!
The OBC on my old Beemer was optimistic by 15%! VW/Skoda ones seem to be spot on.
I suspect the aerodynamics are far worse with the lid off, and so would expect fuel consumption to be worse top down.
While pretty much any petrol engine is more efficient at wider throttle openings (less pumping losses - diesels have 2 advantages for fuel consumption, one of which is they have no throttle and the other being that diesel fuel has about 15% more energy by volume), driving it harder generally means more acceleration and braking (which destroys fuel consumption) and going faster which allows the lousy aerodynamics to screw up the fuel consumption.
All the best
Keith
Think yourselves lucky. Slightly off topic but had a track day at Mallory Park. Full to empty in 3 sessions thats around 45 laps just about 60 miles to a tank ! It is Turbo’d with 5 injectors and running safely rich 
. I can get 25 mpg taking it very easy .
As the maximum speed (claimed) is slightly higher with the roof up, there is definitely more drag with it down.
I should add that my car is running on the 205/45R17 Bridgestones which do nothing whatsoever for efficiency.
I do most of my driving with the roof down -over the last 2/3weeks i have done about 600 miles and got an average of 29.2 MPG .thats driving carefully where required and going for it where appropriate… great fun - I love the mid range pick through the bends.
I have done about 15000 miles now and the car seems to be getting better -engine and gear change seems very smooth - I think its the best car I have ever had…
40.9 off the first full tank, brim to brim. Open roads with no congestion, roof down for two thirds of it. Indicated 40.5.
30.8mpg over the last few months from new, according to the car.
How you achieve 40 I have no idea?? I’m not really lead-footed (occasionally), but the car usually only does short trips which isn’t great for economy…
Short trips are indeed the biggest problem, which is why I have a bicycle.
1800 miles in France last week 80% on motorways at 80mph and the rest town an mountain passes driving returned 33mpg…for a 2.0 i think that was good.
I’d rather be getting 30mpg on a 10 mile commute than the 40mpg I get on a 65 mile commute 
I do sometimes cruise below the speed limit (when I’ve got no one behind to hold up) to save on a bit of fuel, and my ears (oh the road noise!), but I’d never not put the roof down! 
My 1.8 coupe has done 9k miles, getting 37.6 mpg. Mostly long runs, but do enjoy a bit of fun now and again
Second full tank was 40.5mpg brim to brim, so it isn’t a fluke. The OBC seems to under-read by approximately 0.5mpg, which is remarkably good. There were a few shorter trips this time and one long run up to Northampton and then back the same day. The A43 isn’t conducive to good fuel consumption because of all the roundabouts on the dual carriageway section around Brackley. Normally I’d have it up to 70 in third coming out of those, but not on a new engine.
Of course, the car was filled with 5w30 oil at the factory (could even be 5w20 if they build them the same as the US ones) which is going to give 1-2mpg more than the Mobil 1 0w40 I used to put in the Mk1 1.8. The Mk1 would have got 37-38mpg with the same type of driving and ambient temperatures.