You have to look into overall bro. It's best if you check your MZD fuel consumption data analyze. Look at the picture which I've took. The avg FC is hard for you to judge.

Basically, you need to know how you want to control your speed on the highway. Well, basically in city, you can hardly control due to the jam. Your speed have to be constant in order m3 system to show you the accurate display. If you 10 mins step 90 kph then the next 110 kph, then next 50kph, and so on, how can m3 system show you an accurate display, if you get what i mean. It's best if you observe the current FC instead of the average. For my guess, m3 system will take 90kph + 110 kph + 50kph + so on / the time running = average FC.
So far, if at constant speed, yes, i find it 80% accurate. The other 20% base on approximity / roughly (example, m3 shows 780km and u can roughly estimate 750km @ 90kph.)

Here are the pictures of the km I've been travelling from Damansara to Penang with just 1 fuel tank.

Above is the full tank before i travel to Penang. Notice that is already running 24.1km.

Above is the M3 system on economy Fuel consumption menu. Driving on 90 kph @ 4.6 ltr/ 100km

Above driving on 90kph after more than an hour @ 4.5 ltr/ 100km

Above driving on 110kph going back to Damansara.

Above driving on 110kph reaching Damansara.

Above is the total km which i m about to refill

Above refilled. The km updated shows 797km which I'm able to drive at 110kph constantly.

Above is the price I've pump. 34.41 liter @ RM67.10 - RM1.95 per liter
Note that M3 is a 51 liter car. That gives me remaining of 16.59 liter.
Hope this give an overall that m3 is a fuel saving car depends on the speed you are going. Above is 90kph and 110kph day trip.

Well, enjoy your ride with M3 guys. =)

Any idea on Caltex fuel?
I got few friend which in car servicing business, they recommended me to use Caltex.