i know it recalculates constantly per trip. I was just trying to use it as an example comparing the mazda range to the mondeo. The mondeo range was actually accurate. If i filled it up and it said i had a range of say 550 and I kept watch of how many miles I actually did till I emptied the tank, I would always manage to actually get 550 out of it if not more. My point being the range expected from a ful tank on filling up was always at least what I'd actually manage to do.

This advice is OK except for one thing IMHO - 'brim to brim'. Firstly it means you have to empty a full tank to get an answer; secondly I don't really like having a full-full tank anyway.

I suggest (if you do a deal of M'way driving) when the fuel warning light shows, put in (say) 20 litres exactly, then drive till it shows again. Do that two or three times and you will have more data, and will have used about a tankful.

Whether your odometer reads correctly is another issue, but it's unlikely to be far out without stopping altogether.

You cannot rely on the range as it recalculates every trip using the average of the previous trip.

If I have a 10 gallon tank and achived 50mpg on my previous run, then fill up, it will state 500 miles. If I only did 30 mpg it will state 300 miles.

The long term average is the best figure to use

To be fair, that engine, even an older one, should easily get 45mpg, and if mainly used on the motorway and driven properly and at legal speeds, 50 or so. This is backed up in the Real MPG section for this model, even if that represents a 25% drop off in expected vs real mpg.

My nearly 15yo Mazda3 1.6 petrol can get 45mpg on a similar driving pattern and 40-41 average, though I'm not heavy on the gas pedal (but no 'Captain Slow' either).

I find that with my car's trip computer, it starts off (tank fully brimmed) with about 450-570 miles left (depending on the weather at the time, as it bases the estimate on what it has been using recently), it drops slowly at around what it should, then at around the 100-150 miles left, drops at an increasing rate per mile, especially within the last 50-75 miles. Even so, there is likely to be some left as I've had to drive an 'sort-of emergency situations with zero left on the trip computer.

I get between 380 miles (dead of winter) and 470 miles (best weather conditions in late spring/early summer [no heating or A/C, good light), which equates to (using the brim-to-brim method) about 36mpg - 45mpg.

Sounds like either:

1. Some mis-calculation (never rely on the trip computer) in the mpg, which can easily be found by checking the odometer against a know distance via Google maps or similar;

2. A miscalibration of the speedo/odomoeter, which is under-reading the mileage - have the wheel/tyre sizes been checked again what they should be in the handbook? A fault in the odometer or the tyres/wheels changed and the odometer/speedo not recalibrated could've thrown the system out. Unlikely, as the discrepancy would only be in the order of 5-10 tops.

3. Some fault with the engine, emissions/exhaust system (DPF, EGR etc) or the fuel flow and/or fuel level gauge that gives a false reading. Again, unlikely, as the fuel pump at the filling station would also read low. The figure from the filling station should always be used, never the trip computer, if it gives 'fuel used'.

My money's on some serious fault/blockage in the emissions/exhaust system, but it could that and a odometer calibration error.

Unfortunately, the likelihood of problems with Mazda diesels is high as they age unless their usage (not just service) history can be proven 100% and that is mostly decent length trips at 40+mph, with a decent amount at 70. The low mileage for a car of that age would've been a distinct red flag to me unless the usage history could be proven, which it rarely can.

Often best to stick to your existing car - especially diesels, even if that means paying an extra £20 - £40 in VED pa, as that's nothing to spending £000s or more every year or two on repairs because of how it was used/cared for before you bought it. At least with an existing car, you know what it can do. Not anywhere near as bad for petrol-engined cars, as they are far less prone to the exhaust and emissions systems related problems associated with diesels and short trips.

Sadly, an expensive lesson to learn.

To clarify again, my vehicle is the 62L tank with 150bhp. I recorded the mileage at the last fill up, I currently have about 170 ish range left/roughly a third of a tank. When I run it down enough I will be able to record what I've done before I need to fill up again and will be able to update with the real mpg of the car. So it is a slight waiting game until I can update with the mpg everyone is asking for. I am already on the case but I don't have the figure right now.

Some people are saying I am getting hung up on the range when it is about the mpg..

As someone else has said, there is only about a 2 gallon difference between the mazda and my older mondeo. The mondeo would always get between 550-600. The mazda is only getting 420 to a tank. Same driving conditions, mostly motorway.

I don't care what anyone says about mpg (not wanting to be rude) but to me that is blindly obvious something is wrong. The difference of 150 less range is a massive amount

'It's a diesel' being the exact reason I bought the car. I needed a diesel for the increased fuel economy and mpg that it is supposed to get over a petrol on long distance higher speed driving which is exactly what I do.

I went from a 1.6 petrol which suited my needs at the time, going back 10 years now. Then my needs changed and I went to a diesel (mondeo), this was amazing on fuel and mpg, was only costly on road tax at 200 a year. I got the mondeo up to 160000 and it started to become unreliable.

I got rid of the mondeo and thought it might be sensible to go back to petrol due to the governmenta stance on diesels now and the stigma around them. I had a 1.6 petrol again for 6 months, hated it, did not suit my needs and had many issues with the car in question that put me in a massive financial situation. I cut my losses and decided I had to go back to diesel.

I was going to go back to a mondeo as its what I knew but then discovered the mazda which is supposed to have 20 MORE MPG than the mondeo. It is also 170 a year less road tax.

Comments saying its a mazda and it's a diesel are not of any help to me and frankly I find them rude and uncalled for. As its obvious to read that for my needs a petrol is just not suitable and the for financial and economical reasons I am better suited with a diesel.

Before coming on this forum I had only heard good things about mazda and the engine was supposed to be amazing for mpg.