Had a chip box fitted cpl months ago and had it set up in middle of scale , have been getting 27.5 mpg , change box setting to max performance and now getting 29 mpg , anyone else noticed this with there's , can't understand how I can be getting better mpg ? .

Presumably you are getting your MPG figures from the computer which has been confused by your chip box. Things are not what they appear to be. The chip box is usually just an additional resistor plugged into the cars electronic circuit which tells the injectors to over fuel the engine so by turning it to maximum the chip box has just increased the over fuelling to the next level
Below is an extract from one of my previous posts on the subject.

Adding a resistor into this sensor circuit passes the wrong information to the ECU. What was 1500bar is now reported as 1400bar, so the ECU will activate the high pressure injection pump to raise the pressure by 100bar above the sensor value. The extra pressure will force more diesel through the open injector and for a diesel, more fuel = more power. The better chip boxes will have safety factors and tolerances built in, but in all cases, the Injector pump is now doing more work providing the extra pressure, and the Injectors are now holding back greater forces. The Torque and Smoke limiters have been bypassed as the ECU doesn't realise the greater fuel flow, and more interestingly, the MPG is screwed!

Economy and Tuning Boxes

From the original calculations in the ECU, and the Vehicle Road Speed, the ECU will be able to calculate the quantity of fuel used per unit of distance traveled. But what happens if the quantity of diesel injected is higher than the value the ECU is expecting? In this case, the economy will be overestimated as the ECU thinks it is using less diesel than reality. As with most external changes, if one calibrated component is disturbed, the knock on effects will be far reaching.

I decided to do test monitoring mileage, I filled up with fuel, so far I have done 247 miles and I have left in the tank between half and 3/4 , dash is telling me I have 307 miles left but we will see how that really turns out over next few days. However it does seem better so far by quite a bit, and power is awesome, it is also so much smoother . Fingers crossed things stay the same. Rich

Quick update, I got to 503 miles then filled up again , comp said I still had 26 miles left , so all in all better for me by far.

Have you been driving it normally though or driving for economy?

More power = higher pressure at the injectors and more fuel going in so better mpg doesn't follow logic, but if you're happy then hey ho