Just been thinking about these devices and I'm aware that compared to a proper engine map they are a poor cousin but they do have the advantage of being easy to fit and remove if necessary.

But here's a question which I'm not sure has been answered...

They give a better performance due to fooling the system into thinking the actual fuel rail pressure is less than it is.

So with regards to the claimed improved MPG, would it be fair to say that the system will also fool the car's ability to determine the actual fuel consumption.

In other words, is the 'better' fuel economy of these devices down to the fact that the displayed MPG is higher which is no longer an accurate representation (not that it is normally but you get my gist) and that the actual MPG is pretty much the same?

has anybody tested this doing the 'brim to brim' test?

The ECU calculates fuel used by measuring the injector opening times, not the pressure in the rail...

Had one on my old Volvo and it definitely gave more MPG when measured the old way... brim to brim... Pete

__________________________________________________
2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

A tuning box won't give better fuel than a remap, indeed, it's much more likely to use more fuel than standard than a remap is. You also loose the safety buffer with limp mode, sould the engine develop a fault. A good, reputable remap is the only advisable option IMO.  Visiting from DISCO3.CO.UK
Discovery 3 TDV6 Auto HSE Zambezi Silver

OK, that's interesting regarding how the fuel consumption is calculated. Thanks for the clarification.

Ive fitted a tuning box to my TD6, totally transformed the car, it claimed 10% better economy and according to the on board computer its right. the improved mpg comes from more torque and not having to press the throttle as far 03 TD6 vogue....Epsom, very nice
03 TD6 se....gone
94 Vogue se....gone

I had one on my D3 2.7 and made a big diffrence, but the more you go! The more you drink as the saying goes  Current vehicles
4.2 SC FFRR RSE 2010- facelift
Range Rover Evogue Dynamic Lux

Gone
Rejected RRS my14.5 nightmare
Disco B4 XXV convert
Freelander 06 SE
06 RFFRR 4.2 SC Vogue RSE
Audi A8 4.2Tdi Quattro (hated with a vengence!!)
54 FFRR 2005 model V8 4.4 Cobra Variant
53 Mitsubishi Shogun V6