The fuel has a massive difference on economy and power, ive always advised that if you use good fuel it will return you good value for money.
Especialy if tuned / remaped.

The supermarket fuel is a cheap fuel and you gets what you pay for in that respect.

I personaly prefer BP Ultimate diesel over all the main brands, it returns mebetter economy and also ive proved the power differences on the dyno.

Reading about Cetane value of diesel it is intresting and does help you logicly work out why good fuel is normaly better than the lower cetane fuels.
If im correct BP offer a guarenteed 55 cetane.

http://www.bp.com/sectiongenericarticle.do...Id=7009069

Regards Pete 
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