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Don’t worry Leigh, the naysayers will be along soon to tell you you must be imagining things because all fuel is the same, evidenced by the expertise of tanker drivers who’ve ‘seen’ different branded tankers being filled at the same fill points in the distribution depots. Then they’ll quiz you on how you measure your economy – whatever you do don’t admit you used the cars own trip computer readout!
In reality different cars will react differently to different fuels. We can’t, however, know the exact differences between fuels from different outlets (Asda, for example, are not going to admit their fuel is inferior to Tesco’s even if it were to be), except as published (usually by the ‘brands’ and usually for super fuels). Exactly how any given car will react will depend upon the way the engine ecu controls the fuel flow as well as the driving style the vehicle is subject to. However, if a [petrol] vehicle is subject to a change in it’s diet I would expect it to take at least a tank full for the ecu to adjust to any measureable changes (on a car driven moderately most of the time).
I’ll give you an example of that last point. I run a tuned Saab 9-3 2.0T. I run it on Super Unleaded. I buy Super for the improved power and engine cleanliness (which contributes to longevity). I do not measure fuel economy either accurately nor fastidiously. I normally use Shell VPower. Now, through empirical and measured data I have determined that the very best fuel (in my engine) for torque and power output is Tesco Momentum 99 (compared to ‘ordinary’ Tesco unleaded it is +26hp at the wheels). VPower is “only” +18hp at the wheels, so why is this my preferred fuel? This is because I still run the standard intercooler. With 18psi compression available as low as 2300rpm the state of tune is theoretically beyond that which the intercooler can deliver. On a hot day (25C plus) if one uses the power several times in quick succession the air intake temps rise rapidly through the 40s and the tune backs off the boost to protect the engine. This is best tested on the motorway in 6 th (right in the middle of the engines torque band). On ordinary unleaded, the boost backs off after just 3 or 4 runs. On Momentum, the cooler running fuel allows the engine to hold on for 8 -10 runs. On VPower I can push her time after time in succession and have never yet suffered “boost fade”. Long uphill overtakes (eg Fossebridge dip) with their sustained power can also induce boost fade. So, VPower it is for it’s consistant performance (I’d use Momentum for a track day as long as it wasn’t a hot day). BTW I have to go out of my way to get VPower.
My wife runs a C1. Like me she uses Super, but simply because she gets more miles per tank and it also drives smoother (more noticeable effect on a 3 cyl engine than on my Saab which has balancer shafts anyway). Having tried all the major Supers, she consistently gets the most mileage from Esso Supreme (with BP Ultimate nearly the same) and the lowest from Momentum. Lucky, as we have an Esso station in town and a BP station on our commute.
It’s possible that when you changed your car’s diet, one tankful was not enough for the ecu to adjust and you may not have got any benefit as a result. It usually takes my Saab a full tank to adjust, though giving it some ‘clog’ helps this process along. So I would say that if the Tesco stuff is giving you good results and it’s the easiest for you to get, then stick with it. If you wish to change brands as a ‘trial’, then give each one a ‘sporting chance’ and allow at least 5 tanks before assessing results.
Interestingly, I see tankers with yellow and white livery filling the pump tanks on my employers sites. That may or may not mean anything.
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