Don't Panic, a tyre question

The difference is noticeable but negligible. The apparent drop in fuel economy is mostly because of the effect of the tyres on the odometer. As with the speedo under-reading, the odometer will also be recording 6% less miles for the same given distance. This will make mpg seem 6% worse than it was before even if it doesn't change at all!
Anyone who talks about fuel economy in 'miles to half a tank', 'miles on a tenner' etc should be ignored anyway as those are entirely inaccurate ways to measure it.

The effect on performance is the same- it's primarily due to the change in the gearing rather than the extra resistance of the tyres.

Al. wrote: I would like to consider a tyre size of, 235/70/2015, I use my car daily and so does the wife, meaning I don't want to mess with the speedo.

I ran 225 70 15s on a slightly wider rim than stock. It should have made the speedo under read - so when the speedo says 30 real speed is 31ish - but the speedo was over reading anyway and I recall that the speedo was pretty much bang on after fitting the tyres.