The rev counter runs from the alternator, so it's never going to be accurate. The one in Blue never gets above 4,300rpm for example.
In my LSE I get about 29mph/1,000revs once the auto 'box has locked up. My sister gets roughly the same in her TDi Auto and IIRC our TDi manual just sounds like it's reving faster, even though it's (again roughly) the same.
2 quick things, changing the tyres won't have any effect on the readings on your dashboard, they might just make your speedo less accurate though and your truck sluggish in traffic
235/70 have the same rolling radius as a 205/80, but they take more effort to move so will lower your economy with only marginal gains in on-road grip. 225/75 are slightly taller, but generally more expensive.
