In Norway during the summer holidays I drove through a steep tunnel connecting Kristiansund to the mainland. It went down 200 or 250 m, then up again 200 or 250 m at a 10% grade (so a long tunnel; about 4 miles). 80 kmph was allowed. When going up, the engine revved up slightly to about 2500 rpm and the battery assisted. About halfway up the battery assist slowly diminished and the engine revved much higher. I reckon, based on sound etc. that 90 or 95 kmph (60ish miles an hour) must ultimately have been the mk4 top speed at 10% inclines. That was the only time the engine sounded loud. Also, radio reception was blocked so far below ground…