My 79 uses 170/70 r 13, and seems like I my speedo is accurate, however the trip counter is WAY off, if I drive around the block it says I've gone a mile.
About the turning full left. I've noticed on my brat there is a mar on the wheel assembly that seems like it was made by the bolts that hold it to the cross member. I see it more on the left side than the right, but if I hard lock to the left I hear a pop when the bolts push under.
About the fuel gauge, I don't trust mine at all. I filled the tank the other day (10.5 gal, and I was running on fumes) and when I started up it said I had 3/4 of a tank. every time I start it up it reads something different, however it seems like mine is always under what is in the tank.