I did some testing FOR SCIENCE on my trip to SagA. What I did is landed on an icy body and drove around collecting any materials I could find and checking about once a minute to see when I gained 1% in my exploration rank. As soon as I gained that 1% I recalled my ship and didn't do any further SRV exploration until the next test at a further distance from Sol. This also included NO "first exploration" bonuses for materials. Results are below:
2164ly from Sol, drove 81km and collected 69 materials for 1% gain
4112ly from Sol, drove 49km and collected 96 materials for 1% gain
8313ly from Sol, drove 28km and collected 79 materials for 1% gain
16062ly from Sol, drove 33km and collected 72 materials for 1% gain
25760ly from Sol, drove 31km and collected 68 materials for 1% gain
As you can see it does seem like you get more exploration ranking the further away you are from Sol, but it seems to level off somewhat at really far distances. The results 8000ly from Sol aren't much different from the results almost 26,000ly from Sol.
On this final planet 26,000ly from Sol I also tested how far I had to drive without collecting any materials to gain another 1%. I drove 66km before gaining another 1% on that planet, compared to 31km + 68 materials for 1% gain on the same planet. Collecting materials is a significantly faster way of gaining exploration rank than just driving around without collecting materials, but both are pretty ineffective in comparison to "traditional" exploration, ie scanning planets/stars and trading in the cartography data at a station. Driving the SRV is definitely more fun however.
