Complex sentence: Essentially all the experimental evidence that can distinguish between the theories agrees with relativity theory to within experimental measurement accuracy, but the differences between the two are usually very small (except where there are very strong gravity fields and very high speeds).
Split into simpler sentences: Essentially all the experimental evidence that can distinguish between the theories agrees with relativity theory to within experimental measurement accuracy. The differences between the two are usually very small (except where there are very strong gravity fields and very high speeds).