Summary form only given. This paper presents LZAC, a new universal lossless data compression algorithm derived from the popular and widely used LZ77 family. The objective of LZAC is to improve the compression ratios of the LZ77 family while still retaining the family's key characteristics: simple, universal and fast in decoding and economical in memory consumption. LZAC presents two new ideas: composite fixed-variable-length coding and offset-difference coding. The two new concepts enable LZAC to improve the LZ family's compression ratios further without using the complex and slow adaptive arithmetic coding or Huffman coding. The Calgary corpus was used to evaluate LZAC's performance. The results show that LZAC achieves a non-weighted average compression ratio of 2.94 bits/char in an 8-kbyte window which surpasses the performance of any other compression algorithm in the LZ77 and LZ78 families.