Difference in hash rates between Bitcoins and Litecoins calculation
If you are using general purpose mining hardware (i.e. CPU, GPU) then it really depends on lots of factors, not only hardware, but also mining software and even its configuration.

For details on mining hardware you can check the tables for Bitcoin and Litecoin.

But with the inevitable introduction of ASIC miners (already for Bitcoin, and soon for Litecoin) the question is actually irrelevant.

ASIC miners can only work with the hashing function for which they were designed.
So a Bitcoin ASIC miner (SHA256) cannot mine Litecoin (scrypt) at all, and vice-versa.