Q:

The change of the tense in the main and the subordinate clauses

I have found the two sentences in Michael  Swan's Practical English Usage .(page no 556)
1.This discovery means that  we will have to spend less on food.
2 This Discovery will mean that we spend less on food.
It was explained that " if the main verb of a sentence  makes it clear that what kind of time the speaker is talking about , it is not necessary for the same time to be indicated again in subordinate clauses"
The author talks about the tenses in the clauses  but I could  not understand whether the change of the tenses in the 
main and subordinate clauses changes the meaning  of the two sentences given above
I would like to know if  both the sentences mean the same thing or do they differ in meaning

A:

Your question asks about differences in meaning due to changes in the tense, but the example sentences do not only differ in tense.  As pointed out by @Jason Bassford in the comments the inclusion of have to introduces a significant change in meaning.  To remove this difference so that your question is solely about tense we can compare the two sentences:

1. This discovery means that we will spend less on food
2. This discovery will mean that we spend less on food

The effect of these two sentences is the same, in sentence 1 the current meaning (implication) of the discovery is that in the future we will be spending less on food.  In sentence 2 the future meaning (implication) of the discovery will be that (at that time in the future) we will be spending less on food.
So although there is a difference in expression the effective meaning of both sentences is the same - in the future spending on food will be reduced because of this discovery.

