does sound th as d makes any difference in fast speech?

I know the 2 kinds of th sound, the question is does pronouncing, for instance, "that" as "dat" makes any difference to real th sound? I know pronouncing it in slow speech or out of a sentence really does but and about fast speech? in fast speech "that" always sounds as "dat", i'm talking about non-blown d sound.

Solution 1:

In some Irish, Liverpool and West Indian dialects the voiced dental fricative (as in 'that') can sound like 'd'. (A song in Lionel Bart's Maggie May starts, "Dey Don't Do Dat Today Den, Do Dey?").

Both the voiced 'th' and the voiceless one (as in 'thin') are quite rare phonemes, existing in only 4% of the world's languages. Jamaican Patois has no voiced dental fricative, and in London it’s not unusual to hear younger Jamaican-Brits substituting 'v' for the voiced th ('Get anovver one') and ‘f’ for the voiceless one ('I’m in your barfroom.’) This is also what many Cockneys do. It's known as ‘Th-fronting’ and it appears in several dialects of English.

In London at least, ‘v’ is increasingly being used to replace ‘th’ at the start of a word - ‘these’ → ‘vese’ etc. - a trend first noticed in a survey in the early 90s.

So yes, your 'dat' would be noticed and maybe ascribed to a dialect. If you find it hard to say "that", you might try "vat".