i have a 1989 gmc r3500 with a 454 and th400 and the speedometer adapter part number 1565215 went bad.i look for it everywhere but cant find it. does anyone know where to get one? it says 1 to 0.950 so i was thinking if i went from the 41 to a 40 teeth driven gear and then hooked the speedometer cable directly up to it would that make up the difference. also the drive gear has 16 teeth if that helps any.

I found this: https://www.stockwiseauto.com/acdelc...ometer-adapter

thanks but its out of stock

Illusive part # for sure. Consider calling the dealer & have them check worldwide stock & see if there is any other GM part number as a cross reference.
Other than that Keep an eye out on ebay. Google under images sometimes might lead to an available part.

will it be ok if i just hook up the cable directly to the housing and driven gear then? i know it will be inaccurate but it would be better then nothing

Ya on the TH 400 I think it should still work ok. Will be interesting to see how far out it will be though.

That part is 0.950:1 so it will only be off by a little.

tried connecting without the adpapter and you got to put a pretty sharp bend in it to get it to line up.should i get sometyhing like this (Speedometer Cable) that will give me more length so it is bent so bad.

You definitely don't want a sharp bend.