yup 3.90's in the 4 speed trucks.

hmm can you explain then the factory code for my truck,,,,,,,L-43 G102 
it is "possible"  that one or two might have been ordered from the factory as 3/4 tons with 4.88 gears,  never seen it but its possible,  ive seen 4 so far that had the 4.375 r&p code, G102 with the L43  they were all early year, as in 2-79 model short beds,   80 models as far as i can tell and "most" 79 all had 3.90 gears,  think it was a G072 code

if it has a G142  then its a 4.88 a.k.a.  3/4 ton