Transmission slipping?

[76chevyc20454]

My buddy has a 1990 gmc Jimmy or the k5 I would think. It had a 700r4 and he swapped a th400 into it. It's the 350 tbi but at 55 he is running 4300 rpm to 4600. It don't feel like it is slipping, and it shift through all three gears. It has 35 in tires and suppose to be a 3.73 reared . Is there a way to make sure it is slipping? It seems like it pulls and could it be something to do with transfer case? Thank you in advance.

 

[87scotty]

Transfer case is gears and chain I doubt its slippin but I have 410 gears and don't rev that high at 55 my guess so thin wrong in tranny

 

[PrairieDrifter]

I would find out what gear ratio you have first, I don't think 3.73's were very common in 1990. Anything under 3.73 with 35 inch tires without an overdrive trans is gonna scream pretty good going down the road.

 

[76chevyc20454]

We locked it into what we think is 4 hi and it droped to 2400 at 55