chevy truck

Has the transmission ever been serviced? Check the fluid. If it's brown, it definitely needs flushing. If the problem has been occurring for awhile now, you may need to replace or rebuild the transmission. The speedometer issue tells me there is either a problem with the computer or with the vehicle speed sensor.

it had the transmission replaced once. it was just all of a sudden the truck has always shifted GREAT! and the speedometer has never acted up. i got in it to go home and at about 35mph it just started slipping bad then i came to a stop, took off again and it slipped a couple more times then was fine. it has never slipped at all before.

you could try testing replacing the neutral safety switch as that tells the trans controls what gear it is in? i've heard of other vehicles fixing issues by replacing this item. speed sensor also. try to find out if there are any other sensors related to trans function.

How many miles on the new/replacement transmission? Since the speedometer is electronically-controlled, the problem with it is definitely electrical. Make sure the connectors on the back of the dashboard are firmly seated. Do the "wiggle test". Get a multimeter, set it to read resistance and wiggle the wires and connectors while watching the meter closely. You should see no fluctuation in the readings.
The slipping problem could be quite a few different things. Check the fluid for color and smell. If it is brownish or smells burnt, get it flushed. If you have an Aamco near you, they do free diagnostics on transmissions.

i checked the fluid and it is pink! seems normal. both of these problems occured at the same time the truck always shifted perfect and the speedo was just fine. i just drove it again and it will shift to 2nd fine but keeps going back and fourth between 2nd and 3rd and slipping in between shifts

Find somewhere that will do a free diagnostic. Shifting back and forth between 2nd and 3rd tells me either speed sensor or vacuum modulator, if so equipped.

ok thank you for your help

My pleasure