I have an 85 Toyota pickup that’s been having some minor issues till just now.
Over the past month it has trouble hitting that last gear change when it first starts up. Eventually it hits it but the last 2 days now it won’t go past 40mph, it violently shakes when it hits that speed, it starts slowing down even if I press the pedal more, and when I eventually stop the car and turn it off I can smell something burning.
I’m if the two problems have anything to do with each other but I’m taking it to a mechanic friday. Just wanna know what to expect.
Engine? carb or FI? Trans? manual or automatic?
Does the engine speed increase when you press the pedal more?
4 cyl
Carb
Automatic
If you pour in a bottle of the Lucas tranmission additive you might get a few more miles out of it but not many.
It doesn’t increase in speed at all.
Sounds like a transmission problem. First step is to check the trans fluid level, next a proper transmission service, drop pan, drain the old fluid out, new filter, and refill. If that helps, but not 100%, repeat after driving 100 miles. If it still’s acting up after that, time for a pro evaluation at the local inde transmission shop. The clutches in automatic transmissions are just like the ones in manual transmissions, normally wearing parts, don’t last forever. Sometimes sensor problems can cause this too, especially if the vehicle speed sensor is faulty.
Maybe I misread that OP. @shadowfax seems to be reading better than me here. Are you thinking the problem may be a restricted exhaust @shadowfax?
At the time I asked I was thinking it might be a manual transmission that’s slipping, but OP has since clarified a few things 
It certainly could be restricted exhaust… Especially if it’s the original cat…
The smell was burning oil. I couldn’t place it till a coworker smelled it too and mentioned it.
Could an overfilled transmission fluid cause this? I recently overfilled it but didn’t think anything of it.
Of course it could. Check your owners manual for how to check trans fluid level. For some cars it’s hot and running, others cold. Get the level correct and let’s hope that takes care of the problem, or at least helps track it down.