I'm at 3200 or so with 3.73's and 28" rear tires. The wheezer goodwrench motor sounds mean with bullets and headers on the highway. Itll cruise at 85 all day with the air on if you can afford to fuel it. People on c10 forums have threads like this all the time guessing it's stuck in second, or their truck has dump truck gears, but no tach. That's just how it sounds. I've gotten the turd up to the "p" in mph with no issues and more left Most really low geared trucks are 3ott or granny low I-6 farm trucks, mostly 3/4 ton.

 

My 91 Burb is at around 2000 at 70, 4L80E and 3.73s, but the factory 16" E load rated tire is 32" tall. I'd love to snag a gear vendors OD, it could easily pull the extra gear with what I normally tow.

 

Why? yes they may all have a 1:1 3rd gear, but the 4L60 and the 4L80 both have overdrive, and thats the only real problem here. (.70:1 & .75:1 respectively)

OP needs overdrive, period.

He likley has a Th400 so time for a 4l80e swap imo unless you can live with the high rpms

 

Gear vendors OD unit would drop the highway Rs down. Very common mod on the older TH400 trucks.