The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - Difference in Hiway RPM-3.73 to 3.42

**MikeB** · 06-03-2023, 11:40 AM

My 69 C10 with a mild 350 has a 3.08 posi diff. I certainly wouldn't call it a toad, because it will spin both tires in low gear. But the engine is still a little busy for me at a constant 70mph, even with 29.5" tall tires.

I also had an 82 C10 with a mild 350 and a 2.73 open diff. It was great on the highway, but acceleration was nowhere near what you'd get with a 3.73 or 3.42. Still, the 2.73 was just fine for cruising around town and doing pickup stuff.

Both of those engines were very torquey at low-mid RPMs, which is what you need for anything lower numerically than 3.23. If you want better acceleration and have a non-OD trans, you'll have to live with higher revs on the highway.

What are you engine specs, including camshaft duration at .050" tappet lift?

06-03-2023, 11:40 AM	#6
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,435	Re: Difference in Hiway RPM-3.73 to 3.42 My 69 C10 with a mild 350 has a 3.08 posi diff. I certainly wouldn't call it a toad, because it will spin both tires in low gear. But the engine is still a little busy for me at a constant 70mph, even with 29.5" tall tires. I also had an 82 C10 with a mild 350 and a 2.73 open diff. It was great on the highway, but acceleration was nowhere near what you'd get with a 3.73 or 3.42. Still, the 2.73 was just fine for cruising around town and doing pickup stuff. Both of those engines were very torquey at low-mid RPMs, which is what you need for anything lower numerically than 3.23. If you want better acceleration and have a non-OD trans, you'll have to live with higher revs on the highway. What are you engine specs, including camshaft duration at .050" tappet lift? __________________ Mike 1969 C10 LWB -- owned for 34 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 23 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!