If you’re building a 1973–1987 C10 or K10, you’ll inevitably face the TH350 vs TH400 decision. Both are legendary GM automatic transmissions, but they’re designed for very different purposes. Here’s the complete breakdown.
The TH350 (Turbo-Hydramatic 350):
- 3-speed automatic
- Used in most 1/2-ton applications
- Lighter, more compact
- 1.52 first gear ratio
- Max torque handling: ~350 ft-lbs (stock internals)
- Found behind 250/305/350 small blocks
- Great for mild to moderate street builds
The TH400 (Turbo-Hydramatic 400):
- 3-speed automatic
- Used in 3/4-ton and performance applications
- Heavier (about 135 lbs vs 120 lbs for TH350)
- 2.48 first gear ratio (much lower, better off-road)
- Max torque handling: 450+ ft-lbs stock, much more built
- Originally found behind 454 big blocks, commercial trucks
- Better for towing, high-power builds, and off-road
Key Differences at a Glance:
| Feature | TH350 | TH400 |
|---|---|---|
| Gears | 3-speed | 3-speed |
| 1st Gear Ratio | 2.52:1 | 2.48:1 |
| Torque Capacity | ~350 ft-lbs | 450+ ft-lbs |
| Weight | ~120 lbs | ~135 lbs |
| Overdrive | No | No |
| Best For | Street/mild builds | Towing/performance/off-road |
Which Should You Choose?
- Daily driver street truck with stock/mild engine: TH350
- High-horsepower build, towing, or off-road: TH400
- Want overdrive for highway fuel economy: Consider a 4L60E swap instead
The 700R4 Option:
If you want the best of both worlds with overdrive, consider a 700R4 (TH350 dimensions, overdrive 4-speed). This is a common upgrade in C10 builds for better highway fuel economy.
Finding a Good TH400:
Look for units from 3/4-ton trucks, motorhomes, or commercial vehicles – these were usually built tougher. A rebuilt TH400 with a shift kit and better clutches is an excellent choice for any C10 build.
What trans did you use in your Square Body build? TH350, TH400, or did you go a different direction?