Allison 2500 Series
Automatic • 5-speed • Allison Transmission
About the Allison 2500 Series
The Allison 2500 series is Allison's Class 4-6 medium-duty automatic transmission — the dominant medium-duty automatic in North America for over 25 years. Used in school buses, box trucks, delivery vehicles, severe-duty service bodies, and Class 4-6 cabovers. The 2500 is widely considered the most-reliable automatic transmission in commercial trucking — many examples cross 750,000 miles before major overhaul. Allison's reputation for million-mile reliability in commercial duty cycles started with the 1000/2000/2500 series.
Overview
The Allison 2500 series is Allison's Class 4-6 medium-duty automatic transmission — the dominant medium-duty automatic in North America for over 25 years. Used in school buses, box trucks, delivery vehicles, severe-duty service bodies, and Class 4-6 cabovers. The 2500 is widely considered the most-reliable automatic transmission in commercial trucking — many examples cross 750,000 miles before major overhaul. Allison's reputation for million-mile reliability in commercial duty cycles started with the 1000/2000/2500 series.
Specifications
- Manufacturer
- Allison Transmission
- Type
- Automatic
- Gear count
- 5
- Torque capacity
- 660 lb-ft
- Truck classes
- Class 4-6
- Applications
- Medium-duty trucks, school bus, delivery, severe-duty service body
- Production years
- 1998–present
Design and engineering
5-speed automatic with torque converter. Allison electronic control. Multiple PTO configurations for vocational accessories. Allison fluid required (proprietary TES-295 spec).
Generations and updates
Reliability and longevity
Allison 2500 is widely regarded as the gold-standard medium-duty automatic. Million-mile reliability is common in transit bus and severe-duty fleet service.
Common problems
Fluid degradation if intervals are stretched (Allison TES-295 fluid must be changed at OEM intervals — typically 150K-250K miles depending on duty cycle). Solenoid wear at 500K+ miles.
Driver experience
Smooth shifting in stop-and-go, optimized for vocational duty cycles. Less driver involvement than manual — popular in delivery and bus fleets where driver pool varies in skill.
Pros and cons
✓ Pros
- • Gold-standard reliability
- • Million-mile track record
- • Multiple PTO options for vocational
- • Strong dealer support
⚠ Cons
- • Allison-spec fluid required (TES-295) — more expensive than generic ATF
- • Higher cost than competitors
- • Service requires Allison-trained tech