There is no single speed limit based only on tractor weight
For most conventional agricultural tractors in Great Britain, the maximum road speed is 40 km/h, approximately 24.85 mph, commonly described as around 25 mph. The tractor's legal construction category and width matter, as do the trailer, load, road signs and conditions. You cannot choose a lawful speed merely by looking up the machine's weight.
A manufacturer's 50 km/h or 60 km/h transmission rating is a capability, not evidence that the vehicle and trailer meet the legal conditions for higher-speed road travel. Use the lowest limit that applies to the road, tractor, equipment and operating conditions.
| Vehicle category | Maximum speed |
|---|---|
| Most conventional tractors | 40 km/h (approximately 24.85 mph) |
| Tractors meeting higher construction specifications | Up to 40 mph, subject to all applicable requirements |
| Special-type agricultural vehicles over 2.55 m but under 3.5 m wide | 20 mph |
| Special-type agricultural vehicles 3.5 m or wider, within the 4.3 m authorisation | 12 mph |
When can a tractor travel faster?
DfT guidance says some higher-specification tractors may be driven at up to 40 mph. Its summary includes all-wheel suspension, 50% braking efficiency, pneumatic tyres, a speedometer and a horn among the requirements. The full Construction and Use rules, not this abbreviated list, determine eligibility.
Ask the manufacturer or a suitably qualified adviser to confirm the precise machine's compliance. Then confirm the towed equipment and brakes are suitable for the intended legal speed. Do not transfer a higher-speed assumption from a tractor alone to the same tractor with a conventional agricultural trailer attached.
The figures in this article describe the Great Britain framework. Confirm Northern Ireland limits and vehicle requirements with the DVA; they must not be inferred from a GB rule or from an AgriNav setting.
Width can reduce the limit
The width measurement includes tyres, including tyres added after manufacture, and any towed implement or trailer. A conventional-looking tractor on wide tyres can therefore fall into a lower special-type speed category. Measure the actual transport configuration rather than taking a narrow catalogue width.
STGO article 21 places special-type agricultural vehicles over 2.55 metres but under 3.5 metres in the 20 mph band. Exactly 3.5 metres is in the 12 mph band, although the width-based attendant requirement starts only above 3.5 metres. The Part 3 width authorisation ends at 4.3 metres, including lateral projections; going beyond that needs specialist movement advice.
31 tonnes is the combination limit, not a trailer payload
DfT guidance states a 31-tonne limit for agricultural tractor and trailer combinations. The trailer itself is limited to 18.29 tonnes, including load imposed on the tractor through the hitch. Those are not permissions to carry 31 tonnes of cargo or 18.29 tonnes of cargo.
The trailer limit includes its own weight and its load; the combination includes the tractor, trailer and everything carried. The permitted operating weight can be lower because of plated limits, manufacturer ratings, axle limits, tyres, coupling ratings or a route restriction. Always use the lowest applicable limit.
For illustration, a trailer with a tare weight of 5 tonnes leaves at most 13.29 tonnes before reaching the 18.29-tonne trailer total, even before any lower equipment or axle restriction is considered. Do not assume that a farm trailer advertised as a '14-tonne trailer' has a road-legal 14-tonne payload in every configuration.
Check actual loads and axle distribution
A weighbridge reading is more useful than a guess based on trailer volume. Wet grain, soil or silage can change the load considerably. Check how weight is distributed across the tractor and trailer axles and the drawbar; an acceptable total does not guarantee every component is within its rating.
A bridge or local road may impose a lower structural or axle restriction. A vehicle that is lawful on an unrestricted road can still be prohibited from that particular bridge. Do not confuse maximum authorised mass with the actual laden weight used for a structural weight check.
Legal maximum and safe cruising speed are different
A legal maximum is a ceiling, not a target. Hills, corners, traffic, wet surfaces and heavy towing loads can justify a much lower pace. Leave enough stopping distance, especially downhill, and use the manufacturer's braking and transmission guidance.
In AgriNav, the road speed entered for the machine is a planning input, not a certification of its legal speed. Enter a lawful speed for your real combination. Road signs describe limits for traffic on that road; a displayed 60 or 70 does not cancel a lower tractor-specific limit. Journey estimates are not a reason to exceed a safe speed.
Official sources
Written from the official guidance and legislation below. Use the current consolidated legislation and seek advice where classification or exemptions are uncertain.