GarageCalc The gap nobody advertises
You would expect GCM to equal GVM plus the braked towing rating. For almost every popular Australian tow vehicle, it does not. Take the numbers from our vehicle data:
| Vehicle | GVM | Braked | GVM + braked | Actual GCM | Shortfall |
|---|---|---|---|---|---|
| Toyota LandCruiser 300 | 3,280 | 3,500 | 6,780 | 6,750 | 30 kg |
| Ford Ranger 3.0 V6 | 3,280 | 3,500 | 6,780 | 6,400 | 380 kg |
| Toyota Prado 250 (GXL) | 3,150 | 3,500 | 6,650 | 6,600 | 50 kg |
| Isuzu MU-X 3.0 | 2,800 | 3,500 | 6,300 | 5,900 | 400 kg |
| Toyota LandCruiser 79 (2.8) | 3,510 | 3,500 | 7,010 | 7,010 | 0 kg |
The shortfall column is weight you must leave out of the car (or the van) to tow the full rating legally. The 70 Series is the rare exception where the two headline numbers can be reached at once.
The payload trap, worked through
Hook a 3,500 kg van to a LandCruiser 300 with 350 kg on the ball. GCM 6,750 minus the 3,500 kg van means the loaded car may weigh at most 3,250 kg. Its kerb weight is about 2,630 kg, the ball weight adds 350, and you are at 2,980 kg with 270 kg left for driver, passengers, fuel above the kerb spec, and gear. Two adults and a full tank and you are done. The rear axle limit (1,930 kg) usually bites even earlier.
None of that is visible in "3,500 kg towing capacity". It only shows up when you put every number in one place, which is what the caravan towing calculator does.
How to use GCM when shopping
- Subtract your realistic loaded van from GCM. That is your maximum loaded car weight while towing.
- Subtract kerb weight and ball weight from that. What remains is what you can actually carry.
- If the remainder is under about 300 kg, the combination works on paper but not on holiday.
- Remember that GVM upgrades rarely lift GCM. If GCM is the binding limit, only a lighter van or a different tow vehicle fixes it.
Frequently asked questions
Why is GCM not just GVM plus the towing capacity?
Because the manufacturer rates the driveline, cooling and brakes for a total combined load, and for most vehicles that total is less than a fully loaded car plus a full-rated trailer. The two headline numbers were never meant to be reached at the same time. GCM is the number that says which combination is actually allowed.
Can a GVM upgrade raise my GCM?
Usually not. Most engineered GVM upgrades leave GCM unchanged, so they fix payload problems (carrying more in the car) but not towing problems. If your combination fails on GCM, a GVM upgrade will not help; you need less trailer, less in the car, or a different tow vehicle.
Which vehicles can tow their full rating at full GVM?
Only those where GCM equals GVM plus the braked rating. In our Australian data the standout is the Toyota LandCruiser 70 Series (GVM 3,510 + 3,500 = GCM 7,010). Most wagons and utes fall well short: the LandCruiser 300, for instance, has 6,750 kg GCM against 3,280 + 3,500 = 6,780, and the rear axle limit binds even earlier.