Trailer weight is the first and most important factor in determining whether or not your vehicle is suitable for towing the trailer you have in mind. The horsepower-to-weight ratio should be calculated. The basic standard is a ratio of 35:1. That is, 35 pounds of GVW for every horsepower.
To calculate this ratio, multiply your engine's rated horsepower by 35, then subtract the weight of the vehicle, including passengers and luggage. The resulting figure is the ideal maximum trailer weight that you can tow. One point to consider: a numerically higher axle ratio can offset what appears to be a low trailer weight. If the weight of the trailer that you have in mind is somewhat higher than the weight you just calculated, you might consider changing your rear axle ratio to compensate.
For vehicles produced up to the end of 1982, Volvo recommends a maximum trailer weight of 2000 lbs. (908 kgs) with a tongue weight of 200 lbs. (90 kgs). In 1983, the limits were generally raised to a maximum of a 3300 lb. trailer (1500 kgs) and an absolute maximum tongue weight of 200 lbs. (90 kgs). The 780 family is the only exception to this rule, having a maximum recommended trailer weight of 1985 lbs. (900 kgs) with a tongue weight of 110 lbs. (50 kgs). A general rule is that the tongue weight of the trailer should never exceed ten percent of the total weight of the trailer.
Cars with overdrive transmissions must not use the overdrive when towing. Volvo recommends the installation of a transmission oil cooler when towing heavy trailers; this will allow the transmission to operate closer to its normal temperature range. When towing, remember that the engine and transmission are subjected to heavier than normal load; engine temperature should be watched closely for overheating and fluids should be checked regularly.