I have just finished reading a guide to safe towing, specifically in relation to car capabilities and legalities. All 1328 words of it were the most mind-bogglingly complicated that I’ve seen since my maths A-level in 1981.

It seems that the magic ‘rule of thumb’ equation for working out maximum towing weight goes something like: car weight including passengers, divided by calories eaten on the previous day, multiplied by the number of bikes in the household divided by the average mpg, minus 58% of the engine’s capacity plus tyre diameter, divided by the torque ratio airbag coefficient, plus seven (for good luck). Or something like that. You get the picture.

The thing is, my trusty Volvo XC70 has finally bitten the dust. After nine years and 180,000 miles it began wheezing and groaning its last. So I made a deal with it in July: just get us through our three-week summer holiday touring extravaganza and I’ll retire you to a warm garage for the rest of your days.

And bless! It stuck to its side of the bargain – almost – as it coughed its way through Devon and Cornwall, and only finally collapsed as we turned off the M5 towards Glastonbury, 40 minutes from home. And even then it allowed us to creak into a layby alongside a friendly pub (The Knowle Inn, the staff of which very kindly re-opened it and cooked us dinner).

But the time had come to get a replacement. Another Volvo seemed to make sense, and the people at Holdcroft Volvo in both Stoke and Crewe were very helpful.

“You’ll need to know your maximum-towing-weight-ratio-cross-hatch-decimal-equator-multiplier,” said the salesman, or something similar. So it was with that in mind that I headed off to the internet for advice. And simple, it certainly is not!

First, you have to know the fully laden weight of your caravan. How do I know that? It depends on how stocked the cupboards are and how many DVDs and teddy bears the kids have packed. I tried driving onto the bathroom scales. Disaster – even with a scale under each wheel. Then there’s the torque ratio multiplier of the car engine and the two-thirds inverse kerbweight and length rule…

I wrapped my head in a wet towel and locked myself in a darkened room with my old scientific calculator from school. Still no good. In the end I bought the biggest and heaviest car I could find: a sparkly Volvo XC90. And it’s even got a towbar!

How I wish I’d had this expert outfit matching video from Practical Caravan’s Tow Car Editor David Motton!

Visit Martin’s website for information about him, his books and his property training weekends, and follow his adventures on Twitter.