Perhaps it was a good investment!

I'm a great believer in silver linings - no matter how dark the cloud. There's always something positive to say about a dire deed that has had to be done.

Everyone I've mentioned it to has gasped at the £5,000-odd I had to pay to get a new automatic gearbox installed in Fiona. My cousin Rosemary is going on a very nice cruise this winter for less. And I admit to having some glum thoughts, when I consider how repaying the £5,000 bank loan I had to take out will depress my spending-power over the next 30 months to mid-2018. It means not only some holidays postponed. I won't be able to throw money at the house and garden - everything will get just a touch shabbier. Not what I'd had in mind at all. I am not as house-proud as some, but I really had wanted to smarten the place up, and not just keep it clean and tidy.

But maybe that £5,000 odd lately spent on my car will prove to be something of an investment, giving me a tangible return in cash.

Am I talking nonsense? Well, take a good look at this photo, taken when I reached home yesterday evening after a longish cross-country journey from Kent:

I draw your attention to the right-hand dial, below the time, where it says my average fuel consumption with the new gearbox was 37.8 miles per gallon.

Now that's not a remarkable figure as cars go nowadays, but it's astonishing for my car. You must bear in mind that Fiona is big and heavy, she has a 2.4 litre engine, and automatic transmission with permanent all-wheel drive. She's not a light and economical town car. When towing, she averages 23 mpg. When not towing, I've been getting an average of 30 mpg from her when at home, less if I go into towns to any great extent. When on holiday, usually on less crowded roads than Sussex has, I generally do a little better. For instance, I averaged 31.4 mpg  for my seven nights in North Devon in March this year, and 31.6 mpg when I went there again for twelve nights in September. It was 32.7 mpg for my six nights in Lyme Regis in April. The very best achieved this year was 36.1 mpg, pottering around Fife in Scotland for three days in June, but that was completely untypical.

So I was shocked when I saw 37.8 mpg on my dashboard display last night. The very best ever. It must be a fluke.

But then I reflected. Would the new gearbox have been exactly the same as the old one? Fiona was 'born' in 2010. Did Volvo have a vast stockpile of 2010-vintage gearboxes stashed somewhere, gathering dust, just in case they were ever needed? Or was this a more recently-manufactured box, the right one for my 2010 engine, but in some respects different from the original gearbox fitted? Although it's been operating smoothly and quietly, I've noticed that this new box likes to hold a higher gear slightly longer than the old one used to, which would tend to improve fuel consumption. That could be part of it.

But another part must be a change in my driving behaviour. In order to protect the new gearbox from whatever damage I'd unwittingly inflicted on the old, I've decided to drive with less verve and dash, accelerate gently, and generally take things more sedately. And this too must be having had a good effect on Fiona's fuel consumption.

A more efficient new gearbox - or the gearbox imposing fuel-saving behaviour on me? Or both?

But a fuel-consumption improvement of 20% or so seems altogether too remarkable!

I'll have to keep an eye on this, to see whether it's really true. But suppose the combination of a new gearbox and a gentler touch is working wonders? Let me see...I expect my fuel bill for 2015 to be about £2,500. 20% of that is £500. If the price of fuel stays about the same as now, and I do no more mileage than I have in 2015, I can expect to be better off by £500 in 2016 and each year that follows. Hmm. I'm no good at maths, but that sounds like a 10% cash return on £5,000 to me. If I don't squander the fuel money saved, then I'll recover the entire £5,000 I've spent in ten years, which is exactly how long I'd like to hang onto Fiona before she finally gets a Viking Funeral.

I suspect that I'm bamboozling myself with figures here. But for the moment, until somebody points out the flaw in my maths, I feel warmly consoled.