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OMG! I think we have a winning reply! Perhaps a careless omission from my initial post, but the WAS a hot smell coming from the car the first time I noticed the low fuel consumption. I thought it was a brake binding, and the hot smell didn't repeat for my home journey after that one instance, despite still having that 20-something MPG on that return journey, so I thought it was a red herring and didn't mention it. My goodness! That would explain why there were no fault codes and the garage found nothing. I think you have nailed it! 🏆

I've tried to second-guess myself on the driving style, trying to fit the available data into a theory, but it's very much guesswork, and it doesn't explain the HUGE variation in MPG, given a very similar route driven at a similar pace, traffic permitting. (I'm just driving too and from work, after all.)

It's now done 31K miles, and the plugs and coils were replaced in April this year during a full service at ~29K miles following a slight misfire, as was the air filter. The car is a 2019 2.0 TSI SE L. I don't know what happened Gaz, but the issue seems to have gone away for the time being, though it still seems a little thirstier than previously, but nothing that I couldn't put down to a change of driving style. Last tank mpg was about 44 overall, which is fairly typical. The 50 MPG figures are only when I'm on a long run or am deliberately driving for economy. I've just checked it (without having reset my BST clock yet. 🙄) and a motorway run I did today was just over 50mpg, which is normal for it. So I have no idea what it was up to for that brief drop in MPG, but obviously I'll keep an eye on it. Thanks for the feedback, gents. 👍

So my beloved 2.0 TSI SE L suddenly has a huge thirst. The daily commute has gone from late 40's early 50's MPG, to mid to late 20's MPG. No fault codes. Tested at garage and EGR, O2 sensor, Air Con, Secondary Air con, Evap Emissions, Catalyst heating and operation all within normal parameters. Tyre pressures all good. Brakes not binding. I haven't changed my route to work or changed my driving style. Fuel used is still Tesco Momentum from the same garage. I'm starting to think laterally, like could it be something the engine drives, like the water pump or alternator causing drag? Could the MPG-ometer be faulty? (I need to test this next fill-up.) Slightly baffled, but enjoying the challenge of a mystery fault. I'd like to find the problem now, before anything catastrophic happens. Anyone got any suggestions as to vectors of attack on the problem?

Woof! That car's seen some action!

The Octavia is a great family car IMO. VAG cars do like pampering though and servicing can't be skimped on without risk. They're very good so long as you anticipate known issues and head them off. Some have high oil consumption, some have issues with silica bags in the coolant that can split and gum up your heater matrix. The more you learn about your car, the better you'll be able to prevent things going wrong with it. (I've no idea what the dealers are like these days as I ducked out of the dealer network as soon as my warranty was up.)

I say keep the MK3.5 and start saving for the new car now. You can save a fair bit in 3 years, and I assume you already have some savings.

Just buy a car appropriate for your needs.

They are lovely-looking wheels, but now your car seems to sit too high. On the 16" wheels and chubby tyres, the clearance looked right. It looked like the car would happily bump along rough conditions and up and down kerbs without a worry. Now on the 18" wheels, the front end looks like one of those all gas-suspension Citroens with the suspension up on tip-toes.

Some excellent tyres are always a good investment for improving your car's performance. As @nta16 suggested, absolutely tip-top servicing and good-quality fuel will also help you get the best out of your car. For performance, reduce weight if at all possible, and keep your car super clean. Then I suggest you lean-into the low power. With a low-power car you can used all the available engine power more of the time. Conservation of momentum and maintaining speed is important, so you'll learn to drive better, finding the best line, when to brake, how much to brake. You won't inflate your insurance cost (Morning Dan!), and you won't knacker your car by over-stressing components. And when it comes to sell the car, all you've done to it is service it perfectly, kept it clean and used great tyres & good fuel, so you'll get the most money back on the car when it comes to replace it. Seriously, embrace the low power as a feature, not a problem. Don't tune the car, use the car to tune yourself. 👍 (Of course I expect you to completely ignore this advice and fit an induction kit and a noisy exhaust, but that's cool. 😄)

The manager, obviously. 😋 <edit> P.S. I hate to do it, but I have to: It's "To whom?".

You're quite correct that this mod will both offer no performance advantage (and may actually make things worse), as well as be considered a modification by insurers. However it may well give the desired meaty sound your son thinks will make his driving experience more enjoyable. He's unlikely to accept an electronic substitute (but it's worth a shot as a compromise) https://www.maxhaust.com/en/active-sound There are two approaches I'd go for. 1) As a cheap fix, I would try and use the police as an attack vector. Tell him if he draws attention to his car, he may get unwanted attention from the police. He may well be stopped, have to produce his documents, which will involve insurance documents, and his insurers might find out about the modification. If that fails.... 2) I'd reluctantly permit the endeavor, so long as it's done properly at his expense. Tell him you will not oppose his mods, so long as they are done well, are reversible for when the car is sold, the insurers are informed of every change, and he pays for everything. Tell him you will check with the insurers to make sure that they are aware of every change he has made to the car to ensure that the insurance cover is not harmed. A third option is to leave him to get on with it and FAFO. Having to deal with the consequences of foolish endeavors is all part of life. It might be that there are no serious outcomes to the mod, and all he gets is extra noise when he drives. But there are possible negative consequences. Good luck!

What is the problem you think you need to address? The problem, not the difference between the spare tyre and your fitted road tyres.

Hi! Just a few questions. Is the issue better or worse when the car is cold/just started? Is the issue better or worse when the car has been driven for a 15 minutes or more? Is the issue better or worse when the weather is cold? Is the issue better or worse when the weather is warm/hot?

The best advice I can give you on engine swaps, is that if you have to ask for advice, don't do it. But if you really MUST do it, expect the project cost to spiral, expect to have to put-up with some unfortunate trade-offs (which could be benign or downright infuriating), brace yourself for some insurance issues, and expect to have the car for life because it'll be tricky to sell. Edit: I forgot, expect it to take about 5 to 10 times longer than you expected it to take.