If you're stuck between an Audi A4 and an A6, the short version is this: the A4 is the compact executive saloon and Avant, and the A6 is the bigger, plusher full-size executive. They look like cousins in photos, but they're aimed at genuinely different drivers. I strip and sell used Audi parts for a living, so I see both cars at every stage of their life, and this is the question I get asked more than almost any other. Buy the wrong one and you'll either feel boxed in or feel like you've paid for space you never actually use.
Let me give you the straight version so you choose with your eyes open.
Looking for this part? Tell me your Audi model and reg and I'll get you a quote — quality tested used parts with nationwide UK delivery.
What each car actually is
The Audi A4 (the B8 and the later B9 generation most people buy used) is a compact executive. You'll find it as a four-door saloon or the very popular Avant estate. It's the sensible, right-sized Audi: big enough for a family, small enough to thread through town and tuck into a normal parking space. It shares its bones with the A5, so if you want the same kit in a sleeker shape, that's the natural rival.
The Audi A6 (C7 and the newer C8) is a class up. It's a full-size executive saloon, also sold as a roomy Avant, built to swallow motorway miles in near silence. Longer, wider, quieter, heavier. Think company-car-and-airport-runs rather than nipping to the shops.
The quick gut check
- Want a premium Audi that's cheaper to buy and run, and easy to live with day to day? A4.
- Need maximum rear space, the calmest ride and a plug-in hybrid option? A6.
- Carry big loads often? Both do an Avant, but the A6 Avant is the bigger hauler.
Size and practicality
This is where the gap shows. The A4 saloon is around 4,762mm long. The A6 saloon stretches to roughly 4,939mm, so it's close to 200mm longer overall, and most of that extra length lands as rear legroom. Sit three adults across the back of an A6 on a long run and they'll thank you; do the same in an A4 and it's fine for shorter trips but tighter on space.
The A4 is the easier car in the real world though. It's narrower, lighter and simpler to park, and it still takes a child seat or two without drama. For a lot of buyers the A4 is all the car they ever needed, and the A6's extra room sits unused 95% of the time.
What we see on these
Across the two, the A6 brings in the most enquiries for the heavy, pricier items: engines, gearboxes and the larger body panels that come with a full-size car. A4 owners tend to ask for the bits that wear on a busy daily driver — suspension arms, headlights, front-end panels and the usual service parts. On both, electrical and infotainment components come up far more often than people expect.
Boot space
If load space matters, the numbers tell the story:
- A4 Saloon: 460 litres
- A4 Avant: 495 litres (around 1,495 with the seats down)
- A6 Saloon: 530 litres
- A6 Avant: 565 to 586 litres (up to roughly 1,680 seats down)
On paper the saloons aren't miles apart, but in practice the A6's longer, wider load bay swallows buggies, bikes and big shops far more easily. If you're regularly loading bulky stuff, the A6 Avant is the one to want.
Engines and how they drive
Both ranges lean on Audi's familiar 2.0-litre TFSI petrol and 2.0 TDI diesel units, badged by output (35, 40, 45 and so on). In the A4 you'll mostly find the 2.0 TFSI petrols and the long-distance-friendly 2.0 TDI diesels, with the hot S4 using a punchy 3.0-litre V6 if you want real pace. It's a range built to be sensible and broadly affordable to run.
Used Audi A4 & A6 engines
Tested 2.0 TFSI petrol and 2.0 TDI diesel units for both ranges, checked over before they leave us. A major repair doesn't have to mean main-dealer money.
The newer A6 takes the same core engines and adds a 48V mild-hybrid system on the 40 TDI diesel, which lets it coast with the engine off to save fuel. It steps up to a smooth 45 and 55 TFSI petrol and, importantly, a 50 TFSIe plug-in hybrid with around 34 miles of electric range — a real draw for company-car drivers chasing low tax. The A6 is tuned for hushed, effortless cruising. The lighter A4 is the keener steer, with sharper turn-in and tidier body control, while the heavier A6 (over two tonnes) prioritises calm over agility. If you love the way a car corners, the A4 is the more involving drive.
Whatever engine you land on, when something lets go I keep tested used Audi engines for both cars, so a big bill doesn't have to mean dealer prices. The gearbox is the other expensive one to watch, and faults there are among the costliest you can face on either model.
Used Audi A4 & A6 gearboxes
Manual, S tronic and Tiptronic units pulled from low-mileage cars and tested. We match the right code to your model and engine before it ships.
Running costs and reliability
Diesel is the economy champ in both. Real-world figures from Honest John owners put the A4 2.0 TDI at around 44mpg on average (low-30s to mid-50s depending on use) and the A6 40 TDI a touch higher at roughly 49mpg, helped by that mild-hybrid coasting. Petrols sit lower, generally high-30s to mid-40s. The plug-in A6 TFSIe can post huge official numbers, but only if you genuinely plug it in. Being smaller and lighter, the A4 is usually the cheaper car to buy, tax, insure and run overall.
On reliability, both share Audi's strengths and the same headache. The mechanicals are well-proven, but the recurring gripe is electrical and infotainment trouble: flickering MMI screens, phone-pairing faults and the odd sensor warning. What Car's survey found a notable share of C8 A6s reported a fault, with most of those concerning electrics or infotainment, though the A6 improved sharply in later surveys. On newer A6 mild-hybrids it's worth checking the 48V belt-starter generator has no fault history. The lesson on either car is the same: condition and service history matter more than the badge, and an electrical fix is exactly the sort of part I can supply used for a fraction of dealer prices.
Used A4 & A6 headlights & electrics
Genuine xenon, LED and Matrix headlight units plus the electrical bits that fail, all tested. A used unit saves you a fortune over the main dealer.
A4 vs A6 at a glance
| Audi A4 | Audi A6 | |
|---|---|---|
| Class | Compact executive | Full-size executive |
| Body styles | Saloon, Avant estate | Saloon, Avant estate |
| Length | ~4,762mm | ~4,939mm (~200mm longer) |
| Boot (Saloon) | 460 litres | 530 litres |
| Boot (Avant) | 495 litres | 565–586 litres |
| Engines | 2.0 TFSI, 2.0 TDI, S4 3.0 V6 | 40 TDI mild-hybrid, 45/55 TFSI, 50 TFSIe PHEV |
| Real diesel mpg | ~44mpg avg (2.0 TDI) | ~49mpg avg (40 TDI) |
| Typical used price | ~£4,600–£27,000+ | ~£13,000–£40,000+ |
| Best for | Lower costs, agility, easy daily use | Space, comfort, hybrid option |
So which should you buy?
Go for the A4 if you want a premium Audi that's cheaper to buy and run, easy to live with, and genuinely nice to drive. It's the sweet spot for most people, and it's closely related to the Audi A5 parts family, so spares are plentiful and sensibly priced. If you're cross-shopping the A4's smaller and sportier siblings, my A4 vs A3 guide and the A4 vs A5 comparison dig into those pairings.
Pick the A6 if you need proper rear space, the biggest boot, the calmest motorway manners, or that plug-in hybrid for company-car tax. It's the more expensive car to buy and run, but for high-mileage motorway drivers and bigger families it earns its keep. Decide how you'll really use the car 90% of the time and the answer usually picks itself.
Weighing the sleeker, sportier option instead? My Audi A5 vs A6 comparison breaks that pairing down the same way.
Looking for this part? Tell me your Audi model and reg and I'll get you a quote — quality tested used parts with nationwide UK delivery.
Whichever you land on, keeping a used Audi on the road is far cheaper when you fit quality tested used parts. I stock Audi A4 spares and a full range of A6 components, all checked before they leave us, with nationwide UK delivery — so get in touch whenever you need a part.
Sources
- A4 saloon 460L / Avant 495L and A6 saloon 530L / Avant 565–586L boot figures, plus ~200mm length difference — cinch.co.uk and carwow.co.uk
- A4 length 4,762mm and A6 length 4,939mm — automobiledimension.com and cinch.co.uk
- A6 engine line-up: 40 TDI 204hp with 48V mild hybrid, 45/55 TFSI petrol, 50 TFSIe plug-in hybrid (~34 miles EV) — parkers.co.uk and topgear.com
- Real-world diesel mpg: A4 2.0 TDI ~44mpg average, A6 40 TDI ~49mpg average — honestjohn.co.uk and honestjohn.co.uk
- Reliability: A4 nimbler and A6 more refined; A6 C8 electrical/MMI faults common in What Car survey but later improved — autoexpress.co.uk and whatcar.com
- Used pricing: A4 B9 from roughly £4,600 to £27,000+ — parkers.co.uk and autouncle.co.uk
