Fuel consumption

[Vancevr]

One of the main reasons I run with so little gas is because if I forget to shut the gas petcock off all of the the fuel will drain from the carb and there will be none left when i go to drive again. ergo if i dont have much in there, there won't be much spilt. If I know that I will be useing it a lot then obviously i will but some more in. I also use 91 octane ethanol free.

[David Stepney]

Fuel consumption depends on many factors, not least of which is how much intermediate gear work you have to do. Most of my local driving involves quite steep climbs, so the RP tends to give me about 35 -36 mpg (imperial). On longer and flatter runs, I get between 40-45 mpg.

And yes, the best I can say for my petrol gauge is that it nearly works, so I always make sure that the gauge is registering at least half full. So far I haven't run out! However, with a rear tank, all that evaporates when the car stands for a few days is the contents of the carburetor.

[Charles Levien]

None of my three sevens has a usable fuel gauge - the Special has none, the Opal and the van have wildly innacurate installations.

I work on an average consumption of 40 mpg and carry a spare gallon in each.

Like Rick, my Opal gauge is inaccurate — reads just over 4gals when full and empty when there’s still a gallon of fuel in the tank. This is certainly better than no gauge but I also carry a spare gallon plus a length of bamboo as a dip stick, in extremis.
I’m scratching my head to think of some other use I could put this to ……………… any suggestions?
And I too get a bit over 40 mpg.

Charles

[Parazine]

I've been driving Austin Sevens for 45 years now and can share the following on fuel consumption:

1) The fuel consumption is wildly affected by the temperature of the engine. On a long run, 40mpg+ may be possible but on short runs, this figure drops to the low twenties or worse. By a long run, I mean over 40 miles or more in one, continuous go.

2) My Chummy, with lightly tuned engine and SU carburettor can achieve 50+ mpg on a long run. The Top Hat, with a standard engine and 24T2 updraught struggles to get to 40mpg.

3) Later cars are much worse, my Ruby with standard hi-comp, 3 bearing engine could achieve 35mpg at best (i.e., on a long run, engine hot). A later, 1934 Box Saloon was a little better but not much. A 1931 SWB Box I once owned achieved nearly 50mpg, but this was on a non-stop, Norwich to Wiltshire run with a strong NE wind blowing!

4) All my tests were carried out in the same way, fill tank, drive, refill tank to same point and note quantity, divide into mileage recorded. Yes, I had checked the odometer was reading correctly and yes, I was refilling to the same point!

A lot of people will want to shoot me down, but these are cold facts delivered in a long A7 career.

[AustinWood]

The 22FZ carburetor is a pre 1st World War design. It is not sealed and the contents of the float chamber evaporate. This is replenished by the gravity feed from the tank.
The garage here is integral with the house and if the petrol tap is left on I can smell petrol in the house. This implies that a considerable amount is being lost.
We have got into the habit of always turning off the petrol tap when we stop for any length of time.

[Chris KC]

Did you calibrate the stick yourself? And was the car standing level when you checked it?

[Mike Costigan]

As others have said, it probably takes a good ten miles before the engine is fully warm and giving of its best, so your 17 mile round-trip will certainly not give optimum fuel consumption. I assume you are talking about your 1929 chummy and that it is fitted with the standard updraught Zenith? In which case your filler is reasonably central in the car so your dipstick should give a reasonably accurate measurement; however, if you were on a steeply-cambered road driving on the right, that might well tip the car sufficiently for you to not be able to access the last half-gallon or so, as the fuel off-take would then be on the high side of the tank. Here in the UK driving on the left there is usually sufficient camber to drain the tank pretty thoroughly!

As for likely consumption, I can easily better 50mpg on a long steady-speed run (at say 45mph) whilst I would be disappointed to get worse than 40mpg in a day's motoring in the Peak District with perhaps 50% use of second gear (1931 RM saloon with 3-speed gearbox and standard FZB carburettor). I have never checked consumption on short-run local trips, but without getting the engine thoroughly warm (not just water temperature) I would not be surprised if it was no better than 30mpg. (all in Imperial gallons).

[Dave Mann]

I can't remember when I last shut the petrol tap on garaging our RN saloons and there is no smell of petrol in the garage i suspect that you have a  carburetor leaking needle valve.

[Howard Wright]

As for likely consumption, I can easily better 50mpg on a long steady-speed run (at say 45mph) whilst I would be disappointed to get worse than 40mpg in a day's motoring in the Peak District with perhaps 50% use of second gear (1931 RM saloon with 3-speed gearbox and standard FZB carburettor). I have never checked consumption on short-run local trips, but without getting the engine thoroughly warm (not just water temperature) I would not be surprised if it was no better than 30mpg. (all in Imperial gallons).

Hi Mike

I am pleased that someone else gets high mpg figures.  I record mileage and fuel use quite accurately every time I fill the Special up and was almost worried to find it has achieved over 50mpg in the 3000 miles I’ve completed since it’s build two years ago (2000 in the last year!). That includes a few longer runs towing a 100kg trailer!!

It is very light (350kg) and has a tuned engine (Ricardo head, SU and 4 branch manifold). It also has a 5.25 rear axle and Andes second gear which I think helps.

Unfortunately I so enjoy driving the Special that my RK get too little use for me to comment on fuel consumption.

Cheers

Howard

[Chris KC]

I made a very broad estimation of my Ulster's consumption on a recent touring holiday using rough mileage (Google maps) and total of petrol receipts, assuming an average ~1.80/litre at the time. It came out at 47mpg, for a car with 1 1/4" SU and a fairly full load of camping gear. I covered long distances daily at a steady 35 - 45mph depending on the road conditions. If anything I'd say that was a conservative estimate.