Triumph Stag fuel consumption

What owners actually report

These are individual owner reports, not a survey, and they are deliberately not averaged into one number. The conditions attached to each figure are the point: a Stag returning 31 mpg and a Stag returning 20 mpg are usually doing very different things.

Alongside the forum reports, Fuelly aggregates real-world fuel logs kept by Stag owners, currently a small pool of somewhere between 8 and 13 cars covering roughly 49,000 to 51,000 miles between them. It is worth consulting as an aggregator of logged fill-ups rather than as an authority, because the sample is small enough that one heavy-footed contributor moves the headline number. Read the individual cars rather than the average.

Mind the units

A great deal of forum disagreement about Stag economy is really disagreement about arithmetic. British owners quote imperial gallons, American owners quote US gallons, and continental owners quote litres per 100 km. The imperial gallon is the larger of the two gallons, so the same car scores a noticeably higher number in imperial mpg than in US mpg without having changed at all. Every mpg figure on this page is imperial unless it says otherwise.

Litres per 100 km runs the other way round, in that a lower number is better. Converting the country-road report above: one imperial gallon is 4.54609 litres, so 13 litres is 13 ÷ 4.54609 = 2.86 imperial gallons. 100 km is 62.14 miles. That gives 62.14 ÷ 2.86 = roughly 21.7 mpg imperial. The same 13 l/100 km expressed in US mpg would come out at about 18, which is why two owners can describe identical consumption and appear to disagree by three or four miles per gallon.

Work out a trip cost

The owner reports above give a sense of what a Stag returns, but a journey cost depends on the distance, your own mpg and the price at the pump. Use the calculator below to turn those three numbers into a rough cost.

Why the figures vary so widely

A spread from about 20 to about 31 mpg looks like noise. It is not. Three variables account for most of it, and once they are known the reports line up rather well.

Gearbox

For fuel purposes the manual with overdrive and the three-speed Borg-Warner automatic are different cars. Overdrive drops engine speed at a cruise, which is exactly where economy is won, while the three-speed automatic has no such top and holds more revs at the same road speed. Every high figure gathered above comes from a manual overdrive car. That is a strong pattern rather than a measured comparison, and no controlled back-to-back test has been published that this archive can point to. The differences between the boxes are set out on the transmission guide and in manual versus automatic.

Carburettor setup

Few Stags are still as the factory left them. Original twin Zenith-Strombergs, SU conversions, Weber conversions and four-barrel Holley installations all meter fuel differently, and the reports reflect it: the Weber car returned 30 mpg on a gentle continental cruise, while the Holley car tuned for racing by a previous owner returned 22 mpg. A carburettor jetted for power is a carburettor jetted to be rich, and no amount of careful driving recovers that. How the standard instruments work, and the argument for rebuilding them rather than converting, is covered in carburettors and fuel.

Speed

The owner who asks whether the 30-plus reports involve ever exceeding 50 mph makes a fair point, and it deserves stating plainly. Aerodynamic drag rises steeply with speed, and a 1970 convertible with a hood up is not a slippery shape. A steady 50 to 60 mph on a quiet road and a sustained 80 on a motorway are different propositions, even though both get described as motorway driving. Reports at the top of the range and reports at the bottom often describe the same car driven differently.

When heavy consumption is a fault

This is the part no specification page covers. A Stag that has dropped several miles per gallon from its own normal figure is usually telling you something, and the fault is rarely exotic. Work through the list in this order, cheapest first, and judge everything against what the car used to return rather than against somebody else’s number.

• A rich mixture. Sooty black deposits on the plugs, a strong exhaust smell and black smoke on acceleration all point at over-fuelling. Mixture is set warm and after the ignition, not before it. See setting up the twin Strombergs.
• Chokes not fully returning. A cold-start enrichment mechanism that stays partly on once the engine is warm is one of the most common causes of a sudden drop in economy, and it often shows up as a high or hunting hot idle as well. Check the cable, the linkage and the return spring at the carburettor rather than adjusting the mixture to compensate.
• Worn needles and jets. The needle and jet wear against each other over a long life, and a worn pair runs progressively richer. A split air-valve diaphragm has a similar effect on running. Both are inspection and renewal items rather than adjustments.
• Ignition timing. Retarded timing, a seized centrifugal advance or a disconnected or perished vacuum pipe all cost fuel, because the engine has to be driven harder for the same work. Tired plugs, leads or points do the same. See ignition timing explained and plugs, coil and leads.
• Brakes binding. A sticking caliper piston, a seized rear cylinder or a handbrake mechanism that is not releasing means the engine is fighting the car all day. The classic evidence is a wheel that is hot after a drive when its opposite number is cool, or a car that will not roll freely when pushed. See brakes.
• An engine that never warms up. A thermostat stuck open, or removed by a previous owner in a misguided attempt to cure overheating, leaves the engine running below its proper temperature permanently. The mixture stays richer than it should, the oil stays diluted, and economy suffers along with wear. A gauge that never climbs to its usual place is the clue. See overheating and cooling.
• The dull mechanical items. A clogged air filter, dragging or under-inflated tyres and a fuel leak that shows only as a smell after a run are all worth eliminating before anything is dismantled. Servicing basics live under the air filter.

Measure properly before chasing any of it. Brim the tank, note the odometer, drive normally for a few hundred miles of mixed use, brim it again, and work out the figure from that. One tankful measured against a fuel gauge proves nothing on a car of this age.

What this page does not give you

No factory or period road test consumption figure appears here, because none has been verified to the standard this archive uses. Several sites quote one; none of them cites where it came from. If you have a period road test or a factory publication that gives a figure, the contact page is open, and it will be added with the source named.

There is no tank capacity or range here either, for the same reason, and no cost per mile, which would be out of date within months of publication. The honest summary is simple enough: this is a 3-litre V8 designed in the 1960s and built from 1970, and it was never an economical car. It is also not the drinker its reputation suggests. Low twenties in town, mid twenties in ordinary use and around 30 from a manual overdrive car driven gently is the picture the owner reports paint, and a Stag well outside that on the wrong side is usually a Stag with something to fix.

Frequently asked

What mpg does a Triumph Stag get?

Owner reports run from about 20 mpg to about 31 mpg (imperial), and the spread is explained by gearbox, carburettor setup and speed rather than by luck. PistonHeads contributors put town driving at around 20 mpg. Stag Owners Club forum members report 31 mpg over 17,020 miles at around 90 percent motorway use, 30 mpg on a run in a manual overdrive car running a Weber at 100 to 110 km/h, 25 mpg from an owner of two Stags driving gently, 22 mpg from a car with a four-barrel Holley previously tuned for racing, and about 13 litres per 100 km, roughly 21.7 mpg imperial, on country roads with an original engine and SU HIF6 carburettors. Fuelly aggregates real-world fuel logs from a small number of Stags. No single average is worth quoting.

Is a Triumph Stag expensive to run?

On fuel, it is a 3-litre V8 from 1970 and it uses fuel accordingly. Somewhere in the low to high twenties mpg is the normal range, with the low twenties in town and around 30 achievable from a manual overdrive car cruising gently. This archive does not publish a cost per mile, because fuel prices change faster than the page could be maintained, and because running costs on a Stag are dominated by maintenance rather than fuel. Nothing here is tax, insurance or investment advice.

Does a manual Stag use less fuel than an automatic?

The reported figures point that way, though nobody has tested the two back to back under controlled conditions. Every high figure in the owner reports gathered here comes from a manual car with overdrive, which drops engine speed appreciably at a cruise. The alternative is the three-speed Borg-Warner automatic, which has no overdrive top and holds higher revs at the same road speed. Treat the difference as real in direction and unverified in size.

Why is my Stag using more fuel than it should?

Heavy consumption on a Stag is usually a fault rather than a characteristic. The common causes are a rich mixture or worn needles in the Zenith-Strombergs, a choke or cold-start enrichment that is not returning fully, ignition timing that is retarded or an advance mechanism that has seized, brakes binding so the engine is fighting the car, and an engine that never reaches proper operating temperature because the thermostat is stuck open, which keeps the mixture rich for longer than it should be. Black smoke, a strong exhaust smell, sooty plugs or a warm wheel after a drive each point at one of them.