Tyre pressures

The figure

Twenty six pounds per square inch at the front, thirty at the rear, measured cold. In bar that is 1.79 front and 2.07 rear.

Three sources give it. Rimmer Bros publish it in their Stag vehicle information, stating that it applies to standard wheels and tyres at all loadings. A specification aggregator gives the same numbers. And Longstone Classic Tyres publish period tyre manufacturer fitment data showing Michelin quoting 26 and 30 for the 185 section tyre in 1974, and again in 1977 and 1982 for both the 185 and the 175.

One honest caveat before you act on it. The archive has not read that figure in the factory handbook or the workshop manual. Every source above is a trade compilation or a tyre maker's fitment guide, not a facsimile of the document Triumph issued with the car. The figure is well supported and it is very probably right. It is not primary.

Where the archive would go to settle it, and why you can too

The Stag Owners Club publishes scans of the actual factory documents on its own website, under technical manuals. The Driver's Handbook exists there in several editions from the first of April 1970 through to the eighth of 1975, along with the USA handbook, two supplements, the Repair Operation Manual and the workshop manual.

Anyone who opens the tyre pressure page in the first edition and again in the eighth would settle two things at once: the figure itself, from the source that matters, and whether it changed across the three specifications. The supplements are the likely home of any hardtop or sustained high speed note, if one exists. If you do that, the archive would like to hear from you and will credit you.

Where the sources disagree, and they genuinely do

The most publishable disagreement is between two tyre manufacturers.

Both are period manufacturer data, both are reproduced by the same tyre specialist, and they do not merely differ in magnitude. Michelin specifies a four pound rear bias and Pirelli specifies none at all. The archive reports both and does not average them. If you are running Pirellis on a car of that vintage there is an argument for the Pirelli figure, but nobody has published that argument and the archive is not going to invent it.

Why the rear is higher, which is the opposite of what people expect

The Stag has a reputation as a nose heavy car, on the reasonable ground that it has an iron block V8 in the front. The factory pressures say otherwise, and so does the one weight distribution figure the archive could find, which puts it at 47.4 per cent front and 52.6 per cent rear at kerb weight.

If that is right, the Stag is mildly rear biased rather than nose heavy. The engine sits well back, and the fuel tank and boot are behind the rear axle. Which would explain a rear biased pressure specification that otherwise looks like a mistake.

That distribution figure comes from a single specification aggregator of medium reliability, so the archive offers it as the explanation that fits rather than as established fact. A period road test giving front and rear axle weights would settle it, and would be a genuine correction to received Stag wisdom. The archive's suspension and steering guide covers what else decides how the car sits and steers.

Why the generic figure is wrong

Search for Stag tyre pressures and you will find a page stating that street cars are usually around 34 psi, rising to 38 loaded and 40 for sports cars on low profile tyres. It offers no Stag figure at all, and it then tells you to consult the handbook, which undercuts its own number.

It is wrong on magnitude, being some 31 per cent over the factory front figure. But the more serious error is the shape. Every Stag specific source gives a front and rear split. A single universal number cannot express one, and applying it destroys the front to rear relationship the car was set up around. That objection survives even if you think the right front figure is 28 rather than 26.

The reason a general figure cannot work is that pressure is an output of a load calculation rather than a rule of thumb. TyreSafe state that recommended pressures under different operating conditions come from the vehicle handbook or the placard on the car. Toyo's technical guidance on load and inflation tables is more explicit: a replacement tyre must have equal or greater load carrying capacity by inflation compared with the original, and when you change size you find the pressure at which the corresponding load matches.

Run the Stag's numbers and it becomes obvious. Kerb weight is somewhere between about 1,250 and 1,300 kilogrammes depending on which source you take. At the distribution above, each front tyre carries roughly 300 kilogrammes. The original equivalent 185 HR 14 is load index 90, which is 600 kilogrammes at its maximum rated pressure. Each front tyre is therefore carrying about half its rated capacity, which is precisely why a pressure well below the tyre's maximum is correct. The 34 psi figure is borrowed from heavier modern cars on lower profile tyres carrying a far higher fraction of theirs.

The original wheels and tyres

Five and a half J by 14 inch wheels. Steel with Rostyle look trims on non-US cars to 1975, 72 spoke wire wheels on US cars in 1971 and 1972, and the five spoke alloy standard on US cars from 1973 and on all markets from 1976, having been a popular option before that.

Radial from new, 14 inch, H rated, which is 130 miles per hour and comfortably beyond the car. The original is effectively an 80 series tyre with a tall sidewall: the current Michelin MXV-P equivalent is 188 millimetres in section and 650 millimetres in overall diameter.

There is a real disagreement about which size came first. Longstone give 185 R 14 originally with a change to 175 HR 14 around May 1976, and say Triumph made that change because they found the car handled better on the narrower tyre. Vintage Tyres and Blockley also give 185 HR 14 as original with the 175 as a later standard. Against that, Classics World give the original size as 175 and one wheel database lists the 175 as the original equipment size. The archive follows the dated account because it is the most specific and is internally consistent, and records the dissent. The wheels and tyres guide covers sizes, wheel types and tyre age in more detail.

Modern tyres, and one warning

For pre 1976 cars the usual recommendation is the 185 HR 14 Michelin MXV-P, since the original XAS is not currently made in that size. For later cars, the 175 HR 14 XAS. Others available include Vredestein Sprint Classic, Nankang, Blockley and Michelin XWX, in 185, 195 and 205 sections.

One warning worth repeating. Because the correct sizes can be awkward to source, some owners fit van tyres. Do not. Their stiff sidewalls offer inadequate grip on a car of this weight and character.

No tyre specialist, club or manual publishes a pressure for the fifteen inch wheels many Stags now run. One aftermarket wheel database offers a figure, but it gives a single number for both axles, which contradicts every Stag specific source, so the archive will not repeat it. If you have gone to fifteens you are on your own, and the sensible approach is to work from the tyre's load rating rather than from a Stag figure that does not exist.

Checking them properly

Cold means cold. TyreSafe define it as before the car has been driven two miles, and Toyo as at prevailing atmospheric temperature with no build up from running, which in practice means parked three hours or driven under a mile. A tyre checked warm reads high and will leave you under inflated.

On load, note a tension worth being aware of. TyreSafe advise raising pressures when carrying a full complement of passengers or luggage. Rimmer Bros state the Stag figure covers all loadings. The archive has found no factory full load or hardtop figure, and the hardtop adds around a hundred kilogrammes, high up and rearward, to a car already carrying thirty at the back. Nobody publishes a figure for that.

And check the age of the tyres, not just the pressure. The Department for Transport's own basis for banning tyres over ten years old on heavy vehicles was that research shows tyres degrade whether they are used or not, which is exactly the classic car failure: a tyre that is dimensionally perfect and chronologically dangerous. Cars are not covered by that ban. The Federation of British Historic Vehicle Clubs opposed a blanket age limit but states plainly that it does not advocate the use of tyres over ten years old, and that nothing substitutes for careful examination throughout a tyre's life. Tyre age and condition is also one of the things the buying guide asks you to look at on a car you are considering.

What the archive could not establish

The figure read from the factory handbook itself, as above. Any separate full load, hardtop, towing or sustained high speed figure. Any published pressure for fifteen inch wheels. Whether the factory figure remains appropriate for a modern radial of the same size: the argument tyre specialists make for running classics above period pressures is explicitly about pre war cars on crossplies, and nobody extends it to a 1970s radial. The speed rating of the earliest cars, since the 1972 Pirelli data carries no speed letter while everything later says H. Whether the spare should be run higher, on which no authoritative source was found. And the common rule of thumb about pressure changing with ambient temperature, which appears only on retailer blogs and could not be sourced to any tyre maker or safety body, so the principle is given here and the coefficient is not.

One thing this archive is currently the only source for

This site's own frequently asked questions page states that many owners prefer 28 psi at the front. That is very likely true and it is repeated back by Google's own generated answer, which attributes it to a Triumph forum. But when the archive went looking for a citable source, every forum where it would be discussed was unreachable, and the only place the claim could be found was this site.

So it is recorded here as owner practice that the archive has not yet been able to source, rather than as a figure to act on. If you run 28 at the front and can point to where that advice comes from, the archive would like to know, and will cite it.