That’s my understanding.

As in, the distance in mm (+ or -) from the centreline of the wheel, irrespective of actual wheel width, correct? I regularly struggle with this concept and corroboration would be much appreciated  Paul

I got it wrong, what I thought were ET figures on the wheel specs are actually listed as offset sizes, I thought they were the same thing.

Wise decision.  But of course 

Note that unlike offset and set-back, the ET number is influenced by the wheel width. I always need to use an online calculator to figure it out. FWIW, I do know that ET13 on a 6” rim is pretty much perfect on a Vitesse but a not quite enough on the front of a square tail GT6 or Spitfire.

Thanks all Revolutions do look good on a gt6. Very Nice looking car. Can't see 4 spoke revo's listed at Rimmer Brothers. Think the Revolite's are a good price. Their minilite's come in three different offsets ET20 for the 5", ET25 for the 5.5 and ET16 for the 6". The 6" Revolites are ET16. Think i have the 5" on the Vitesse, so ET20 no markings visable on the outside so need to remove a wheel and have s look. Finally had the hernia op a couple of weeks ago so no jacking up cars at the moment. 

To fair Nick, Revolution like the genuine Minilite used to have what they called road and competition versions of their wheels. The competion wheels supposedly being lighter. As these were apparently still aluminium alloy and not magnesium one can only assume this was achieved by thining down the rim section. Might be this giving differences in fit.

They're very nice. Suit the car. Agree about the sleeve nuts. Have them for the 15" X 6" on the Scimitar. They are bloody heavy and getting the nut to start on the stud is a real pain. Revolution still do 13x5.5 for the small chassis cars. But significantly more expensive than the JBW version.

Modern reproduction 13 x 5.5" Revolutions from Rimmers or Canleys in 2011. Fitted with tapered seat nuts on M12 studs. I had sleeve nuts on my previous set of Cobra Superslots and found them a bit of a PITA. Ian

Ok, agreed. I think there may be some intermediate Revo styles @ ET20, which are neither the true originals (which did fit without spacers) or the current Revolites which should (at ET16) fit without spacers.