Spacesaver - predictable outcome?

[GeoffW]

I asked my local Honda garage for the spec for a spacesaver, but they couldn't tell me. I emailed Honda UK a couple of times - no response. There are plenty of mk3 ones on Ebay, but my worry was that the mk4 weighs about 200kg more than a mk3, and I particularly needed the spec for the tyre.
I gave up and found an original matching mk4 alloy wheel on Ebay (£50, v good condition), and when I had a pair of new tyres fitted to the car, I had one of the still legal old ones put on the used wheel. I put it in the car when going on a long, or important journey. 

[Lincolnshire Rambler]

Thank you . I’m going to just buy a full sized second hand wheel and tyre then!

[Lord Voltermore]

One further thought on domed wheel nuts.   The offiical Honda approved spacesaver may differ from some others  only in that it uses the original domed nuts.     There may be a much more commonly available , and maybe significantly cheaper model   which is identical in every important respect, other than needing tapered wheel nuts. Which dont cost much.       
You do see quite a few spacesavers  offered for sale that  have probably been sitting in a car boot for 10 years or more.  The tyre may have deteriorated. .     You can buy new tyres for spacesaver wheels but  IIRC they tend to be quite expensive. 

[5thcivic]

Will a spacesaver interfere with the tyre deflation calculations, or because just the right diameter not make a difference in revolution terms?

[Kremmen]

More than likely

[Lord Voltermore]

Definately. Spacesavers can be a  significantly different rolling circumference.     Its not a good idea to put one on the front.  Better to use it to replace a good rear wheel then use that to replace the front  .( Unless you know for sure you are only going a short distance  at very slow speed.) Double the work but safer.    Someone near me had her car on a spacesaver for nearly a year 

[coldstart]

Will a spacesaver interfere with the tyre deflation calculations, or because just the right diameter not make a difference in revolution terms?

Yes, it will - just recalibrate the deflation warning system (like you would do after any other change of wheel or tyre) and you are safe again.
Please keep in mind that spacesaver wheels are just an emergency stand-in and are not meant for prolonged journeys (hence better to have a full spare wheel on board if you want to carry a spare).