FIESTA TYRE SIZES

It is just me being pedantic, but I think Iain’s column in the January issue has a typo when he talks about changing from 195/50 x 15 tyres to 175/65 x 15s on his Fiesta. This would increase the rolling radius by over 5%. I suspect he is planning to swap his 6J x 15 wheels for 5.5J x 14in wheels with 175/65 x 14 tyres for a better ride. These have a rolling radius only 1% over the ‘bigger’ wheel/tyre combo currently fitted.

Thanks for another bumper issue which I have only briefly scanned. It already brought back memories, including one of a DAF 33 belonging to an ex-girlfriend’s dad when I was at university (her mum was Dutch). Ian Thow

Iain Ayre replies: The aspect ratio is a ratio, so a 65% ratio on a smaller tyre might have the same rolling radius. In any case, plus 5% would just mean the speedo is accurate rather than overreading as usual, and infinitesimally better fuel efficiency. Ultimately, it depends on what the original tyre size was. I don’t have WiFi yet so I don’t know that, but 195/50 doesn’t sound likely for a cheap shopper. It even has steel wheels with plakky Walmart hubcaps, so the tyres new would have been the cheapest.

When I see something that seems not right, I usually go on-line for answers (often disappearing down a rabbit hole as I explore various dead ends). In this case, since I do have WiFi access it was easy for me to check the normal fitment by plugging Iain’s registration number into Kwikfit, so I am pretty sure 195/50 is standard. Next, on wheel-size.com I looked for data on alternate wheel sizes for his model and verified that 14in was a possible fitment. Although there is a slight (16mm) increase in rolling wall height with 175/65 x 15s, the gain in ride comfort that he expects will be minimal, possibly not even noticeable. For comfort he should drop the wheel size to 14in and, if he wants a 175 tyre, go for 175/70 which will also be up on rolling radius (4%) compared to his current fitment, but will have a significantly taller wall due to the smaller wheel diameter. I used alloywheels.com/tyreCalculator for tyre radius comparisons. I do agree with Iain’s comments, although he does his car a disservice in calling it a 'cheap shopper' – the cheap ‘shoppers’ were the 1.25 petrol (55kW) or 50kW diesels, not the 1.4 petrol (59kW) which he has. Incidentally, the hubcaps it wears in the magazine photo are the actual original Ford fitment for the standard steel wheels. Ian Thow

For those who missed the January issue, Iain introduced his 2005 Fiesta and asked about its potential as a future classic – Ed