Tyre Sizing

Competition engines and ancillaries - general discussion
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Posts: 590
Joined: November 26th, 2012, 3:50 pm
Location: Dumfries, Scotland

Tyre Sizing

Post by Will01 » February 20th, 2015, 1:17 pm

Hi Guys
I am looking to get some tyres to fit my Ford Capri where i have 15" Alloy Wheels.
The Front Rim width is 8.5" from face to face & the Rear Rim width is 9.5" from face to face(outer faces)
I do not want any stretched looking tyres, but i am also looking for something that is fairly close to the original diameter of the car, if not a bit smaller.
I am not overly concerned about speedo error as the car is being built as a Track/Race car.
So far i am looking at 215 or 225 for Front & 235 or 245 for Rear.
I have had a look online but cannot find a decent comparision calculator, does anyone know of a good online calculator that they can recommend?
Obviously once i have found a size i need to buy tyres as i am looking at refurbishing the wheels currently in order to get the car rolling on its wheels this summer. I need to do this as i need to simulate whether or not i have enough clearance in the standard arches to fit these wheels. They are alot wider than most would run but they should work well with the other modifications on the car. I have a set of cut down leaf springs to help me simulate full bump and where approx the wheel will be. This will also help me decide if i need to look into designing a Watts linkage straight away.
It may be that i have to look into either narrowing the rear axle, flaring the arches or buying new outer rims (3 piece wheels)to fit correctly.
All help much appreciated.
FYI Standard wheel size on a Capri is 165/65/13 (steels) or 185/70/13 (4spoke alloys) on 4 cylinder or 205/60/13 on 2.8 or 195/50/15 for Capri 280.

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Joined: July 28th, 2010, 3:09 pm
Location: Leiden, The Netherlands

Re: Tyre Sizing

Post by TomLouwrier » February 22nd, 2015, 6:49 pm

hi Will,

There's quite a few decent calculators on line, I can send you some links by PM.
For my own purposes I just calculate the effective roll circumference with:

[circumference in m] = (0,93*(2*[tread width in mm]*[height series in %]/100)+[rim diameter in inches]*25,4)*PI/1000

Checked it dozens of times against OEM data and other online sources like those calculators, works within a couple of percent.
So a
165/65/13 (65? not 85 or 80%?) would be 1,66m
165/80/13 gives 1.81m
165/85/13 gives 1.86m
185/70/13 gives 1.79m
205/60/13 gives 1.76m
195/50/15 gives 1.77m
215/50/15 gives 1.82m
225/50/15 gives 1.85m
235/50/15 gives 1.88m
235/45/15 gives 1.82m
245/50/15 gives 1.91m
245/45/15 gives 1.84m

All theory so far.
The trick of course is in finding a size that is actually available in the market.


Posts: 590
Joined: November 26th, 2012, 3:50 pm
Location: Dumfries, Scotland

Re: Tyre Sizing

Post by Will01 » February 22nd, 2015, 7:03 pm

Much appreciated for the help, a pm with some links would be great thanks Tom.
Finding an available size in the market that is not ridiculous and has plenty of options i do think will be the test.
As for sizing the 215/50/15 & 235/45/15 look like a balanced size, although it doesn't really matter to me if the rears are slightly bigger in diameter than the fronts. The sizes are also close enough for the Speedo although again this really doesn't bother me just as long as it works to pass an MOT.
I have a GPS speedo which i will be fitting to the car if required

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