Text copy below from the location picture

The Italian Job tunnel car chase scene was filmed in Coventry September 1968 using the newly constructed sewer system but where the Minis are seen entering and exiting was filmed in Turin

Did they achieve a complete loop?

It couldn’t be done in the Coventry sewer because of inspection points in the section they used for filming and the tunnel inside diameter at only ten foot but these were not the only reason they couldn't complete a loop other problems was the lack of lighting and in places the flat roof sections as well as the three maintenance / inspection holes and low level water affecting grip.

The most likely section to try was from the starting point where they lowered the vehicles down into the sewer and finihing just passed the second manhole where the tunnel starts to curve light leaving the rest of the tunnel to slow down.

They also only had around 300 feet of straight tunnel to use with other obstacles to avoid, there is also a twelve inch channel protruded down into the tunnel to allow a small brook above to flow across close to manhole 3.

It is reported they did try a loop after filming, Rémy and his stunt team wanted to try it but poor lighting made it very difficult as they had to be directly beneath manhole 1 on the correct trajectory to avoid hitting manhole 2 further down the tunnel.

After a couple of failed attempts they abandoned trying but they did go fairly well prepared with each Mini having a full role cage and extra front lighting but this still proved inadequate so they gave up which is a shame as there are no press pictures covering this which is proof they had a fairly well controlled zone at sewer pipe level.

David Salomone has said four vehicles were in the tunnel, the three Minis the Police car and a mini or mini moke driven by himself used for filming.

Hope this makes sense and will update when I can.

P. S on the driving experience since 2004 we would stop at the exact location of filming in Stoke Aldermoor parking on 'The Linfield' which overlooks the recreational Sowe Valley and walk above the tunnel on the same path the Mini took from the start where they lowered the Minis to the Copsewood end just five feet below the ground pointing out the inspection covers as they curved around the River Sowe a few meters away. We would also be soon surrounded by the locals asking if we were there for filming more sewer scenes.
I was born only a couple of miles away in Dane Road, Barras Heath, Coventry about a couple of miles away from where my Father crew up in Stoke when his father moved away from Swansea for work (that's not Homes under the Hammer rebuilding Stoke on TV).
My mother also crew up in Dane Road Coventry with her parents who owned the house my Mother and Father were renting along with her sister and her husband Mr & Mrs Alan Jury.... just a few doors away. Soon after I was born my father had to move away for work returning a few years later leaving Essex for nearby Balsall Common when I was Six to be closer to family and friends.

I went to the Butts college in Coventry day release when I was serving a five year Sign Writing apprenticeship with K. L Display Studios in Kenilworth with master sign writer Artist from Coventry Noble Key who painted all the Queens Coronation Paintings for the Swanswell display in Coventry 1953... picture quality of Nobby at the top of this page is not the best due to storage damage...