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Rear Wheels

This Morgan original had 165-R15 Radial tyres on 72 spoke 5″ wire wheels.
In 2001 I changed to Bridgestone tyres 185×65-R15 and extended the rear wings with some 15mm.
With the turbo power, the grip was not good so further extension was needed.
I got 6″ 72 spoke wire wheels and decided for what. I found the best Tyre for general purpose, Continental Premium Contact 2 in 215×60 R15. It give +2,82% (measured 2.5%) on the speed to RPM. Normally you should stay within +/- 2,5%, but longer gearing was preferred.
In this configuration the rear wings should be further extended with 2×30 mm. As you can see on the pictures this requires a lot of extra work: Welding extensions, filling old holes, a lot of fairing and painting.
Over all the result was beautiful and worth the work.

Wire wheels Circumreference Speed Speed Diff

Tyre Size Calculator:

From this Calculator the 185/65 R15 should be 1952,5 mm but measures is 1895 mm
The 215/60 R15 should be 2007 mm but measured is 1943 mm

Using the measured circumference following speed cal is done:

Tyres mm 5g 3000RPM 5g 6500RPM
185×65 R15 – 5″ 1895 mm 101,5 km/h 219,8 km/h
215×60 R15 – 6″ 1943 mm 104 km/h 225,4 km/h + 2,5%
Car simulations show that with the current drag coefficient of 0,68 ‘as open’ (its like a bus) there should be enough power to reach a top speed of 225 km/h (139,8 mph). It is not tested and of no interests.

Wirel wheelsCRSpeed
5g 3000 RPM
5g 6000 RPM
185×65 R15 – 5″189,5 mm101.5 km/h219.8 km/h0
215×60 R15 -6″194,3 mm104.0 km/h225.4 km/h+2.5%