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Old 22nd October 2019, 23:16   #2
Andy_with_a_screwdriver
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Rover 75 Tourer CDTi

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Quote:
Originally Posted by meterman66 View Post
Hi


Can't get my head around if that's better for mpg or not

Thanks
Col

The Mrs is watching re-runs of the British bake off on TV, therefore I was feeling a bit bored so here goes.

It should in theory be better with 18" wheels (circumference approx. 2072 mm) Calculated by the following:
18" x 25.4 mm = 457.2 mm
(255/100)x45 = 101.25 mm
101.25 x 2 = 202.5 mm
457.2 + 202.5 = 659.7 mm the diameter of wheel and tyre.

pi x diameter = circumference
3.141 x 659.7 = 2072.12mm

Repeat for 16" wheel with different values.

Every 1rpm of the driveshaft will take about the car about 53 mm further along the road compared to 1 rpm with the 16" wheel (circumference approx. 2019mm)

To calculate distance travelled in an hours driving at a constant driveshaft rotation of 1000rpm.
1000 x 60 min = 60,000 revs per hour
60,000 rph x 53mm = 3,180,000 mm
3,180,000 divide 1000 = 3,180 metres or 3.18 km
3.18 km divided by 1.6 = 1.99 miles further with 18" wheels.


To calculate velocity in mph:
60,000 driveshaft rph x 2.072m circumference = 124,320 meters per hour travelled.
124,320 divide by 1600 = 77.7 mph

So at a constant 77.7 mph you would in theory travel 1.99 miles further for the same number of engine revolutions, about 2.56% more efficient.

However this is all probably negated if you open the window for 20 minutes!

Make of it what you will.

Last edited by Andy_with_a_screwdriver; 23rd October 2019 at 08:45..
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