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Old 22nd October 2019, 18:23   #1
meterman66
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Default Wheel tyre sizes

Hi
I'm thinking of changing to larger wheels could somebody advise me if they will be better or detrimental to mpg.

Original size is: 215 55 16

Replacement is: 225 45 18

Odometer readins are as stated:
16" wheels at 50 mph
18" wheels read 51.27 mph

16" wheels at 100 mph
18" wheels read 102.55 mph

Overall diameter is 16.8 mm larger on the 18" which equates to +2.55%

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

Thanks
Col
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Old 23rd October 2019, 00:16   #2
Andy_with_a_screwdriver
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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 09:45..
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Old 23rd October 2019, 10:29   #3
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However it is similar to driving in a slightly higher gear and the engine will be working a little bit harder. I do not reckon you will notice any difference in mpg.
What you will notice is that the ride is harder.
In my opinion I would stay on the 16" wheels
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Old 23rd October 2019, 10:42   #4
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The same amount of work is done in moving around, whether the car runs on 15" wheels or 18" wheels. The amount of work done controls the fuel consumption.



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Old 23rd October 2019, 11:00   #5
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The same amount of work is done in moving around, whether the car runs on 15" wheels or 18" wheels. The amount of work done controls the fuel consumption.



TC
Sorry to disagree though the normal formula is W = MV.
If you do an experiment at a constant speed. Once in the correct gear and once in a gear that is miles too high for the speed you are doing then you will find the former is giving the best consumption.
Or another way to look at it is if you are trying to accelerate at low speed in too high a gear how you have to floor the accelerator to get the car moving
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Old 23rd October 2019, 11:40   #6
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W=MV is not an equation I'm familiar with. Do you have a web reference for it?

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Old 23rd October 2019, 11:42   #7
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Work = Mass x Velocity ?
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Old 23rd October 2019, 11:55   #8
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Quote:
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Work = Mass x Velocity ?

But it doesn't.


Anyway, whatever version of physics you wish to apply, altering the wheel rim size has no effect on mpg.



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Old 23rd October 2019, 12:14   #9
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Originally Posted by dattrike View Post
Work = Mass x Velocity ?

Agreed. Seem to recall this from "A" physics.
However this is a simplistic formula used when you are pushing/pulling an inanimate object. Would not apply without modification to an engine with a non linear power curve operating through a gearbox with rations.
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Old 23rd October 2019, 12:16   #10
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Quote:
Originally Posted by T-Cut View Post


Anyway, whatever version of physics you wish to apply, altering the wheel rim size has no effect on mpg.



TC

Agreed which is what I said in my first post.
In your first post you said it was 2.56% more efficient which I thought you meant that there would be a fuel saving.
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