Wheel tyre sizes

[meterman66]

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:shrug:

Thanks
Col

[Andy_with_a_screwdriver]

Hi


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

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. :cool:

[another_clean_sheet]

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

[T-Cut]

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

[another_clean_sheet]

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

[T-Cut]

W=MV is not an equation I'm familiar with. Do you have a web reference for it?

TC

[dattrike]

Work = Mass x Velocity ?

[T-Cut]

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.



TC

[another_clean_sheet]

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.

[another_clean_sheet]

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.

[RoverP480]

Without knowing the specific fuel consumption curve for the engine , one cannot really say what the effect will be by altering the engine revs at the same road speed . If say at 70mph the engine is running at its optimum speed for fuel consumption, altering it either way will make it worse. If though it moves the engine speed to a more favourable part of the curve it will be better, but if it moves to a less favourable part it will be worse. One is also moving it about on the torque and power curve as well so you end up with multiple effects . I suggest moving it by 2.5 % is unlikely to show a noticeable effect. One is more likely to notice the ride changing.

[meterman66]

Hi

I'm only thinking of changing as I've just bought my latest conny with 16" forks that really don't do it for me in the looks dept and tyres are all pretty close to the limit and require replacement.

As there are a few sets of 18s on tinternet with newish tyres at around the £100-£150 price range so was going to kill 2 birds with 1 stone so to spk.

Many thanks for all your input

Col

[T-Cut]

To be sure your chosen wheel/tyre combination complies with the spedometer regulations and for your insurance to remain valid, I'd suggest you stay within the sizes recommended in the Handbooks.


https://www.mediafire.com/convkey/e156/cr187e5ul1k5fm86g.jpg


2004 Handbook (introduced 18" rims)
https://www.mediafire.com/convkey/52c7/8i2wwhnbw7cfslb6g.jpg

[MarkS]

Honest John recommends that tyres should be a minimum of '55 profile to avoid rim damage due to potholes in the UK. Seems sensible advice. A blowout at high speed due to pothole damage does not bear contemplating, simply for a 'nicer' look. You only have one life.

But it is your choice.

[peelaaa]

The 18 inch do fill the wheel arches better but car will be higher off ground unless the springs are shorter.
My car had 18 inch when I bought it but went back to the 16 inch . Car sits at correct height now and ride much better.
I am sure the mpg was slightly worse with the 18 inch but then it may have been the tyres.