General MPG 16s vs 18s

[Fergie]

Hi all,

I just brim to brimmed the tank for the first time after changing to 225/40/18s instead of the 205/50/16s.

My car is a 'tuned' 1.9 diesel 150 which on the 16s (B rated) would normally return high 40s mpg from a tank of general driving (esso fuel).

On the 18s (E rear and B front I think) with added fuel booster it returned low 40s - difference of about 10% in economy (esso fuel).

I was just wondering what sort of economy others are getting these days on different wheels? Would be great if anyone else has comparisons between different tyre sizes?

 

[F123C]

I think in general fuel consumption increases ( less mpg) as you go up in width of tread due to increased rolling resistance. There can also be errors introduced into the odometer/trip meter readings due to changes in turns per mile between the different tyre sizes. I noticed you said the car was fitted with 205/50x16, are you sure they weren't 205/55x16.

Al.

 

[Fergie]

It might have been - it was certainly the correct Bravo size 16s. If I remember rightly, the rolling diameter of the 18s is slightly larger. One other interesting thing is my speedo still slightly over reads, but not as much as it did on the 16s (comparing to gps speed)

 

[Fergie]

Just to clarify the tyres are 205/55/16 and 222/40/18.

The difference in rolling radius is about 0.8%. So basically about 0.8 mile per 100.

 

[RobBravo]

Weigh them, I bet the weight difference is significant.

Everytime you accelerate, you are (for example) moving 4 wheels at 10KG each

If the wheels are then larger and heavier, and come in at 15kg, you are effectively having to move a further 4 wheels at 5KG, or 20kg.

20KG is not a lot at all, but rotational mass is much harder to move than static mass.

For example, moving a car with a 1 tonne concrete block is much easier than moving 1 tonne of water, because the water isn't static and therefore contains more energy when accelerating or braking. I hope that makes sense

It's complicated, but as a reference I lost just over 2mpg when I went from super light 16 steelies to 18" alloys. I could happily get 42mpg from my 1.4 T-Jet but now I struggle to get 40mpg.

That's the reason why touring/ racing cars use ultra light magnesium alloys, and that's down to rotational mass (acceleration, braking, handling and so on)

 

[Whaley247]

I had 19s on a 1.4 T150 was only getting low 30 but to be fair it was when I first got the car so was a little heavy footed