Wheel change makes massive difference in fuel consumption

[garycat]

I finally made the wheel change that I discussed here

Do these wheels/tyres fit?? Read this first! Offset/fitment answers within..

but on a long journey I make frequently, my consumption has gone from ~66 MPG to ~56 MPG. Is this to be expected?

I'd hate to go back to the old wheels as I really like the look of the new ones, but 10 MPG is ridiculous

 

[190]

You were always going to have worse fuel economy with wider tyres due to increased rolling resistance, more aero dynamic drag and presumably new vs worn tyres but altogether that shouldn't have been more than a couple of %. On top of that there might be different rolling resistance ratings of the old vs new tyres although again this is minor. I don't see how changing tyres could account for a 10% change short of being grossly out of alignment.

 

[John]

Or the tyre pressure is very low on the replacement wheel tyres...

 

[Wolfie1]

Have you done an online rolling circumference checker on your wheel/tyre sizes as if there is a massive difference it will change your mpg significantly

 

[KillerHERTZ]

I doubt it will have changed much if at all, just your car is having trouble calculating it.

Check how many miles your getting from a full tank now compared to before, I doubt its changed much (if at all!)

 

[garycat]

Thanks for the replies, I'll give the pressures a check. Regarding the diameter, according to Tyre Size Calculator the fronts are 0.4% bigger and the rears 0.8% smaller so I doubt the calculations make a vast difference.

I haven't had a chance to use a full tank since the change.

 

[Bobby Dazzler]

I don't suppose you weighed the new and old wheels and tyres did you? Wheel/tyre weight can make a noticeable difference to acceleration and fuel consumption, but if they're genuine Mercedes wheels then I'd be surprised if there was much in it.

 

[garycat]

I don't suppose you weighed the new and old wheels and tyres did you? Wheel/tyre weight can make a noticeable difference to acceleration and fuel consumption, but if they're genuine Mercedes wheels then I'd be surprised if there was much in it.

I did actually, old wheel & tyre - 14.7KG
new wheel & tyre - 20.8 KG (rear)

 

[Petrol Pete]

5 KG per wheel ! that's an extra 10kg (I'm good at mafs me) of un sprung weight on each axle, not sure about MPG but that must have an effect on handling. Un sprung weight is one of the first things race car (bike) designers try to get to a minimum.

Do you notice any effect ?

 

[190]

I still don't see that a real 10% change is possible. Even if all the minor effects were cumulative I doubt it would add up to as much as 5 %. 10% reduction in pressure for example only drops MPG by 0.8%

 

[Bobby Dazzler]

I did actually, old wheel & tyre - 14.7KG
new wheel & tyre - 20.8 KG (rear)

That may well be the reason for at least part of the increase in fuel consumption

 

[GeeJayW]

10mpg off 66mpg (15%) reduction in fuel economy is a lot. The extra weight, width, side wall stiffness/rolling resistance will all contribute to real changes in fuel economy. The change in rolling radius will also affect the calculated fuel economy. Very easy to lose 10%, but 15% does seem high.

 

[LTD]

Driving style will also affect things

Are you testing out the extra grip ?

 

[190]

If the extra grip is used MPG should improve due to carrying more speed through the bends

 

[garycat]

As a follow up to this post, I changed my wheels back to the original 16" and immediately saw my MPG back up at 66-68 on my regular long motorway trips. The best I saw with the AMG 17" wheels was 59. Its just a shame that the AMGs look so much better but I can't ignore that difference in consumption, and I always get a smile seeing over 1000 miles on my range display.

 

[190]

It's a surprise that slightly wider tyres made so much difference. It'd not just an in increase in rolling resistance that results from a wider tyre but also an increase in aerodynamic drag. Even the wheel design itself impacts on drag. Your original 16" wheels look to me like the ones MB claimed to be low drag on the Blue efficiency models. I do know that on my BE, decreasing tyre width by only 10mm and adopting that same wheel design provided sufficient improvement to drop the car into the next lower road tax band.

 

[garycat]

It's a surprise that slightly wider tyres made so much difference. It'd not just an in increase in rolling resistance that results from a wider tyre but also an increase in aerodynamic drag. Even the wheel design itself impacts on drag. Your original 16" wheels look to me like the ones MB claimed to be low drag on the Blue efficiency models. I do know that on my BE, decreasing tyre width by only 10mm and adopting that same wheel design provided sufficient improvement to drop the car into the next lower road tax band.

Yes it is a BlueEfficiency model, and the tyres are Michelin Energy Saver, compared to the Michelin Pilot Sports on the larger wheels.

 

[CJD]

Your new wheels look miles better to me.
If you're still getting 59 mpg, I could live with that.
But if you need to get the better mpg, you may have to bite the bullet and swap back.

 

[190]

Yes it is a BlueEfficiency model, and the tyres are Michelin Energy Saver, compared to the Michelin Pilot Sports on the larger wheels.

I have those same 16" wheels with Michelin energy tyres so after your experience I'll be sticking with them.

 

[Codger49]

Perhaps Michelin Energy Saver tyres really do save some energy?