Larger wheels for MPG?
 

I wonder if you have tried going to larger diameter for lower cruising RPMS?

Is there a benefit? Is it worth the cost?

I have 14" steel wheels on a Ford Focus 1.8tddi and I am considering picking up a set of 15" alloy wheels.

I will see what has been done to my car - I know often when the larger rims go on they try to match the overall rolling diameter by using a lower profile tyre, but what if you just learned the speed offset and kept the extra size to lower lower cruising RPMS?

I am aware that the mph readout will be different.

I am aware that the car will be less torquey feeling as the larger diameter makes it harder to transfer the power. (in crude terms).

Appreciate any thoughts!

Plenty of time to think this over - just put a new set on. 185/65/14

I must add that the guys at Ivan Kerr Tyres in Carrick have been amazing to deal with. I had a ballache (to put it mildly) with a busted rim, it's replacement and the 3rd replacement! Anyway, 3 days of faffing and it is sorted. They put up with a wild load of tyre changes and didn't charge me more than the cost of a new tyre and fitting. Brilliant! (they also didn't mock my car smelling like a chippie!)

I also thought about this. Nearly due a set of tires and noticed my speedo reads a little high so going up a few mm in diameter might help bring it down a little, while maybe helping mpg?

That's interesting - my speedo reads about 4mph high as well!

Don't rely on the speedometer, look at the odometer over a distance of 10+ miles. Some cars, like my Civic, have the speedo read a few MPH high, but the odometer is exactly what it should be.

this might help on the speedometer calculation...

http://www.miata.net/garage/tirecalc.html

as far as whether or not there is benefit, it's a mixed bag of issues here, so it could help or hurt.

you'll need to be more specific about the suggested alteration to determine if you'll see improvement or not.

wheel weight, diameter, tire width, aspect ratio, and tire design as well will all contribute whether or not there will be benefit.

and... believe it or not, many alloy wheels weigh more than their steel counterparts, so an exact wheel choice will also be needed to help determine if your new wheel is eco friend or foe.

...or, if you want to "calculate" for yourself:

MPH = [ 60 / (G×A) ]·[ RPM / rpm ]

...where:

MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile.
_60 = constant, minutes-per-hour
__G = transmission gear ratio, ie: 1.01:1, etc.
__A = axle ratio, ie: 3.55:1, etc.

Increasing tire OD almost always backfires and MPG is reduced.

It might work if you were a long-haul trucker who can maintain the same speed hours on end, but most of us have to deal with stop-and-go situations.

Rotational moment of inertia goes up with the square of diameter. That means those four "flywheels" have to absorb more energy to increase their RPM. Until the tire RPM increases road speed doesn't increase.

Another evergreen myth that deserves a sticky.

We need to watch how that toothy green box out in Cali does with it's new shoes...

Dave, I think i see what your saying, but wouldn't the tread of the short tire and the tread of the tall tire be going the same ft/sec. at x MPH. The axle would see more torque but at a lower RPM. If the tall tire was wider, more aero drag, or heavier more vehicle weight, or caused more air under the vehicle, then i could see obvious losses. But what if the tall tire was the same width, same weight, the vehicle height could be lowered to stock. And the engine efficiency at the lower RPM was the same. Wouldn't the MPG be the same. Of course, how often could we find a tire that weighted the same.
On the other hand i could see where a narrower, lighter tire that lower the engine RPM to more efficient speed could be beneficial.

Some gear ratios and other numbers for you:

1.8L Endura-DI 90 PS
1st 3.667
2nd 2.136
3rd 1.448
4th 1.028
5th 0.767
Reverse 3.727
Final drive 3.410

1.8L Endura-DI 75 PS
1st 3.667
2nd 2.047
3rd 1.258
4th 0.864
5th 0.674
Reverse 3.727
Final drive 3.560

shortest 5th 0.674
shortest FD 3.410

Standard fitment rpm/70mph in 5th 90 PS 75 PS 0.674 5th & 3.410 Finaldrive
185/65 R14 1873mm 2622 2405 2304
195/55 R15 1871mm 2625 [+3] 2408 [+3] 2306
195/60 R15 1932mm 2542 [-80] 2332 [-73] 2234

Alternative 15” tyre
195/65 R15 1993mm 2464 [-158] 2260 [-204] 2166
[Dimensions in mm are circumference]

I would keep the 185/65 R14 steel wheels, or go for 15” alloys and tyre for looks, they will weigh about the same, maybe more or less. 195/60 R15 would lower revs at 70 mph marginally, 195/55 R15 would be basically identical to the 185/65 R14. Worthwhile gains could be had by using 195/65 R15, though beware the extra rotating mass and squidgy handling, especially turning into corners. I would recommend looking at the cost of the tyres as this could be a deciding factor.

A much larger reduction in cruising revs is to be found by changing final drives/ 5th gear depending which box you have to start with.