Wheel size VS Economy

tambox · Post by **tambox** » Sat Apr 27, 2013 12:38 pm

I have two sets of wheels for my EJ22 LPG L wagon, did some economy runs to compare the difference.
I regularly drive the same route, use same servos etc, identical trips. The route consists of freeway/highway to winding mountain roads all bitumen.
I have a set of 27x8.5, soft off road tyres and a set of hard road tyres 185/70/14.
Driving this route when traffic/weather/temp are similar I get better fuel economy with the small wheels.
Less accelerator movement with the small wheels, but higher revs.
The tyre difference is about 8%, the small ones use 6% less fuel.
Why???
Soft tyres? motor happier with quicker responce? higher rpm better?

Anybody else done similar?

pezimm · Post by **pezimm** » Sat Apr 27, 2013 1:04 pm

Hi tambox,

I have similar experience with my Forester. On its original tyres (215/55R17) I used to average 10.5l/100km, while with my off-road tyres 215/65R16 the best I can get is around 11.3l/100km.

I think there are a few explanations for the change in my case:
1) Off-road tyres are heavier and run steel rims, as opposed to lighter original tyres and alloy wheels
2) The gearing is very sligthly off with the bigger tyres
3) The odometer is sligthly off with the bigger tyres, so that it registers a smaller distance than actually travelled

2 and 3 are minor. I believe the biggest difference is the heavier unsprung weight.

Pedro.

steptoe · Post by **steptoe** » Sat Apr 27, 2013 3:59 pm

Watch a vacuum gauge and see how often it dives lower while drivng bigger wheels, with you planting your foot further to go same....

Watching vac gauge while still drivng at same speeds I gained advantage from 27 to 30MPG including cold air only ducting for 465 four barrel Holley on 250ci Ford six

tambox · Post by **tambox** » Sat Apr 27, 2013 5:18 pm

When i put big wheels on the turbo L, it used a lot more fuel, BUT, unlike the example above, it was when we were going away, loaded up with tent on roof.
Yes Jonno I watched the vac gauge then, to try to get the best out of it, but it was happier running a bit of not vac, about 1-2psi. Hence poor fuel economy.

steptoe · Post by **steptoe** » Sat Apr 27, 2013 9:16 pm

Think EA82 7.7:1 turbs comp ratio running 2- 3 psi boost is supposed to be similar to a NA comp ratio in the EA82, but it feels stronger to me. Ain't the turbo there for efficiency ?

Brumby Kid · Post by **Brumby Kid** » Sun Apr 28, 2013 12:18 am

Boost and efficiency do not go in the same sentence!
Just ask tweety

steptoe · Post by **steptoe** » Sun Apr 28, 2013 7:25 am

and, not to try steer thread off course - you'd think to lower a compression ratio for the turbo would bring about an inefficency of its own while not under any boost which is why I think the two or three pounds should bring it to about same as carby or efi NA economyy / efficiency levels ??

This would be a good thread to post up Nominal Overall Tyre Diameters eh?
To at least show a 195.50.15 is much the same NOTD as the placarded 175.70.13's on our MY and L Series beasts

And this is a good explain how, with warnings about accuracy due to factors...

http://www.tirerack.com/tires/tiretech/ ... p?techid=7

Subydoug · Post by **Subydoug** » Sun Apr 28, 2013 9:54 am

I think steptoes right about the vac gauge. bigger tires bog the engine down more, though there is probably a point where smaller tyres start becoming less efficent due to revs. Narrower tyres? would they help economy?

I always had the impression that turbos are only more efficent off boost during constant speed, like down a HWY. Unfortunately power always comes at the cost of burning more fuel. Going for power and economy means motorcycle

.

Regards

Doug

niterida · Post by **niterida** » Sun Apr 28, 2013 12:16 pm

Softer flexier sidewalls would also have a lot to do with it. The softer 4WD tyres will have a lot more rolling resistance and hence use a lot more fuel.

steptoe · Post by **steptoe** » Sun Apr 28, 2013 1:03 pm

I guess the economy was/is to be able to use a smaller engine capacity yet to have greater power on tap than an NA when necessary - this is all pre stop start tech/ cylinder isolate tech/ total injector cut theory. Pre angel gear theory too

LIFTED · Post by **LIFTED** » Sun Apr 28, 2013 11:17 pm

I put some larger 98 Outback rims and tyres on my 92 Liberty for a few weeks and my fuel consumption increased. The wheels were 10% larger I took this into consideration when calculating the distance travelled and fuel used. I put the stock smaller steel rims and wheels back on and the fuel economy improved.

taza · Post by **taza** » Mon Apr 29, 2013 12:03 pm

Well on my Foresters I am running 225/70r15 on steel rims. After weighing them and weighing my stock alloys with 205/70r15 they are 8kg a wheel heavier. That's 32kg more in unsung weight. I have noticed before my engine was dying I averaged 460km to 54l with these wheels and with the stoxkers I averaged 620km to the same 55l.
Will be changing back to the alloy wheels with big tyres soon just to help. Going from an ej20Na to ej25 I think will help with fuel economy as the engine won't have to work as hard to do the same job. .

pezimm · Post by **pezimm** » Mon May 06, 2013 7:37 pm

^Hi Taza, my weight difference is not so big, 23kg for AT/Steelies and 21kg for HT/Alloys...

Pedro.

Davidov · Post by **Davidov** » Mon May 06, 2013 11:00 pm

pezimm wrote:^Hi Taza, my weight difference is not so big, 23kg for AT/Steelies and 21kg for HT/Alloys...

Pedro.

They are some heavy HT/s with alloys

My alloy/ Geos were about 24kg each, and my alloys with HTs were 16kg each with low profiles but now probably about 18kg each with stock size rubber.

Tazas wheels are heavier and are about 27kg each.

60766244 · Post by **60766244** » Tue May 07, 2013 10:46 pm

After lifting my Brumby 2" and fitting some 14" rims and removing the 13"s (don't rage now) I have noticed a large increase in my fuel consumption. Before I could get 250km before the needle hit half on the tank, I got 170km this time. Thank F*** for cheap fuel these last few weeks. Might have to put the 13"s back on or get those alloys I have with some new tyres.

YouTube · Post by **El_Freddo** » Wed May 22, 2013 11:56 pm

niterida wrote:Softer flexier sidewalls would also have a lot to do with it. The softer 4WD tyres will have a lot more rolling resistance and hence use a lot more fuel.

My thoughts would be that 4wd tyres have a tougher side wall construction that's not necessarily soft - hence why I've killed about 4 road tyres with a hole/pinch in the side wall. I'm yet to tear a side wall in one of my offroading tyres - hopefully this never happens!

60766244 wrote:Before I could get 250km before the needle hit half on the tank, I got 170km this time. Thank F*** for cheap fuel these last few weeks. Might have to put the 13"s back on or get those alloys I have with some new tyres.

I'm guessing with the 13 and 14 inch rims we're also talking about a different sized tyre diametre? It is possible to have a 14 inch rim and a stock tyre diametre.
With your fuel usage calculations I hope you've added the missing distance from the larger tyres. On Ruby Scoo the speedo is out 18%, hence adding 18% of the displayed distance on the trip metre - once you do this your fuel economy readings become closer to what they used to be, at least mine do.

I'm not really wanting to weigh a rim tyre combination I'm running at the moment - I just know they're bloody heavy! But that said I'm not about to run a track day in Ruby S, so I'm not fussed, offroad ability is what I'm aiming for.

Cheers

Bennie

60766244 · Post by **60766244** » Thu May 23, 2013 1:59 pm

El_Freddo wrote: I'm guessing with the 13 and 14 inch rims we're also talking about a different sized tyre diametre? It is possible to have a 14 inch rim and a stock tyre diametre.
With your fuel usage calculations I hope you've added the missing distance from the larger tyres. On Ruby Scoo the speedo is out 18%, hence adding 18% of the displayed distance on the trip metre - once you do this your fuel economy readings become closer to what they used to be, at least mine do.

I'm not really wanting to weigh a rim tyre combination I'm running at the moment - I just know they're bloody heavy! But that said I'm not about to run a track day in Ruby S, so I'm not fussed, offroad ability is what I'm aiming for.

Cheers

Bennie

Yeah I have factored in the change in the speedo/odo as well, still hitting me. The tyres are much larger overall, though I can't remember the overall outer diameter, they make the old wheels look puny.

Once these pirellis run out I'll go down to something slightly smaller I think.

Venom · Post by **Venom** » Thu May 23, 2013 3:21 pm

I think the extra wheel/tyre weight contributes almost as much to fuel economy as the tyre size/gearing change. The car i drive in mount isa gets ridiculous fuel economy for a 30 year old Subaru. It's completely stock except for some very very lightweight aluminium 13" rims. I get under 8 liters per hundred km's every fortnight, guaranteed.