bigger tyres for fuel economy

DOn't worry about the spare wheel - just keep standard size on the carrier and increase the size of your road wheels. You'll get heaps better economy and in the rare event of needing to use a spare don't go too fast.

Bigger tyres will not give you better economy, except if you were driving long distances with a tail wind!!!

The bigger they are, the more effort it takes for the motor to get them rolling. Given that most people use their vehicles in a manner such that there is a lot of stopping and starting involved, then you will actually use MORE fuel with the bigger tyres.

There is also the issue of braking (takes more braking effort to pull up bigger rolling diameter wheels/tyres too).

Having said that, I am running 35" tyres on my Patrol, but only cos I've got oodles of torque from the 6.5 V8 and wanted to try to keep the revs down. Because of this torque factor, I can actually claim to use about the same amount of fuel tht I would use if I had 32" tyres.

To answer your last question, you should be able to fit 285/75R16s on your spare wheel carrier.

There are other reason why fitting larger tyres IS a good choice; they all revolve around more ground clearance.....so if you go off the beaton track very much, then the 285s could be a good option anyway (just not for saving fuel)

Cheers

Roachie

Hi jt,
I have a GU 4.2 TD and have put 35" mud tyres on. I found that on the open road i will actually get better economy than running 31" tyres because the revs are lower, but for start/stop driving in town economy is worse.

cheers
Brett

How much better economy were you expecting?
It will be barely 5% or probably less and you wont even know until you have done a few thousand klms at least.
Bigger tyres can have an adverse effect because of the wind and rolling resistance.
I would get the bigger tyres for better offroad capability and ride,but for better fuel economy it will be pot luck.

Those that claim you are getting better economy fail to undersdtand that the larger tyres are probably effecting the accuracy of your odometer and your not actually travelling further on the same amount of fuel.

But your odometer says you are. Get a GPS and mesaure how far you travel and see if it the same as odometer.

Jt

Fatter tyre won't help you much even though it larger (285/75/16).

The tyre you need and which I run is 255/85/16.

These few choices though - I have Cooper st and Maxiss versions.

Fitting them will bring your speedo to virtually correct and overcome the inbuilt 6% Nissan speedo error.

These tyres are actually larger dia than 285's at 33 1/3 inch and there profile is such that they fit better under the guards and on the spare wheel carrier.

Ideally they should be fitted to specifically Trak 11 16 X 7 steel rims

Robin Miller

Lets forget about rolling resistance, wind resistance, greater rotational mass, and engine out of sweet spot (although they all play a roll in worsening fuel consumption).

Also, lets forget about acceleration and just look at what it takes to maintain a particular speed on level ground. Lets say arbitrarily, for simplicity sake, that it takes 100 lbs of force where each rear tyre meets the pavement (refer to diagram) to maintain a velocity of 100km/h on level ground. With a 30 inch diameter tyre:
D=15 in (or 1.25 ft)
So, T=(100 lb) X (1.25 ft)
T=125 lb/ft, or the amount of torque required at each rear wheel to maintain the vehicle at 100km/h on level ground.

Now, lets look at a 35 inch tire:
D=17.5 in (or 1.46 ft)
So, T=(100 lb) X (1.46 ft)
T=146 lb/ft, or the amount of torque required at each rear wheel to maintain the vehicle at 100km/h on level ground.
Now it is easy to see why our engines have to work harder, and thus burn more diesel, to maintain a particular speed.

Now tack on the rolling resistance, wind resistance, greater rotational mass, and engine out of sweet spot and it just worsens the picture. Yes, you do "cover more ground" for each revolution of a tyre, but ultimately you will see a sacrifice in economy because the engine has to work that much harder to get that one revolution.

Another factor is tyre pressure. A larger tyre has more pressure variation and larger rolling resistance with lower pressure.

Even if you have adjusted your speedometer to accomodate for larger tyres, you will still see a consistent drop in economy compared to smaller tyres.

I run 255/85/16 on my truck. As a poster already stated this is a good size, though selection is limited, because it is higher with more clearance but not as wide as 265. Nevertheless the additional height will cost economy. I have reprogrammed my speedo and sitting on 80km/h my engine revs at 1300 in 6th gear. This is not as it was intended because the transmission shifts to 5th with the slightest acceleration. Not so with the original tyres.. there the engine revs above 1400 and with careful acceleration the transmission stays in 6th.

The characteristic of the auto transmission changes completely and in my case larger tyres do void the warranty (mind you I do not get any warranty on a US truck anyway here)..

In order to really adjust for larger tires the shift characteristics of the tranny has to be adjusted. The learning program in my Allison does not compensate for that and even with a TechII I can not adjust the tranny only the ABS speed sensors, which is another factor to consider when running larger tyres.

I do not exactly know how much I lost in economy since I never really checked it on a complete tank with same conditions. Too many variables ... power tune, towing, fuel quality ... etc..

I have a gutt feeling that I lost between 1 and 1.5 l/100km in economy, but thats just a feeling not a fact.. others have reported 2 - 3 mpg loss from 28" to 35" tyres on the same vehicle as I have.

You need to understand what you do to your vehicle when you mount larger tyres and understand why it is illegal to do so beyond the given limits in the ADR.

good luck
gmd

Like Micks reply I have to say BS, I been there, tried that ....don't work, what you do in effect is raise the gearing up higher and the engine has to work just that little bitsy more , but if you think your on a roll then go for it

Mmmmm......... I would have to say that anyone who gives complex data to back up there claim is wrong.......Why because average poeple don't understand it so you must be wrong...LOL

Lets throw another 2 factors into it:-

The higher the vehicle is of the ground the less aeodymanic it becomes.

Do you use less power with bigger wheels because the rotational mass has increased and multiplied the torque.

Now a thing I got told last week on this forum was you can produce all the data and read all the books but you are still wrong because data and books are wrong and in real life it is differant.

Thats my 2 cent worth.

Richard