Lift kits & tyres affecting economy?

Anything you do to let more air under the car will affect fuel economy.

When Land Rover started putting front air dams on Range Rovers they claimed that the air dam improved fuel economy by 1MPG out of 20MPG on highway or 5%.

And I recall recently that ford claimed the air dam on the new Ranger improved economy by a considerable %. AFAIR 6%.

Tyre pattern will also affect economy with Mud tyres the worst.

However these all pale into insignificance compared with a full roof rack, or towing a caravan.
Regards Philip A

Tyre diameter can also affect fuel economy as it changes the gearing. Not only that it will also affect the odometer reading.

I put larger mud tyres and a modest lift on a TD courier I had, fuel economy changed by 2 liters/100 ks.
Speedo change slightly, but most noticeable was a drop in power, bigger wheels/tyres will make it feel you are carrying some sort of weight, not a big difference but noticeable.

Philby
Larger dia tryes have some negative effect but most also increase the width of the tyre. Pushing wider tyres along the road also causes more fuel usage. If you decide on a bit bigger dia try to choose minimum width increase or non at all if possible.

As mentioned, more air under the vehicle can cause more fuel usage but it can also cause less vacuuming effect of heat from the engine bay because the designed "suction" or negative pressure generated doesn't pull as much airflow from under the engine area.
May contribute to overheating at higher speed while loaded. Then it relies on the fan and frontal 'in flow' only. Hopefully you don't reach this critical point.
Some vehicles have a horizontal strip to force air down and away so a degree of hot air extraction is preserved. None of these are fitted to the lower edges of Parliament house though.

It's hard to calculate your economy when the odometer reading is out. Larger tyres will mean you are doing less kilometres... 5-10% in some cases (good for resale :-)). Therefore your fuel economy will change by that much unless you adjust it to real kilometres in your calculations. You also have the increased rolling resistance and the gearing effect making your car gutless up hills. The larger wheel diameter also increases torque pressure on axles and cv's making them more prone to breakage.

There is a happy medium with everything raising dosn't automatically mean it will use more fuel a 2" lift might be ok where a 3" or 4" might not it's trial and error I have raised several 4X4's over the yrs by 2" without any change in economy 1990 ,1996 cruisers a 2001 disco & 1991 GQ. Tyres are the same I put 32" tyres on a the 1991 raised petrol GQ and it got better economy. My current turbo diesel GQ has 33" all terrains and gets 0.15 lph better than the 31" tyres it use to have and the tacho reading is down by 50rpm at 100kph every vehicle is different you just have to find what suits your car. Maybe start with a 2" lift and go up one size in tyres don't do anything to radical just small changes you'll be surprised how much difference it will make.

This has been heavily discussed on the 4wd forums and subject of tests in the magazines.

BIG lifts and BIG tyres badly effect fuel consumption no question.

Small lifts the issue varies depending on how bad the aerodynamics of the vehicle are to start with.

Small changes in tyre size can make a big difference.

Lost of people who fit 31's to larger 4wds that actually came out with taller tyres like landcrusers & patrols find it effects fuel economy badly.

If ya fitting bigger heavier wheels & tyres, that will clearly effect your round town economy because all the extra rotating mass you have to accelerate.

Then there is the issue of pushing the car off the sweet spot....light cars with planty of power and good aerodynamics will get good economy over a wide range of speeds.

Heavy 4wds, which lets face it are never over powered or particularly aerodynamic are optomised in gearing and torque curve to run best at a couple of speeds......

mess with the gearing and all that does not work well.

With the changes in modifications rules in every state but NSW and comming there..ya better off staying under 50mm lift and 50mm tyre diameter increase....

And realy...how much do you actually gain in tyre diameter increase, you can rarley get the whole 50mm in the tyres available and once aired down off road you might gain 10 or 15mm.

There is a certain 4wd identity, known for driving a green truck and wearing a beard....in addition to not drinking much beer as it may appear also has more or less standard diameter tyres and a very modest lift...yet takes his truck almost anywhere the boys with big tyres and big lifts goes.

A modest lift and better tyres ( not necessarily bigger tyres), will save you money every way you turn and get you most places you want to go.

cheers

I have a 4" lift and 35" Tyres on a 2003 4.2TDI patrol. Diesel went from 14.5ltr per 100 to 20. Fitted straight through exhaust and better flowing turbo (TD05 18g) now getting 13.5ltr per 100. Using a GPS and 170ltr range to average consumption.

Thank you everyone for your advice. I have much to think about it seems...

Mate don't let it become over-complicated.
On your Prado, if you are doing a bit of decent offroading, then a 2 or 3" lift will be one of the best things you can to do improve it's capability offroad. As for using more fuel with a small liftkit- absolutely insignificant compared to the other benefits.
As for larger tyres... a lot of people dont grasp the basics.... No matter what width tyre you have, be it a 185 or 285, at the same tyre pressure you will have EXACTLY the same amount of tyre contacting the road. Wider tyres spread the load out a little wider, and narrow tyres spread it out lengthways.
As for larger diameter tyres, now you are talking about something that will have an influence on performance and economy.
Your Prado with it's D4D will love running 31s and even 33s, because it has over 400nM of torque to turn them. If you do a lot of outback touring, then they may even give you similar economy if not better because they will gear your car up so it revs lower, and the torque will keep 'em rolling.
If you had a gutless 4WD then rolling heavier rubber will make it struggle and burn extra fuel for sure.
When i swapped the 31s to 33s on my petrol 80 series, i lost on average 100kms range per tank. Definitely noticeable. But now my latest 80 series with a 6.5L TD Chev with 700nM of torque will spin 35s and tow a camper without affecdtinf econamy anywhere near as much as a smaller 4WD.

Mate, you are over thinking it.

Like all of us you own a vehicle with the aerodynamics of a besser block and worry about the burr on the edge of it?

If you have a use for the lift and the different tyres just go do it.

Then drive at 90 km/hr on the highway and you'll recover the increased fuel usage if in fact it actually increased anyway in spades.