Fuel Comparison: Diesel, petrol or LPG?

With the fuel debate raging, we take two 4x4s and crunch the numbers for diesel, petrol and LPG.

Fuel Comparison test: Diesel, petrol or LPG?

It’s been several years since 4X4 Australia compared fuel types.

This article was originally published in the August 2014 issue of 4x4 Australia. 

In fact, the last time we compared economy and costs was back in 2006, when fuel prices skyrocketed – we took diesel and petrol versions of the then-new and still-current Nissan Navara and Toyota Hilux for a comprehensive ’burbs, bush and beach bash all in the name of research.

Since then, the Australian Government’s LPG technology incentive rebate has come and gone.

Hilux and trailerAlso, several 4x4 vehicle manufacturers have reduced the availability of petrol engines or deleted them altogether, a decision driven largely by the mining sector’s need for diesel (minimal fire risk) and the ever increasing economy and power outputs from turbo-diesel engines.

With the popularity of Aussie travel increasing, and with plenty of budget-conscious buyers looking to buy a second-hand 4x4, we figured it was time for another look at fuels: petrol, diesel and this time, LPG for outback and bush travelling.

We needed vehicles to run the numbers and Toyota’s Hilux is a good choice. It’s available with 3.0-litre four-cylinder turbo-diesel engines and is one of the few commercial vehicles to continue offering buyers a grunty petrol option: a 4.0-litre quad-cam four-valve V6 petrol, but only on the top-spec SR5.

Filling up at service stationIts popular diesel, the D-4D 3.0-litre, four-cylinder, four-valve turbo rattler develops 126kW at 3600rpm and 343Nm at 1400rpm. It ladles out strong performance lower in the rev range, making it a more confident and capable tow vehicle than some newer-design engines. It’s backed by a five-speed manual or a five-speed automatic (as shown here), new for Hilux and replacing the four-speeder of the previous model.

Next to it, Toyota’s 4.0-litre DOHC four-valve all-alloy V6 develops a stout 175kW at 5200rpm and 376Nm at 3800rpm.

Thanks to the tech tricks of variable cam timing and dual-runner intake manifold, the lusty V6 manages to fill-out the lower part of the torque curve far better than its ‘revvy’ specifications and smaller-than-V8 capacity suggest. It certainly doesn’t need much clutch-feathering to get moving.

Engines comparedPetrol engines can be converted to run on cheaper, greener, liquefied petroleum gas (LPG) but this is not a showroom option on any 4x4 vehicle. So for this task, we used a camper-equipped 2005 Hilux SR 4WD 4.0-litre petrol V6 fitted with an aftermarket Impco/BRG sequential vapour LPG injection system to provide dual-fuel operation.

This LPG installation features one injector per cylinder and sequential injection, just like a standard petrol injection system. In fact, the Impco set-up has an engine-bay mounted electronic control unit to interface with the Toyota’s ECU; each LPG injector copies a petrol injector’s behaviour to provide precise LPG delivery and engine outputs and driveability that are identical to petrol.

Even when in LPG mode, starting and warm-up happen on petrol. When the engine reaches operating temperature it almost seamlessly switches to LPG. If the LPG tank runs out, the system sounds a ‘no LPG’ warning to the driver and reverts to petrol operation.

Filling up gasThis dual-fuel conversion provides reduced fuel costs, petrol servicing regimes (and prices) and extended range – very handy benefits for 4x4 owners.

The engine is also protected by a Valvemaster system to prevent or reduce valve seat damage as LPG burns hotter and is more aggressive on the exhaust valves. Note that while Toyota (and other manufacturers) offer an LPG option on some models, this one is an aftermarket system and is not endorsed or warranted by Toyota.

We laid-out an on-road test route in western Victoria. Our beginning and end point was Dimboola’s service station where we had a bit of space to pull-in and easily refuel with a caravan on the back.

Our loop was around 130km of mostly open 100km/h highway with a sprinkling of urban running through the small town of Horsham.

Caravan towingThis allowed the three fuel types, in the two vehicles, to be assessed in real-world – and almost exact – conditions back-to-back. There was little change in elevation over the loop – in other words, there were no steep hills – and our aim was to derive comparative consumption figures between the fuel types in identical conditions which, with other costs such as servicing and LPG conversion pricing, would allow us to determine comparative costs.

Measuring of the fuel used on each test loop was critical so it was important to reduce or eliminate what the white-coats refer to as introduced errors. Each tank was carefully and precisely topped-up before and after each test to provide an exact fuel use while using the same roads and even the same bowsers kept things scientific.

Towing the trailerEach fuel type was used around our loop with nothing on the tail, an 800kg Cub camper trailer and a 2.2-tonne caravan. The two vehicles carried different hardware but were piloted by the same driver and, as mentioned, each test loop was replicated as exactly as possible right down to idling time at, and acceleration from, traffic lights and 100-to-40km/h deceleration for road works.

Acceleration requires significant fuel burn – more than idling at lights – so replicating these situations each time was critical to the validity of the test.

Most people who have been around 4x4s, towing and travelling for a while won’t be surprised to learn that the diesel Hilux’s fuel use was, on numbers alone, the most economical in all three situations, whether towing a trailer, a ’van or nothing.

Petrol 18.9L = 14.5L/100km 24.7L = 19.0L/100km 32.3L = 25.8L/100km
Diesel  12.0L = 9.2L/100km 16.4L = 12.6L/100km 23.9L = 19.1L/100km
LPG  25.7L = 19.7L/100km 31.0L = 23.8L/100km 43.4L = 34.7L/100km

But what of actual running costs? Petrol was 156.9c, diesel 161.9c and LPG 92.9c. Without doing the sums it’s easy to see that diesel’s similar bowser price but more frugal burn means it’s the pick of the pair… but by how much?

And is LPG a viable alternative to diesel for overall running costs?

9.2L/100km = $14.90 12.6L/100km = $20.40 19.1L/100km = $30.92
14.5L/100km = $22.75 19.0L/100km = $29.81 25.8L/100km = $40.48
19.7L/100km = $18.30 23.8L/100km = $22.11 34.7L/100km = $32.23

So, what’s the outcome of all this? Firstly, there’s no doubt that diesel is the fuel of choice for many of us for good reason – vehicles that use it are more frugal, especially when doing the hard yards towing.

What we haven’t analysed is the increased servicing costs of diesel vehicles compared to petrol and dual-fuel machines. This varies by brand and model but in general, diesels need more frequent servicing, with more expensive oil, and more of it, plus higher labour costs. This eats away at any day-to-day savings created by greater fuel economy.

Driving the Toyota HiluxFixed price servicing as offered by most new car manufacturers these days takes the guess work out of estimating you overall service costs when comparing between fuel and/or vehicles. Similarly there is the extra cost of an LPG conversion: around $4000 for the clever dual-fuel EFI system as featured here. For the past six years, the Australian government has offered a rebate for LPG conversions as it is a demonstrably cleaner fuel than petrol and (especially) diesel

Unfortunately this rebate came on an end on June 30 and with an increase in the tax/excise on LPG, this fuel is having its decades of outstanding appeal for cost-conscious (and more environmentally aware) motorists eroded by fiscal policy.

While we’re talking costs, we paid rural prices for our three fuels. Closer to town, prices are lower – especially, proportionately, for LPG which means even further savings which brings LPG even closer to the per-100km cost of diesel but with petrol-vehicle servicing costs.

Diesel has wide appeal for good reason but do the sums and an LPG conversion on an older petrol-powered ute or wagon can make owning a 4x4 vehicle more affordable on a dual-fuel conversion than on diesel in the long run.

Living with LPG

LPG provides economical running but reduces absolute range. A valve-saver system is necessary but cheap (around $100 plus fitting) protection against valve seat recession for many engines due to the potentially damaging hot-burning nature of LPG. New-generation LPG systems start and warm-up the engine on petrol so the standard petrol injectors are flushed with fresh fuel at every start-up to prevent seizing and gumming: keep the petrol in the tank fresh by consuming it all and refilling (even if it is to only 1/3 or ½ full) every month or so in suburbia.

Glenn filling up with LPG


It’s possible to install LPG on a diesel vehicle, too, but the outcome is different to that on a petrol/LPG dual-fuel vehicle. According to Diesel Gas Australia, introducing a small amount of LPG to a diesel-burning engine allows it to produce more power and torque with less diesel consumption. The LPG works as a catalyst, introduced by a single injector into the intake manifold at part-throttle and above (not at idle) to improve the burning of the diesel. It does not replace diesel completely, as LPG does in a petrol-powered engine. This better burn of the diesel thanks to the presence of LPG boosts torque and power. As the LPG is a ‘power adder’ fuel, its consumption in a diesel vehicle is a small proportion of the distillate burn. Of course, we couldn’t test this in our test loop, but more grunt and better economy is worth crunching the numbers for!

Diesel and LPG prices


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