There's been some interesting
views expressed on a recent thread about "skinny versus fat" tyres and the fuel economy thereof.
However, that discussion did raise (sort of) the interesting issue of whether more powerful engines on modern 4WDs have resulted in better off-road ability or not.
The reason I say this is that one result of more powerful engines is that the vehicle is capable of higher speed.
However, to achieve this, the tyres have to have a much higher speed rating. In general terms, this means fatter tyres, lower profile, softer compounds, and thinner sidewalls.
Accompanying this has been the trend to larger diameter rims (so even lower profiles) and the use of alloys (easily damaged compared to steel rims). Alloys mean lighter
wheels with a lower moment of inertia so faster acceleration and braking. Plus most say they look better with more pose value!
For reasons discussed in the other post, all of these factors (IMO) have a negative impact on off-road ability.
However, as Cackles has rightly noted in the other thread, everything is a compromise at some point.
If you want a good towing vehicle, you need (want?) good engine power and torque. However, you don’t really need high speed! Few people tow anything of size at 130 kph!
But if you do want “quiet”
well-behaved on-road tyres that can sit at 150 kph with good fuel economy, then that becomes your key criteria.
However, if you want good off-road tyres,
well then that’s another matter altogether...
Extra engine power (and even torque to a lesser extent) is largely useless off-road. The most important thing is the overall transmission reduction ratio, which, if anything, has been getting poorer over the years, as this allows for higher vehicle speeds and lower engine rpm so better fuel economy.
Unfortunately (or perhaps fortunately), it is illegal for tyre fitters (or even yourself) to fit tyres to a vehicle that have a lower load or speed rating than the OEM tyres even if you never intend to go at those high speeds. Hence even if you say to yourself “I don’t plan going more than 130 kph”, but your original tyres had a 200 kph speed rating, it’s illegal to fit 130 kph tyres.
If you have fitted tyres that don’t meet the OEM speed or load rating, and you had a tyre failure (or even an accident involving stopping or steering issues), then you could be in strife with the constabulary, or at least with your insurance company.
It’s interesting to note the comments from one of the world’s most experienced overland expeditioners of modern times, Tom Sheppard.
Tom is an ex RAF test pilot who’s done over 100 000
miles (160 000 km) crossing deserts and overlanding over 40 years (he’s in his 70s now). He led the first coast to coast crossing of the Sahara Desert, and even since 2001 has conducted six SOLO (!) Sahara expeditions! He’s very experienced in serious deserts with both smaller 4WDs and 4WD trucks. He states in his book “Vehicle dependent expedition guide” that a power to GVM ratio of 30 BHP/tonne (he works in horsepower!) is plenty for even the Sahara. My Earthcruiser is 4.5 tonnes and 110 kW so has a ratio of 32.8 BHP/tonne, and even at 6 tonnes GVM (exactly the same vehicle can be purchased with 6 t GVM) has a ratio of 24.8 BHP/tonne. Compare this to the legendary Mercedes Unimog which has “only” 17 to 21 BHP/tonne (depending on the model), and the Pinzgauer 4x4 and 6x6 which have power to weight ratios of 30 and 24 respectively.
Ironically, the most capable 4WD vehicles have low power to weight ratios! Why? Because it isn’t important off-road.
By contrast, a new 200 series turbo diesel L/C wagon provides 195 kW (261 HP) for a GVM of 3300 kg or a power to weight of 59 BHP/tonne! A RAV4 has 81 BHP/tonne!
Even the oil companies (and Army) get their big trucks across deserts and the answer is basically the big (diameter, not width) tyres and low ratios not big engines.
However, now consider the L/C wagon’s gearing. Its 1st low overall reduction is a rather pitiful 34.1. The EarthCruiser’s ratio is 54.3! At a fast idle, the EC is doing only around 2 kph in 1st low. At peak engine torque (471 N.m at 1600 rpm) it's doing 4 kph.
However, the L/C 200 series has 285/65 R17 116H tyres (I think) which means they are rated at 1275 kg per tyre and 210 kph! This speed rating is needed because its big donk can get it up to that speed, at least theoretically!
It’s interesting to note that the earlier L/C 100 series wagon used 275/70R16 114T which are rated for 1180 kg per tyre at 190 kph. Note how the tyre width has gone up, the profile has come down, and the required speed and load rating has gone up from the 100 series to 200 series.
My EC uses Michelin XZL 255/100R16 which are rated to 1700 kg each tyre, but at only 110 kph!