Ha, I'm no engineer, and no JLR rep either. My 2 cents is that if the handbook on the v8 says not to load a roof rack, I'd not do it. I'm looking at the 6cyl. anyway, but even with the v6, I've not bought because I'm waiting for 18" rims + solid roof.

 

I am an engineer in this field, although not for JLR. The load a high-cg vehicle can carry is directly related to its track width and lateral grip capacity. Roof load in particular is a huge challenge to design for, and the tire’s peak grip will be determined early in a program based on the vehicles loading requirements. Every 0.01g of lateral grip reduces the load that a vehicle like this can safely carry by a non-negligible amount. I’ve personally run these tests myself in similar vehicles.

It looks like the V8 defender’s peak grip is around 0.78g vs the P400’s ~0.7g. +12% lateral grip is *huge* in this context even without considering the extra powertrain weight. You’d be surprised at how differently the car will perform in emergency maneuvers when fully loaded with that much extra grip.

I’m new to the Land Rover community and I’ve been blown away by my Defender. I had a random opportunity to drive one through work and a month later I had one in my garage. LR did an amazing job engineering these trucks to be hugely capable, comfortable and fun, but they aren’t immune to the laws of physics.

 

It’s not that rim diameter is the limiting factor for top speed, it’s more that JLR assumes that someone buying the 22” wheel is not interested in off-road so they optimized and certified the 22” tire for higher speed, most obviously by choosing a tire with a higher speed rating. This is common among vehicles like this.

 

You may be aware that most really fast cars run 20" rims and can do 190 mph.. It's the tires that limit top speed not the wheel diameter ..

 

I'm guessing they're not running 32" tires.

Sidewall height is a very important variable for speed performance. As sidewall reduces, so too does lateral flex of the tire during cornering. Minimizing lateral flex is desirable and a very important feature in a speed/performance driven design. One major reason behind the 20-22" tim, speed rating discrepancy.


An example diagram but this is applicable to any tire.

 

My AMG GLE 63s runs 32" tires and will do 170 mph.. You never mentioned the sidewall height, just wheel size..

 

At no time did I say what a vehicle and tire will or won’t do. That’s irrelevant. Well done that vehicle.

I would posit that there are few 32” tires with a speed rating of 170mph+ and a non-low profile aspect ratio.

My guess is most vehicles built for speed are running low profile tires with diameters less than 32” as this is a design metric that makes sense given the laws of physics.

The question earlier was regarding speed rating and you appeared puzzled where someone was getting such erroneous data.

As tire diameter is an OEM constant here, rim size is the variable in question for regulatory panels’ machinations and their criteria for speed rating of vehicles and tires in these instances.

The tire rating rather than the vehicle’s being the likely governing factor.

My underpants let alone my tires are not rated for 170mph so what do I know.