I've researched this issue, but haven't been able to sort out a clear answer.

I assume that owners install 18 inch wheels and large tires to gain ground clearance, but my question is: what does that do your vehicle otherwise?

Do larger wheels/tires impact gas mileage and what does it do to general drive-ability? Is the vehicle any less safe on the highway, etc?

 

16" rims are usually desired with the larger tires. more rubber
mileage will suffer as your little v8 is working harder.
a change in gear ratio is advised if you go big enough. most guys do 4:11 with 33s
safety and drivability are easily maintained. you will notice some things more and some things less. for example; a bent axle on 29" tires might never be noticed. throw some 35's on and it looks like you are about to lose a wheel. you will need to assure that you brake lines and components match your lift

 

Expect mods like bigger tires, roof racks, winch bumpers and suspension lifts to increase fuel consumption. I notice more wind and tire noise, but no noticeable effect on safety.

 

Thanks for responses. That's just what I needed to know to make a decision on new tires.

I don't want to affect my gas mileage adversely and since I already have a two inch lift from new medium duty TF coils and shocks, I think I'll stay with my 245 75R16 tires and call it good.

Plans include some long distance road trips in the next couple of years and mileage is definitely an issue. Plus we'll be pulling a small offroad trailer, so...

 

The slightly bigger tyres on my D2 raise the gearing slightly so that it revs slower for a given speed, making it a bit more refined (in theory). It also means that my speedo now over reads slightly so I need to be careful about speed limits (100km/h on the speedo gives me 103 according to my sat nav)

 

x1 this is one main feature of larger tyres without diff ratio compensation as the speedo under reads actual true speed but being the standard speedo/tire setup reads between 5-10% faster than actual ground speed (by law) it is unlikely to be too far OTT. It can be calculated easily with the use of a satnav as Richard has stated or simple math. The other downsides are lower MPG and higher CoG also a worsening of highway handling at higher speeds (over 60mph) plus any tire imbalance is magnified somewhat. Road noise will increase anyways using AT or even chunkier tires over the standard highway tires so additional wind noise won't be significantly noticeable.