The Honda handle guy had a similar question that I've been researching, and [MENTION=96008]Engineer[/MENTION] 's response was also thoughtful and useful. However, despite all Engineer's considerations, some rough ballpark understanding of how much sidewall height is lost when a typical 285/70R17 tire is mounted at 40psi on a 5th gen would be nice for a starting point of understanding. I'm Googling looking for an answer, but I get inundated with more typical posts such as asking whether 285s will rub.
There are a couple reasons why knowing this relevant. One relates to choosing a 255/80R17 vs a 285/70R17. The 255/80R17 option seems great from a lift perspective. On paper, it gives more lift than a 285/70R17. However, I've seen a few people mention the 255/80R17 looses about half an inch diameter when installed. I presume that loss is entirely on the bottom of the tire, so the diff really doesn't get nearly as much of a lift as the original spec would suggest. I assume the 0.5" sidewall height loss is on a rated E tire since that's what the common Cooper ST Maxx and newer Falken Wildpeak come as, but that's just my assumption.
Now what about a typical 285/70R17 tire? Does it loose roughly half an inch on the sidewall when installed? Sure Engineer's points below are valid, but it's also valid to start analysis like I'm inquiring about with some ballpark estimate.
I Googled '5th gen 4runner 285 diameter loaded' and was immediately swamped with irrelevant results. Given people don't often talk about actual installed diameter (or even the sidewall loss being on the side of the tire touching the ground), I figure plowing through posts about whether a 285 will rub is going to be a futile waste of time. Also, I'm not active enough on this forum to create my own thread yet. I don't mean to hijack, but I thought my question was also in line with the spirit of this thread (analyzing tire diameter).
Lastly, one thought is a great way to measure what we all probably truly care about is simply stick a measuring tape between the ground and the lowest part of the differential. Do this before and after swapping tires. I plan to do that going from worn Cooper AT3s 265/70R17s to whatever tire I buy next. I can even post my inflated PSI, the ambient air temp, and state the tires are at ambient temp
Thanks! -Dan
I think this would be about like asking OTD pricing - you need to include wheel width, tire pressure, weight of the truck, how was it measured, etc - (eye balling with a tape measure on the ground to the top of the tire would give me 5 different results if I asked 5 people....)
