Tire Size Differences (235 vs 285)

[wicked1]

What are some general differences between tire widths?
The person who built my truck originally had 285/75r16's. But had switched to 235/85r16's by the time I got it.
I know the previous owner went through some optimizations for improving fuel mileage, and I'm assuming he made this change for that, but I'm not sure.
Some local off-road guys were saying my tires looked too narrow.. But I've had no problems in mud or soft sand.
I'm read some general info about it.. Cutting through mud, vs floating on top, etc.. But am wondering if anyone here has personal experience.

Thanks.

 

[ramblinChet]

My suggestion is to run the tire originally specified by the manufacturer. Provided your vehicle fully loaded does not exceed GVWR you should be perfectly fine.

It sounds like the previous owner went cheap and saved about 35% when it came to purchasing new tires. The original 285/75R16s are approximately 33 x 11.5 tires whereas the current 235/85R16s are approximately 32 x 9.5. Make sure you purchase the correct size tires but also the ones with the correct load index, speed rating, etc.

 

[Mighty Dodge Ram]

You don’t mention which truck it is, year, make, etc. Some older Ford 250/350 came stock with 235s. My ‘02 Chevy came from the factory with 245s.

 

[fish more]

Your truck should have a tire info sticker on the driver's door jam area, this sticker will tell you the tire size the truck was equipped with from the factory.

 

[wicked1]

It's a 2015 tacoma. Looks like stock is 245/75r16. But this truck is nothing resembling stock.

I doubt the previous owner did anything to save money.. (a rich trust fund kid from NY had it built at Mainline Overland.. then lived in Baja cali for a couple years in it).

I haven't weighed it, but I'd assume I'm over the GVWR. Heavier suspension. Heavy flat bed. Extra fuel tank. The camper. Winch, etc, etc..

Like I mentioned, I know the guy went through a bunch of optimizations for getting better highway mileage.. I'm assuming that's why he changed them. Less rolling resistance. Less rotational mass. But otoh, I've been reading some offroad people prefer skinny tires..

I'm just wondering what I'd feel or notice being different if I went back to the wider tires. (basically a tire I was wanting to switch to isn't available in anything smaller than the 285 size, so was thinking about switching.. But I can't afford to give up highway fuel mileage)

 

[ramblinChet]

You can load up, fuel up, and stop by a nearby truck stop to have it weighed so you are sure.

And there is virtually nothing you can do to "optimize" a truck with a camper for highway mileage, lol. Aerodynamically, it's a big dirty brick. Rolling resistance is somewhat impacted by tire pressure - size has virtually zero impact. Diameter and rotational mass only truly matter during acceleration and braking - once you are up to speed those are very minor considerations.

About the only variable you have control over with regards to highway fuel mileage is your speed and that has the largest impact of all. In order to perform the calculation you will need your frontal area in square feet, coefficient of drag, how your vehicle weighs and then your speed.

For example, my truck has a front area of approximately 40 ft^2, drag of 0.75, and weight 7,500. These are just close approximations.

At 60 MPH my truck requires about 60 HP to keep it moving (45 HP drag & 15 HP rolling resistance).
At 70 MPH my truck requires about 90 HP to keep it moving (70 HP drag & 20 HP rolling resistance).
At 80 MPH my truck requires about 125 HP to keep it moving (105 HP drag & 20 HP for rolling resistance).

So, if I increase my speed by 33% (60 to 80) it requires over 100% additional HP (60 to 125) with most of that being associated with drag, and almost none to rolling resistance.

I know V-8 engines enjoy running around 1750-1850 RPMs on the highway for maximum efficiency. There are may variables that go into those calculations but you can research your engine to find it's ideal range and adjust your tire size and gearing to place you in that sweet spot. Much to consider although you see how the dominant factor is what speed you decide to travel at.

 

[ntsqd]

My 20-odd years experience driving an '84 Xcab Mini in all sorts of weather and over all sorts of terrain is that a truck of that size does not need the width of the 285. I ran mine on 33-9.50's until those got too difficult to get and then moved to 33-10.50's. Normal street pressure for those tires was 30 psi and really aired down for the super soft stuff was 19 psi. I never, ever felt any need to go lower and as I got more experience with the truck and those tires the less I even bothered to air down. 30 psi delivered the dirt traction that I needed 90%+ of the time. Had a ~33" tall, 8.0-8.5 width tire option existed I would have happily run that.

Infernal Combustion engines have their best Brake Specific Fuel Consumption at their Torque Peak. That is to say they make the most power for the least fuel at the Torque peak. With a throttled engine (gasser) the rpm of the torque peak gets moved around a bit by the throttle angle (quoted peak is at WOT) and I don't know enough about it to predict where it moves. I try to operate them at a couple hundred rpm lower than peak torque as from years of observation that seems to be about the right place.
FWIW the LSx engines family likes to rev more than it's earlier cousins did. I suspect that Ford's modular engines do too. Don't know anything about the Dodge/Ram gas engines. The 6BT diesel's sweet spot rpm is 1750. No idea about the PSD's or the D-M's.

 

[Beach]

If you ever get into deep soft sand you'll want more width than a 235 profile tire for more floatation.

 

[wicked1]

After much reading, I'm sticking w/ the 235's. Not worth new wheels and other changes to move up to 285.
And, a lot of off-road people love their 'pizza cutter' wheels, apparently.
I drove in deep mud this past weekend without any problems. Been to beaches and sand dunes, and no problems.

Wish it were like a video game, where I could just press a button and swap parts to try them and see what works best!