Tire size philosophies and reality

[barshnik]

Just a few thoughts, lets assume equal D2 2" lift, stock 16" rims, with different tire configurations. What am I missing? Trying to get a better idea what might be ideal for varying conditions rather than speciality stuff (oh, I know, 265x75's).

Larger diameter (265x75x16 or larger)
Pluses -
- more diff & steering parts clearance for rocks & mud
- looks cooler?
Minuses -
- possible gearing problems with steep hills, less towing capacity if needed
- worse gas mileage
- possible rubbing, reduced turn radius, etc
- speedo reads low
- possible reduction in side-angle capability (higher center of gravity)
- more expensive
- more strain on drivetrain

Wider tread (265 or wider)
Pluses -
- better traction on smooth rock
- more forgiving of wheel placement
- more stable on-road
- better floatation on soft sand
Minuses -
- more prone to sidewall damage in tight spots
- more floatation not always good, re: water, snow, even gravel/dirt
- may require reduced turning radius

Not everyone does heavy duty rock climbing, I wonder if popular tire sizes have grown a little too large for all-purpose offroading?

After recently fitting 265x75x16 after having 255x70x16's for so long in so many different conditions, I wonder if maybe larger is not always better. I've raised my center of gravity over 3/4" (over 1 1/2" difference in diameter) - just how big of an effect does this have on side-slope rollover angle, for example? DISCLAIMER: I've been out of town since the change, have not yet wheeled with the larger tires, maybe this will all become obvious the first time out when I get back...

Please ignore without comment if you feel this subject has been beaten to death, but in all my searches, I've seen no real test results or calculations on the effects of common lifts (2 or 3"), or tire sizes larger than stock on side-slope rollover, for example.

John F
LV NV

 

[Rich Lee]

John,

There are so many variables, It's hard to say what your outcome would be.

Here (for comparison sake) is the evolution of my 2000 D2. SE7 with SLS (no ACE)

Stock suspension & goodyear 255/65-16 wrangler HPs...fair performance, better than my 95 D1 with Billsteins & 255/65-16 Michelin XPCs. Goodyears were noisy and trecherous in the wet.

Changed tires to 255/70-16 Yokohama Geolander AT Plus II. Immediate improvement in handling, but the tires felt soft for the truck.


Changed to 245/75-16 Geolander AT plus IIs . Better handling on & off road (due to stiffer 3-ply sidewall and higher weight & pressure ratings). No significant loss in acceleration & braking over stock tires.

Upgraded suspension to OME 751 springs in front & 763s in rear (chucked the rear SLS) with firm OME shocks. Better handling in all conditions, but "firmer" ride on road. total lift about 2 1/2" front & rear. with adventure rack & plywood flooring it felt no tippier than before the lift on sidehills. So much is dependant upon how heavily laden your truck is. I have not yet disconnected my swaybars. The rack changes handling and drag more than the lift does.

Upgraded divetrain to 4.11 gears and Detroit Trutrac diffs front & rear. Felt like an immediate 40 hp gain, but revs were a bit high over 70mph.

Upgraded tires to 265/75-16 Bridgestone AT Revo (for non-mud) or 265/75-16 ProComp Mud-Terrains (for mud).
Really feels right with the new gearing. acceleration is still better than stock and traction is outstanding. The stability is better than stock, especially with the Revos, which stick like glue on all pavement and do extremely well on gravel, sand, rock & snow. I trimmed the front bumper, but did not need to trim the rear (as on the D1 with the same setup). There is no rubbing at full articulation. I think the stability is more dependent upon the tire I have used than the sizes I have used. The better ride of the OME springs seems to compensate for the higher center of gravity, at least if the lift is in the 2"to 2 1/2" range.

I commute "briskly" every day on some of the steepest, twistiest paved roads in the SF bay area (2500' elevation change in 6 miles), so predictable, stable handling is important to me. With good springs & shocks and a moderate 2" to 2 1/2" lift plus good 265/75-16 tires, you can't go wrong (except for a slight gas milage penalty).

Add 4.11 gears and TruTracs from Great Basin Rovers ($2500 for both, ready to install) and you have a truck that feels way more sure-footed and agile than stock, on or off road.

Hope this helps.

Rich

 

[craig]

You can widen your track to compensate for the higher center of gravity by purchasing rims with less backspacing (to stick the tire out further). I went wider by 40mm on each wheel back when I had 255/70s and was amazed how well the truck handled (with BFG MT KMs, and an OME 2" HD graduated spring lift). I've since gone to a 3" RTE lift and 265/75s (and the same 40mm/wheel offset) and it drives pretty much like it did when it was stock (except for a minor castor issue). I'd like to have a slightly wider track again to compensate. It really makes an amazing difference in the lateral stability of the vehicle.

Stiffer springs also help, but that will reduce the flexy nature of your suspension. It is a bit of a balancing act to find good lateral stability and a flexy suspension.

--Craig

 

[Gary P]

This may sound stupid...but how does increasing the rolling diameter increase gas mileage? i.e. LESS revolutions per mile.
I've got 265/75x16's on my Disco 1 Tdi at present, and MPG hasnt really changed much either way. . .
But I can remember changing the 6.50's to 7.50's on my SWB series IIA and improving the MPG...

 

[JohnGA]

Gary, it's all about the gearing. When you increase the tire size, the engine is working harder to move it. If you got better MPG by increasing the size it was probabaly undergeared for the tires that were on it.

 

[barshnik]

I'm still wondering how much of an effect on side rollover angle (whatever the 'official' name for this is) the very common mod of a 2" or 3" lift (either OME or RT) with 265x75x16's. Anyone know if any static test has ever been done on this?

I'm not crazy about going wider with less backspacing wheels, either - we sometimes do trails where even an inch extra width would be a bit of a problem...

I used to know the advertised max side angle for Discos but can't remember now - lets say 44 degrees. Would the mod listed above reduce that to, what, 42 degrees, 39 degrees, what?

We rarely have clearance issues, but we do often find myself on scarey-steep side angled mining roads. I know that dynamics play a big part, but a static test would be a good starting point.

John F
LV NV

 

[kennith]

My DII has 265/75 mud terrains, around 2.5 inches of lift and no swaybars. The bushings are at 70,000 miles, less the shock bushings, which changed with the shocks.

She will still take a steeper side tilt than the LR test track at dealerships. I always neglect to bring any sort of angle measurement tool along to see exactly what I am doing, but I'm sure the puckermeter keeps me well away from 45 degrees.

Cheers,

Kennith

 

[craig]

If you are worried about roll-over, then you should think long and hard about your position on not increasing your track. If you can't afford to go 1" wider, you probably shouldn't be thinking about putting a wider tire on, and should rule out airing your tires down. I'm pretty skeptical that a 1" wider track would ever prevent you from successfully navigating an obstacle. Unless you are running Pucker Ridge , it seems like a pretty thin reason for not wanting to go with a wider track. The reality is, if you face a lot of side slopes, the wider track will help you FAR more than it will prevent you from falling off of a cliff, or from squeezing between rocks/trees.

Nobody is going to have an answer to your question about max side angle. It varies radically based on springs, shocks, swaybar/no swaybar, tire size, tire width, track width, etc.

Also, If you don't have clearance problems, why do you want to put on bigger tires (and the lift necessary to make it happen)?

 

[barshnik]

<<If you are worried about roll-over, then you should think long and hard about your position on not increasing your track.>>

Thanks to you, I'm going to...

<<if you face a lot of side slopes, the wider track will help you FAR more than it will prevent you from falling off of a cliff, or from squeezing between rocks/trees.>>

That sounds right...

<<Also, If you don't have clearance problems, why do you want to put on bigger tires (and the lift necessary to make it happen)?>>

The lift has been done for a couple of years, I've grown fond of the current height and more secure feel the OME's gave me for better or worse. I will have to give a bit of thought to replacing the 265's with, maybe, 255x70's (size I had on before needing replacements.) Thanks to Firestones '30 day test drive', I can still do this.

Thanks for your insightful comments.

John F