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dragonriot
10-11-2006, 09:43
Most of us here at The Diesel Page most likely use our trucks for towing. Some others may own a diesel because of the cost of gasoline, and still others may own a diesel for recreational purposes (off-roading.) The towing gurus here will probably say they don't need or want bigger tires, as they have been pretty well optimized for doing their specific jobs, but those of us who drive for mileage or for fun could benefit from a bit of Tire Tech. If you are an occasional towing guy, or a towing guy who has a buttload of power and still running on 265/75R16 tires with 4.11 gearing, you may want to pay attention too.

The whole idea of MORE POWER! is to get the most overall performance out of our trucks. While you may require some "trimming" to get bigger tires to fit, the lowered cruise RPMs are often worth the sacrifice. Plus, 33" tires look sweet under a set of Bushwacker Fender Flares... In this short article, I'll discuss the benefits of running a 285/75R16 tire in all of our trucks, duallies and towing guys included.

In all of the previous MORE POWER! articles, engine performance has been the key topic of conversation, which is fitting considering this is a Tech forum for GM Diesel Engines... There is a lack of other topics relating to the general performance of our trucks, including suspension upgrades (not necessarily lift kits), tires, wheels, and improving aerodynamics. While I am not an expert on any of these thing, I have worked with all of these upgrades at one time or another at the shop I worked at, or on the 86 and 70 K5 Blazers I used to own. Perhaps the worst issue with GM trucks is aerodynamics, but that's another article...

Consider your current highway cruising RPM. We'll use 265/75R16s as the base tire size, and we'll look at 4.11, 3.73, and 3.42 gearing. If you have 4.11 gears and an overdrive transmission on your tow rig, then your cruising speed at 55MPH will be about 1800RPM according to the calculator at www.4lo.com. By adding 1.2" (285/75R16 nominal diameter) to your tire diameter, your new speed at 1800RPM is 57MPH, or 1730RPM at 55MPH... a noticable difference. Of course, by adding bigger tires, you effectively lower your rear end gearing. With 4.11s and 33s, you would have a theoretical 3.86:1 axle ratio. At 65MPH with the same specs above, you'd be cruising along at a very comfortable 2050RPM, a full 150RPM lower than your normal cruising RPMs at 65MPH, netting a decent savings on fuel, and extending the life of your engine. (I've just realized through my calculations that I have 4.11s, not 3.42s like I originally thought.)

Moving on to 3.73 and 3.42 gearing, more comonly found in the SUVs and maybe a few of the 2500 trucks, changing to 33" tires will lower the rear end ratio to 3.50 and 3.21 respectively. While not optimal for getting a load moving, having a 3.21 rear end instead of a 3.42 rear end will get you a few extra miles per gallon, and your truck will look pretty cool too. Based on the calculator, at 55MPH with 3.42 gears and 265/75R16s (31.6" nominal diameter) you would be turning about 1500RPM in Overdrive. Moving up to 32.8" nominal will drop your RPMs to about 1445RPM, not bad. At 65, with 31.6" tires, you should be spinning at 1775RPM, and with 32.8" tires, it drops to 1700RPM. Similar numbers will be found with 3.73 gearing, about halfway between the calculated values for 4.11 and 3.42.

With a bit of trimming, and a set of Bushwacker Flares, you can even fit 305/75R16s under our fenders with no lift. Those are 34" nominal size tires, so you can imagine the huge drop in RPMs they would get you. The only issues you may find with tires this size would include reduced handling ability due to a higher center of gravity, and a slightly rougher ride because of the bigger tread of the taller tire. Also, you might get a bit of floatation in snow due to the extra width of the tire, which is not at all desireable.

In any case, you can see that adding a slightly larger tire can effectively improve fuel mileage to a point... there does come a time when you start losing fuel mileage because it takes more power to get your tires moving in the first place, and taller tires are heavier tires, so it requires more power to keep them turning. Up to 305/75R16s, you should still gain mileage and lower your RPMs when cruising, and even still have enough power (with 4.11s or 4.56s) to get pretty much any load moving. Past that size, extreme lifts and suspension alterations become necessary and very expensive.

Craig M
10-11-2006, 11:10
The premise of the discussion is good, but not sure if it works in the real world. With friction and rolling mass neglected bigger diameter tires will decrease rpm's. How much fuel mileage is gained by a couple of hundred rpm reduction is unknown, but I do not think it is close 2 mpg. In actuallity we cannot neglect friction and mass. The wider tire has more contact area and weight. The increased truck height may or may not affect wind resistance of the vehicle (It does affect center of gravity of the rig). All the guys that I know who have added bigger tires and done any raise to their trucks have seen a loss of fuel mileage. Skinny large diameter tires will give us the best mpg, but my 285-50R20's look so much cooler than 235-75R16's.

dragonriot
10-11-2006, 19:57
Haha... that's awesome...

I agree with you on the wider tires causing a higher drag coefficient, but I know I got better mileage on my gasser with 33s than I did with 31s... and the 33s were 2 inches wider...

murphyslaw
10-11-2006, 21:09
adjusting the tire highth is good for fine tuning, but wider does cause more resistance I have two sets of 38" tires one set is 15" wide and the other is 11" wide. I get 2 more mpg with the skinny tires. these tires are the same brand and tread design so I know its not just the difference in "tires". its the difference in the width of tire. also wider is heavyer= more rotating mass= harder to turn= more fuel to turn them(yeah i know you could have figured that out but im bored).

DA BIG ONE
10-12-2006, 04:30
I've gone to 3.42 diffs and 33+" tires and found tire compound and tire tread must be considered.

Then there is the weight/drag increase from say aftermarket bumpers, winches so thing go to hell quickly as mpg is concerned.

I"m on 285/75/16 silent armor's now and they seem to be ideal for this heavy burb on highway and off road, alignment mustbe right on spec.

Another consideration is tranny, it's condition, and if you can use an aftermarket controller w/it?

I'd ponder to say H-1 wheels w/37" tires can be used with frt hub dulie adapter and perhaps a 14 bolt ff from a van in rear using bushwacker cutout's.

moondoggie
10-12-2006, 10:16
Good Day!

I changed my 95 pickup from ~ 8% oversize tires (285/75R16) to ~ 4% oversize tires (235/85R16) at virtually the same time as I installed the Gear Vendors Aux. OD. So, my effective rear ratio went from ~ 3.81 to ~ 3.07 (with the GV engaged). It's hard to be sure what effect it had on my mpg, but my best guess is it went up 2 mpg. (I am EXTREMEMLY anal about mpg - I check every tank, & take any odo error >= 1% into account in my calculations.)

I am not in any way doubting anything you've said, but please remember: my mpg only improved ~ 2 mpg with an rpm drop of ~ 19%. To achieve this same rpm drop with just tires, they'd have to be 37.8" tall. That size tire would probably be pretty heavy, & unless the aspect ratio was a lot higher than 75%, they'd be awful wide too. The tire change arguement would carry much more weight if accompanied by hard, irrefutable data. (I have all my mpg data, & even with that I'm not comfortable making hard mpg claims. I've been involved in scientifically collecting data for too long to stick my neck out very far. I just present my data to whoever's interested & let them draw their own conclusions.)

Please take this in the spirit given. I'd like to pursue a technical, NOT emotional, discussion of mpg. Heck, I'd put cat pee in my tank if I could prove it improved my mpg. ;)

Blessings!