Tire size - Fuel Mileage
 

I have been told by several die hard diesel guys that going from the stock tire to a 35" tall tire should show gains in the mpg department.

Right now I am running 285/75/16 tires, if I move to 34x10.5 Super Swamper LTB's would that pose a gain in mileage since the tire is staying the same width, just gaining in height?

Or would I be better going back to stock tire height to gain better mileage?

I have a 93 W350 single cab that is my towing truck which pulls a 27' car hauler with an 84 Ramcharger built for offroading. I do alot of traveling to offroad meets and I want the max mpg out of my cummins. Is there anything else I can do (remember I am looking for fuel efficiency not power)? I currently have have an AFE magnum intake setup and a Banks stinger setup. Thanks for any suggestions.

Re:Tire size - Fuel Mileage
 

No experiences with larger then stock tires?

Re:Tire size - Fuel Mileage
 

Changin tar sizes is like changin gear ratios. Putting on a taller tire may adversely affect your towing ability. If you have 4:10 diff. gears installing 35's may improve your mileage running unladen. Tread design affects mileage as does width of the tire - something to do with rollin resistence.<br>Swampers are not known for their mileage and wear characteristics. Beeg lugs slappin pavement don't help mileage!<br>Check out the threads in the towing/RV forum for mileage figgurs.<br>M .o2 worth, Chris

Re:Tire size - Fuel Mileage
 

I tend to agree... you'd best plan on losing some MPG rather than gaining...<br>Those tires take a fair bit more effort to turn and keep turning.<br>This is based on an '82' Toyota shortbox.... don't laugh....<br>That truck was a full balance/magnafluxed/ported &amp; polished/twin Webber/supercharged 5spd conversion with middle of the road gearing.<br>The mileage took a serious dump when we put the big 'dogs' on for our hunting trip each year.<br>BTW, that little rocket would turn 6000 RPM and hum all day without so much as a burp.<br><br>On the up side we got thru water that broke over the hood and hit the windshield... nasty..<br><br>bob.<br>

Re:Tire size - Fuel Mileage
 

This is my exp. Not much but it also confers with the above. When I went from the stock tires to B/F Goodrich I went to 33 inch tires. This is the max I could put on my stock suspention. In face the left tire will hit the fender on a right hand turn with a little bump. I would say without measuring it is about one to one and a half inches from the lower front of the fender.<br><br>When I went to the taller tires I didn't notice much difference in milage but I noticed a big difference in acceleration. I have 354's in er and it really gave er longer legs. If I were goin to do it again I would consider a lower rear with the taller tires. I do like the looks of them better than the stockers though. When pullin it doesn't take much for me to have to drop down to 4th to keep the 12k fifth wheel rollin. At 60mph I am only turnin about 1600 rpm. Unless I am on the highway most of my drivein is in 4th with the fifth.

Re:Tire size - Fuel Mileage
 

I don't think a toyota qualifies as an example, main reason the torque behind the diesel.<br><br>The main argument of the guys telling me mileage will improve with taller but same width tires was that the torque output of a cummins is so high. That is why I say the toyota isn't a good example. In one of my Ramchargers, I dropped in a 440 and with 40x17 offroad tires and 4.56 gears and I was getting 12 to 14 mpg, where most stock guys were getting less then that with a Ramcharger (10 to 12). That is why I do see their point with the torque and tall tires.<br><br>I have 3.54 gears in the cummins right now, they had said if it was 4.10's to diffenetely get 35&quot; tall tires, but with only being 3.54 should I go back to stock tire height? I am running 285/75/16 mud tires now.<br><br>Thanks for the replies!

Re:Tire size - Fuel Mileage
 

When I went from the stockers to 33x12.5s I saw about a 1/2mpg decrease BUT it corrected my speedo ;D [eyecrazy]<br><br>As soon as I can get a lift kit I'll be going to 35s and possible 4:10s<br><br>Jay

Re:Tire size - Fuel Mileage
 

What is your fuel mileage at? Right now my 285/75/16 are roughly 33x11's and I get 18 mpg with trips running in the 75 mph range. I see why you lost mileage, mainly due to using a wider tire, more surface area equaling more friction and drag. Anyone using any 33&quot; or larger tire in the 10.5&quot; wide range?

Re:Tire size - Fuel Mileage
 

I consider my 285/75/16s as 33X10.50s and found that my mileage stayed about the same. The speedo is correct with 3.54s. My fuel mileage is only 15mpg with an automatique, but I'm pulling lotsa long grades at 70mph from 2,400ft to over 7,000ft.

Re:Tire size - Fuel Mileage
 

what size are stock tires?

Re:Tire size - Fuel Mileage
 

235/85/16 ? I think!

Re:Tire size - Fuel Mileage
 

the guys running the 33x12.5 tires are claiming loss in mpg, but since the true height isn't 33&quot; it shows that they are losing mpg because of tire width.<br><br>You having the same size tire as I which is 33&quot; tall but more narrow and you said you stayed the same.<br><br>The LTB's I am talking about are only 1&quot; different in height then the 285/75/16. I guess they really wouldn't hurt then?<br><br>I guess nobody has recorded better mileage by increasing tire height and keeping same width. Stock tires if 234/85/16 are about like 32x9.5's, so going up an inch and wider an inch netted nothing. Maybe because the height gained but the width took away?

Re:Tire size - Fuel Mileage
 

The Firestones that were on my truck were shorter and narrower than that, they were leettle. They also had a very mild highway type tread design. I suspect that tread design plays a big role in mileage, that's why I would avoid swampers. The tires I have now are a typical 4 row AT tread, a little more resistence than the other tires - I needed that type of tire for driving on snow and ice.<br>Chris

Re:Tire size - Fuel Mileage
 

although it is true that a wider tire puts more contact patch to the ground thus increasing friction, there are many things that come into affect. when you combine all these things together even if the tire is the same width or larger you increase rolling resistance.. the increased mass or what is refered to as unspung weight also increases. all these things unsually lead to a decrease in fuel mileage. not to mention by raising the truck you increase the drag. drag increases exponentialy above 55 mph that is why the federal government use that speed limit. your best gas mileage is usually gotten at the lowest rpm peak torque is generated at. so determine what size tire will put the engine at peak torque at a given speed and that should give u the best mileage at that speed.

Re:Tire size - Fuel Mileage
 

to me unsprung weight and even rolling resistance from going 1&quot; taller and wider shouldn't matter to a diesel. The diesel is a different animal then gassers. I think people base alot of assumptions on gas engine experiences, but to me those are irrelevent since the diesel is making almost twice the torque outpower (power).<br><br>I am not sure if this is true, but to me a cummins is much like a gas engine truck that has really really low gears, by going with taller gears or tires, mileage can be improved. At least that is what I have been told, my main reason for asking everyone for experiences with cummins and larger tires is to see if that is true.<br><br>Thanks all for the replies, and to thuse who will post more.

Re:Tire size - Fuel Mileage
 

Don't forget the bigger tires are gonna take more effort to stop, and will really make a difference trying to stop while towing.

Re:Tire size - Fuel Mileage
 

RCC_SaMiaM,<br><br>Re: the toyota example....<br><br>The only time the torque would be a factor is in the initial stage of &quot;launch&quot; so to speak....<br>After that, all the effort is a result of HP, not torque, at least that's my take on it.<br>My thinking is this... you have the low end grunt of the Cummins that can never be matched by the type of engine I described, that's very true... but... once the vehicle is moving at anything near highway speeds you are now in the HP range of both engines.... otherwise you'd be doing all your driving in the 1500-1800 RPM range to make use of torque.<br>Wouldn't you see the benefit(s) of HP in this case???<br>The dodge CTD makes it's torque, as noted above, in the 1500-1800 RPM range, and all it's HP in the 2000 + range RPM.<br>The vehicle I was referring to did the same thing except the torque range was correspondingly higher in RPM (and far less torque), and the HP was also in correspondingly higher ranges.<br>Our average running RPM with a camper, utility trailer/boat/full supplies for 2 weeks was approx. 4000 RPM or higher.<br>We'd pull a posted 8%+ grade in 4th gear turning 4500-5500 RPM... right in the upper middle of the HP band for his engine build.<br>No way on this planet could he hope to produce the gut buster low end power of the CTD, but at the upper ends, this thing was a full blown tornado in a soup can....<br><br>BTW, no flame intended here, just wondering if I was on the right/wrong track is all..<br><br>On a lighter note, have any of you ridden in an old DC3???<br>Well, this thing was a nasty butt monster that sounded like the inside of a DC3 on acid..... I resorted to earplugs to just survive a 4 hour trip without total brain fade and bleeding ear drums....... but it did surprise a few guys ... even in the bush..<br><br>Bob.

Re:Tire size - Fuel Mileage
 

I see your point, but I thought the cummins made good torque throughout the powerband unlike a gas engine. And since the rpm limit of a diesel is so low, I don't really see hp being a factor in a heavy diesel truck. I do see your point, but I think it makes more sense with a car rather then a heavy diesel. But I could be wrong. I am looking to gain fuel efficiency, especially while towing. Thanks for your input!

Re:Tire size - Fuel Mileage
 

Hey, no harm no foul.... I'm looking to learn too.. ;D<br><br>bob.

Re:Tire size - Fuel Mileage
 

Increasing tire size without increasing width should most likey cause a increase in mileage. The truck will roll the same as far as rolling resistance. I have a 3.60 rear end ratio, but feel it would do better in mileage if it were a 3.05 or such. I get 20 mph with stock tires. By changing tires (or rear end ratio), I believe I could get 23-24 mph. I pulled a 10,000 lb fifth wheel and had no problem taking off with the stock tires. Probably wouldn't notice it either with bigger ones. I have had a lot of Ford powerstroke diesel owners complain about mileage. They have told me horror stories of anything from 8 mpg to 14 mpg. I went down to the Ford dealer and looked at some powerstrokes. All on the truck lot had 4.10 or greater rear end ratios. Personally, myself, I am not into pulling RAILROAD CARS, I like to get the best of both worlds. Pull adequately and get outstanding mileage (compared to gassers).<br>

Re:Tire size - Fuel Mileage
 

This is what I thought, but wanted to make sure. Thanks.

Re:Tire size - Fuel Mileage
 

Not meaning to insult anyone ... but ... I hate to break this to you ... diesels are still subject to the laws of physics ::) ;D. <br><br>This may help in your decision making:<br><br>RPM = (336 x MPH x RGR x ODR) / TD<br><br>where<br><br>RGR = Rear Gear Ratio<br>ODR = OverDrive Ratio<br>TD = Tire Diameter (actual, not nominal)<br><br>Say you're looking for a TD to get the RPM at 80MPH in the ~1800RPM TQ/fuel mileage sweetspot with 4.10RGR &amp; ~0.75ODR. Solving for TD:<br><br>TD = (336 x MPH x RGR x ODR) / RPM<br>___= (336 x 80 x 4.1 x .75) / 1800<br>___= 45.92&quot;<br><br>Waaaaay too biiiiiig!!!<br><br>Alternatives are to either re-gear, slow down or install an auxiliary OD.<br><br>Re-gearing to 3.54's yields 39.65&quot; -- still waaaay too biiiiig! <br><br>Slowing down to 70MPH yields 40.18&quot; -- still waaaay too biiiig! <br><br>Adding something like a Gear Vendors OD with 0.78 OD yields 35.82&quot; -- that's more like it. In other words, close to being practical.<br><br>So ... if a RPM just slightly out of the engine's sweet spot is acceptable, allowing it to increase to 2000RPM &amp; adding the GV OD would produce a desired TD of ~32.2&quot;. This could easily be achieved with a 255/85 tire which is ~33&quot; in diameter. It's only 20mm (~0.79&quot;) wider than a 235 but the sidewall is ~217mm vs ~200mm which translates to a ~1.34&quot; (17mm x 2 / 25.4) taller sidewall overall. If that isn't acceptable there's at least one 235/85 that I know of that matches the 32.2&quot; TD &amp; that's the Michelin XPS Rib tire. There may be others. IMO the very slight increase in the tire contact patch would have either no or a negligible impact on fuel mileage. As for the &quot;... added wind resistance ...&quot;. I don't think so! In either case the reduced RPM's would more than offset either.<br><br>But ... all the above goes right out the window when much wider &amp; more aggresive tires are mounted. 1st of all there's the obvious added friction -- tire-to-ground &amp; tire-to-air (wind resistance) &amp; then in stop-n-go type driving there's the added mass that has to be continually accelerated. For these reasons fuel mileage WILL SUFFER (decrease)!<br><br>One more note on air resistance: a truck body's aerodynamics DON&quot;T change with height. At least not at the stock vs big tire heights. Might be a little different story if the truck started off 'slammed' to the ground with a full ground effects pkg &amp; then was raised but otherwise ... fugedaboutit!<br><br>Good luck.

Re:Tire size - Fuel Mileage
 

OK you want real world experience.... My 2001 came with 265 75 r 16's and got 17 mpg the way I drive. <br><br>I lifted it and put Super Swampers (dreadful tires) 36X14.5X16.5's on it and my milage dropped down to 15. BTW those tires actually measure 34.5 tall and 12.5 at the widest treads! They kinda fib there.<br><br>I loved the RPM drop and the loss of MPG was minimal in my opinion. I don't pull much but the truck had to come out of OD alot more because it was lower RPM and the tires seemed to bog it down some but not too terrible. Factor in that I am higher up in the wind too.<br><br>The Super Swampers I was glad to get rid of... OMG they were so noisey and rough. There was never any sanctuary in the cab of my truck...NEVER! They caught ruts in the road and the rodeo was on! My truck has dual shocks AND dual stabilizers.<br><br>I put the Parneli Jones Dirt Grips on now and they are measuring a little taller almost 35&quot; and milage came up .5. Ride is MUCH better, Noise is almost non-exsistant, steering is easier, and they do well enough off road.<br><br>Answer to your question... Will it lower your RPM's and give you better highway speeds? You bet ya (my comment) But will you want to go very fast with Super Swampers after they aren't new and the ride has dimemished? <br><br>With the 4:10's I reccomend going with bigger tires or higher gears. I would go bigger tires! The hell with the milage if you lose any it won't be much. <br><br>Since I have made my &quot;Bombs&quot; I don't have to come out of OD anymore. My truck has 3.55 gears so I am now geared way up there. I think they say 3:22 is what I am geared equivalent to.<br><br>My gearing is different but I won't tell you there was no difference.<br><br>Off the line when totally stock, I noticed a huge difference when I put the 36&quot; tires on. Now I boil them [laugh]