Redleg

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

Bushy

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.

RCC_SaMiaM

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!

Bushy

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

Den052

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>

RCC_SaMiaM

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

Suburbanite

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.

James Lucas

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