Mileage

[michaelsgreene]

I am a commercial transporter of RVs averaging about 60% loaded with every conceivable 5th wheel, trailer, horse trailer, etc.



A major problem has always been mileage. I get around 12 mpg, averaging loaded and unloaded together.



Existing:

98 12 valve, #10 TST plate, waste gate plugged, 250 hp injectors, pyro, 4. 10/4. 11 diff, 215/85 tires. 63 mph = 2400 rpm



While we were replacing the timing gear housing, we discussed a number of options to improve mileage. The main objective seems to be to reduce engine rpm at cruise. (Isn't it?) Power is plenty, but the engine is just running way too fast. Here's what it boiled down to:



22% reduction by adding a Gear Vendors overdrive. Cost about $3000 installed.



18% reduction by switching to 255/85 tires. Cost about $200 more than smaller size (6 tires). These tires are about 33. 5 inches tall compared to 28 inches for the existing tires. 33. 5/28 = 19. 5%.



15% reduction by changing diff gears. $2200 (front and rear)



It seems to me that the answer is obvious here. The biggest bang/buck would be the tires, so I went down to Les Schwab to price tires and several of the guys there totally insisted that changing the tire size would not improve the mileage at cruise and would likely decrease it. Reasons included: increased weight of the tires, increased rotating mass requiring more HP to keep it turning, more throttle required because of the lower overall ratio.



I stood there with my mouth hanging open trying to follow the logic and finally left. I next went to the dealer and asked their "diesel" guy about it. He said pretty much the same thing. The higher ratio just means you have to put your foot into it more to maintain speed. He said that claims of improvement brought about by any means of changing the overall ratio are most likely because the driver has not bothered to account for speedometer error.



I asked another driver with considerable experience my question and he said it MIGHT help a little, but don't go past 235/85. I asked why not and he said it would cause trouble.



Here's what I think: At a given speed, any change to the overall ratio that reduces the engine speed by a certain percentage will improve mileage by at least that percentage as long as you are above the most efficient rpm of the engine, which a recent TDR article puts at 1500 rpm. The wind resistance is the same, the weight is virtually the same. All you are doing is running the engine slower. In addition because my engine is so drastically above the most efficient rpm, I believe that my improvement would be substantially greater than just the strait ratios would suggest.



Have I missed something here? Has there been an amendment to the laws of physics?



Help me out, please!



Michael

 

[BK]

there is truth to the fact that if you have more rubber on the road your rolling resistance does go up. During develop of EV's back in the mid 70's , we did tire testing to find the best ones to run on the conversions. We could directly measure the KW being used to keep certain speeds.



On my own trucks, I've seen my milage go down of v6's and 4 bangers when I went to taller AND wider tires with agressive treads.



But...



With our CTD, your making lots of unused torque at the low end if your foot is half way into it. The slight increase in rolling resistance with a wider tire probably would be offset by using the torque your making anyway. The only way to loose on the deal is if you stayed way off the pedal, drove for mileage and efficiency with making very limited power, when driving in stop and go.



If you going to replace the tires go for a very stiff sidewall. Very highway oriented tread pattern, very high wear rating, narrow but slightly taller tire. Make sure you run the proper air pressure.

With the 4. 10's your probably more than fine. From what I remember reading in some threads here, some guys have reported milage increases with 3. 55 and 285's.



The best I've ever done with my CTD while not towing and really being extremely nice the the pedal was almost 20mpg. Mostly highway, with some milage through town. I've seen my milage vary with the same tires if I didn't airup when towing, or didn't air up enough after coming off the beach.

With towing and keeping my pedal down to the floor I usually get 13-13. 5 mpg. With a ton of chest pounding racing at the lights I hit an all time low of 12. 5, that's the lowest I've seen.

When not towing and running from light to light... around town. . I get from 13. 5 to 15 depending on how nice I was to the pedal. I really have to be nice to get 15mpg around town, coasting to lights, staying with the slower traffic, not getting annoyed and passing the sleeping drivers in the left and right lanes.



What cetane rating are you able to buy? Myself and a buddy with a PS both have seen our milage go up with some rare higher cetane fuel we've come across locally.

 

[LSMITH]

The 235's will help, and the 255's are likely to help too. The 24V would be right at home at 22-2400 RPM, but your 12V wants to turn less RPM, 1600-1900 would give you better economy. You can make the power to pull your loads with a 12V at those RPM too.



My vote? Buy the tires, the 255's. If it does not work you are out 50K miles or so of lower economy (not much if any) but it will look good doing it . If it works you are way better off money-wise.

 

[Oil Ape]

Actually the tire people are right... I used to work in a shop and some people just didn't believe us when we told them...



Bigger tires doesn't amount to any better efficiency and often the opposite is true. The reason they recommended possible going to the 235/85 R16 is it is considered the pizza cutter of the bunch. Now don't get me wrong I love this tire and I'm currently spinning them on my truck.



Here is the breakdown in comparison to the 235/85:

245/75 R16 Stock tire on some, shorter, and fatter

255/85 R16 Taller but is several inches wider

265/75 R16 Slightly shorter and several inches wider

285/75 R16 Tall like the 255 but wider yet

295/75 R16 Tall and wide



The taller and narrower the tire the better your efficiency, granted the engine doesn't require much addition power to spin them. Almost all of the other optional tires are big, heavy, and wide, and the shear width in an aerodynamic sense eat up any efficiency gained by the height. Not only that but those bigger tires often use lower air pressures thus flattening out more and maximising the contact with the road decreasing overall efficiency.



Now the question you are now going to ask is what about race cars? their tires are wide? They are using them for traction on hot asphalt and they need them to hang on in the corner and transmit their power to the ground. Unless you are using your truck for drag racing or driving on the beach or mudbog, you don't need that wide tire and it will eat your lunch on the highway. The 235/85 is also much better as a snow and mud tire in that it applies more pressure per square inch on the ground and gives you better traction and cut through ability.



Now the downside, the 235/85 R16 is only designed to be run on a rim width between 6" and 7" and anything wider, like the ever popular 16X8, is too wide and they will void the warrantee on any tire that is run on that rim. The original rims on my truck were actually 6. 5" but I'm running Eagle Alloy Series 055 that are 7" wide. Alcoa also makes a 7" wide wheel. If you are running OEM rims check the drivers side door tag for actual width.



I really don't think tires will actually give you any appretiable boost in fuel efficiency and my advice would be to sell that truck and buy one already equiped with the 3. 55 differentials

 

[Archive]

The law of physics

You sound conviced... .

at 12 mpg I think it's worth a try. Sure, it's not my money, but I'm in agreement with you... . 2400 RPM is fairly high as a constant and your engine is beefed enough to pull at a lower RPM.



For cost reasons I would try the taller tires first and see how much it drops your RPM and the impact it has on your mileage.



If that's a positive, then look at the Gear Vendor unit.



One thing is for certain... if you don't do something, you'll continue to get the same results.



My 2 cents worth !

 

[michaelsgreene]

Thanks for all the answers. There seems to me a mix of opinions. Let me clarify:



I don't care (much) what it looks like.



I drive 100% on the highway.



I have gobs of power.



I have 6 inch rims and no money for new ones.





Michael

 

[Oil Ape]

You must be a dually. The only tire option you have then with a 6" rim is the 235/85 R16. Any of the other tires require a wider than 7" rim or they too void the warrantee of the tire. If you are indeed a dually the wider taller tires also would have spacing problems and would rub against each other burning up the sidewalls. What is the standard tire on that beast? 225/75 R16?



Honestly the 3. 55 trucks are pulling machines. I have a friend out of Lyman Wyoming that delivers trailers for a living and often pulls double trailers all the time and he mistakenly bought one with with 4. 10 rear ends and after about a year swapped for one with 3. 55. Much better efficiency and with just that efficiency, if you are driving quite a bit will pay for itself.



I have a 2500 with 3. 55 and highway mileage under 70 mph is about 21-22. Best I've gotten empty is about 24 with a good tail wind. Over 70 mph I average about 19-20 empty. City and trailers drop it probably into the midteens but I don't have any numbers to prove it.

 

[dan_gilson]

I saw previously that you said you didn't want new rims but...



19. 5" rims with low rolling resistance commercial truck tires might buy you some. You can get larger diameter tires that are not that much wider. They will increase your weight carrying margin when you are loaded. I was figuring it would take about 300K miles to break even on a new set if Rickson wheels and tires.



You have a 12V so, you should be better milage at a lower RPM. 2400 seems a little high and will definitely rob you of some economy. The closer you can get to say 2150-2200 the better. 2200 is published peak HP on a 12V. It drops off from there. Even a little lower might be better to give you some overhead for passing and such.



There was page that I took from one of the forums that converted tire size to RPM. It is pretty close. If you can not find it PM me and I will post my revised version on my site and point you to it.



The 24V tends to run better at a high RPM.

 

[Steve Roseman]

Best Fix

The best fix of all that won't cost anything... . SLOW DOWN!

I know you've got 4. 10 gears, but how fast are you going at 2,400 rpm?

 

[michaelsgreene]

I have looked at the Rickson wheels/tires, but the payback is just too long.



I spent a year running 70 - 75 (almost redline) getting 6 - 8 mph. I was clued in to the speed issue and now drive 63 all the time and mileage has improved to 12.



As mentioned before, I drive 63 mph all the time. RPM at that speed is 2400.



Peak horsepower and peak efficiency are two very different things. Peak efficiency for the 12-valve was listed in a recent TDR article as being 1500 rpm.



Btw, these are actually "read the odometer and divide the by how many gallons you put in" figures, not the crap that comes off that fancy meter on the ceiling of the newer rigs.



Michael

 

[dan_gilson]

1500RPM seems way to low. It is below max torque and well below where the HP comes in. Typically peak effeciency is where the torque and HP curves cross. I was suggesting that you try to get closer to it.



The stock 12V smoke above 2250RPM. This means that they are passing unburnt flue. This is maybe not your case with the plugged waste gate. I would take a peak in the Rt side mirror at cruze and see if you have any of the black stuff vsible. The HP curve also is going down quickly after 2200RPM. Torque is also dropped way off. So above 2200RPM is definitely not as efficient.



On my 96 12V I was getting ~22 Mpg unloaded. I have a pretty light foot and it had a 3. 55 rear end. My dad had an older 12V with 4. 10 rear end. He was getting around 14 Mpg with a 11k# trailer. He was getting 17 unloaded on a 2x dually. So, with your mods you are probably in the range when loaded. Unloaded I would expect more as long as you are not on the go pedal hard.



I have some of the same weirdness in my 24V. It gets 17Mpg no matter what, driven hard, driven soft. It seems to get better milage in the hills. Occasionally when in the hills I get 19Mpg. The high the temp gauge the better it seems to like it. The 24V is suppose to be more efficient and slightly better in fuel economy than the 12V. But I have not seen any of it. The 2 training wheels that I have on the 24V compared to the 12V will knock off some but I can not believe 5Mpg.

 

[Maximus]

When I put 31575R16 tires on my truck I lost about 3 MPG. This was taking the tire size in to account for mileage and before I added a 2" lift. I do have a 24v w/3. 54 LSD.

 

[Lsfarm]

Friction losses

Michael: I believe that the tire shop guys are used to putting tires on gas vehicles. There is often a loss of overall fuel mileage when installing too tall tires on a gas vehicle since they just don't have the torque the Cummins has and have to compensate with more throttle.



Where you will gain mpg is by losing some of the internal engine friction with lower rpm. If you had 3. 55 gears already I would say stay where you are, but with your 4. 10's I would definitely try the tall tires, and just make sure the sidewalls don't rub on the duals. Like someone else said, you will either gain or at least have the same for 50K or so, the life of the new tires.



Have you gone to fully synthetic lubricants for your truck? This can be worth a few percent in fuel mileage, especially in very cold weather.



Let us know how it works out.



Greg L

 

[jsnyder]

Michael,



When my stock tires went to pot on me I put on 265/75 R16s (because Costco wouldn't put on 235/85's ). I don't know that I've really seen that much difference loaded or unloaded.



I never really hand calculated with the old tires. Did it for kicks with the new ones, taking into account the difference in the odometer, and found the the trip meter wasn't too accurate.



Anyway, I'm rambling and not answering your question . With the 4. 10's it might help you. It will definitely lower your RPM's. Based on a gear calculator it dropped my 3. 55 to 3. 3 I believe. I think I went out of the peak efficiency range for the towing I do, but then again I can't verify that because it would be basing against trip meter numbers.



I'd say give it a shot. One thing on tire size, you'll find a much better selection (and price) on 235/85's versus 255/85's. Also, most 255's don't seem to be 10 plys.



Jason

 

[michaelsgreene]

Some interesting comments from an article in TDR, issue 33, Aug/Sep/Oct 2001, pg. 28 (Robert Patton).



"In layman's terms, brake specific fuel consumption is the efficiency of an engine... BSFC tells you how much fuel it takes your engine to produce each horsepower. Look at the BSFC for the ... (12V)... is lowest at 1500/1600 rpm. "



"What does this mean to you? ... it would be best to put your engine on cruise control at a speed corresponding to 1500/1600 rpm At (this speed) you've (also) got the torque peak of the engine available to keep your speed constant as you encounter a grade"



"The 24-valve is an entirely different story. ... the ... data on the 24-valve engine is not a single curve - rather a collection of fuel maps that differ based on timing, engine load, throttle position, etc"



(My comment: the only possible problem I can find with this approach is that BSFC is measure with wide open throttle position. )



Michael

 

[klenger]

I don't think that the milage gain by changing gear ratios (while not chaning tires) is proportional to gearing reduction. In other words, chaning from 4. 10's to 3. 55's (which is about 15% reduction in RPM) will not increase your milage by 15%, especially if you are towing or city driving some of the time. I think an average of 7% increase in milage is more reasonable. RPM is not the only factor in milage; wind resistance at higher speeds becomes a bigger factor.



For the money, I think your best options are:

1) Slow down if possible (65 MPH works for me).



2) Taller tires w/o adding width, if possible.



3) Synthetic engine oil and axle lube.



4) Buy a little more fue with the money you didn't spend on gear changes.

 

[michaelsgreene]

Thanks for the comments, but:



I already only drive 63 mph. Much slower is just plain dangerous on the freeway.



Wind resistance will not change, because I will not be going any different speed. Any of the changes I mentioned in the original post will have absolutely no effect except to slow down the engine rpm.



Ovbiously there will be changes during acceleration and on grades as a trade-off for changing the overall ratio. Since I spend the vaste majority of my time just cruising on the flat, that is where I will gain the most benefit, even if I do have to downshift for a grade.



Thanks and keep the comments coming.



Michael

 

[Big White Beast]

Michael...

I went through the gear change thing twice. Looking to stay in the power band more so than for mileage.



Had 4:10's originally and swapped for a 3:54. Absolutely no change in mileage. This is with a mix a freeway and town driving. Finally spent the money on 3:73's and found the best combo. Still absolutely no change in mileage. Again a mix of driving.



Stock my truck got 11. 5 empty and 10. 5 MPG after I had it loaded down.



After modifications I upped it to a pretty steady 12. 5 MPG. Best I ever got was when I unloaded it and drove to Phoenix to have my transmission mods done. That tank got me 14 MPG.



Disclaimer: I'm not one to drive for mileage!



Other than tires which may or may not help any it would be hard to recoup any costs for gears with mileage. Saving maybe $1 or $2 per tank is a lot of tanks to pay for any mods.



Garrett

 

[Steve Roseman]

Michael,

Are you sure you're speedometer is acurate? I'm not doubting you, but 2,400 rpm seems aweful high for 63 mph.

I know the overdrive ratio in the autos is higher, but with 3. 54 gears I'm running 90 mph at 2,400 rpm.

What's everyone with 4. 10's and 5 speeds pull for rpms at highway speed?

 

[michaelsgreene]

The speedo actually indicates 1 mph less than the GPS does. Also verified using mileposts over a 100 mile span as being within 2%.



63 mph = 2400 rpm



Absolute top speed is 75 - 80



No, really!



Michael