2nd Gen Non-Engine/Transmission Mileage Test... Proof is in the Pudding

[jponder]

I ordered my 285/70R19. 5 tires today. They will drop my rpm by 11% or so and I will be very tall geared with a 6 speed and 3. 54. I'd like to do a mileage test to see the effect that will have. I will drive home to La. with my michelin 265s on a 16 inch rim and then change the tires and drive back. My initial thought was to drive slow but I realize that wouldnt give the average/normal driver much info. I love to drive slow to break my personal records( I can do that latter) In the interest of giving info that might be beneficial to the average driver I thought I would ask what most drivers cruise at with an upper limit of 70. In fact I will be using GPS for average speed as I dont use mileage killing cruise control. Since I use a throttle cable I will have to have a target avg MPH as I will be hitting high speeds going down hills and will let the truck slow climbing hills. Is average speed of 60 to low for people to gain any value from? How about 65? I will be running just a VA with no mods and the altitude drop from Atlanta to Amite, La is only about 600ft over 500 miles. I am very interested in the outcome. I thought some about building a plyboard cover that would come up to the cab and feather to the tailgate down. I may just save that for another trip. I dont have the front lift yet so I may just run the back tires back and carry the other tire with me. This will make up some for the 100 lbs or so of the 2 michelins I will leave in La. If any one has any ideas on how to make this more accurate and to get better info let me know and I will do it. I am proud of the mileage my truck gets but I can tell you straight up I will report EXACTLY the mileage numbers I get. I will be surprised if taller tires dont help me but if they dont I will tell the tale.

 

[LSMITH]

Not to be rough but your expirement means little to me. I run fairly hard as I collect mileage AND hourly pay. Time is money for me and my customers so I need to get there, I average over 70 mph on a 400 mile trip with a speed limit of 65 or less the whole way... ... .



I raised my RPM at cruise recently from 1950 to 2075 ( 80 MPH)and did so with tires 1. 5"+ narrower, it cost me 1+ MPG.

 

[jponder]

Dont worry about being rough. I mean I'm going to do it anyway and I need to pick some MPH to at least see if taller tires will help Mileage. I think it will, but who knows. I think at high speeds like what you are talking about the advantage of a throttle cable vs cruise is mitigated some what for instrance at 100 MPH there would probably be little difference. The only other option I would have would be to pick some average RPM but that wouldnt yield anything aluable would it? I am going to take your response as a vote for avg speed =70. I can do that. I sure will get their quicker at 70. Remember this isnt about money or if you can justify time vs speed or any of that, its just about at speed X will taller tires improve mileage period thats all the question is to me

 

[BigUgly]

JP;

I'd sure be interested in hearing how it turns out. I've done both the bigger tires and the Timing (EZ instead of VA). I never realy noticed any diffrence at 73 mph (my cruising speed). I think this is mostly due to the 4. 10s, at 73 MPH (65 indicated), I'm right at 2000 RPM. I think that any milage I picked up going from 2150 to 2000 was negligable. Now, at times, I've benn forced to cruise at 65 MPH or even 60 MPH, when I have, I've noticed a serious MPG change with just a small drop in RPM. I think it would be good to have hard proof of the effects at a lower RPM. I say, set the "Hillbilly Cruise Control" at 70 MPH on the level and let us know!

Josiah

 

[jponder]

Originally posted by BigUgly

JP;

I'd sure be interested in hearing how it turns out. I've done both the bigger tires and the Timing (EZ instead of VA). I never realy noticed any diffrence at 73 mph (my cruising speed). I think this is mostly due to the 4. 10s, at 73 MPH (65 indicated), I'm right at 2000 RPM. I think that any milage I picked up going from 2150 to 2000 was negligable. Now, at times, I've benn forced to cruise at 65 MPH or even 60 MPH, when I have, I've noticed a serious MPG change with just a small drop in RPM. I think it would be good to have hard proof of the effects at a lower RPM. I say, set the "Hillbilly Cruise Control" at 70 MPH on the level and let us know!

Josiah

Who you calling hillbilly Doesnt deem to be much interest in it. It seems like there was some post about the effects of larger tires effect on mileage. I know there was some post on effect of timing boxes. I could go ahead and run the bigger tires and check mileage change with timing box Vs no timing box. Is there any interest out there in that? I havent made a run for mileage in awhile. From what you said you think there is a big change somewhere lower than 2000 K huh?



LSmith> Do you mean you went to a smaller thinner tire and lost 1 MPG? that is what I would have expected but I think Ifflem said that people going to larger tires experienced no gain and that would go counter to what I would expect.



If anyone has any interest in either change in mileage due to bigger tires or change in mileage due to adding a timing box, chime in and I will do a run to check one or the other. Unfortunately I cant do both with one round trip.

 

[illflem]

I'd be interested in seeing if the taller tires make a difference.

I'd say to get real world numbers go the posted speed limit.

 

[jponder]

Originally posted by illflem

I'd be interested in seeing if the taller tires make a difference.

I'd say to get real world numbers go the posted speed limit.

I was afraid you would say that. I makes sense though, once I put the big tire on I'm not going to want to take them off to check mileage. It will be another week or so till I get the big tires and I need to get the front end leveling kit before then. I geuss I'll get the lift and make sure I dont need to go higher then put the big tires in the bed and drive the small ones home and change them. I'm going to leave the small tires in La anyway. That will make a difference of about 600 Lbs and that may skew the test in favor of the small tires unless I put some weight in the truck to compensate for the return trip. My son is shipping out to Afganistan on the 15th so I need to go see him before then. What do you think ifflem timing box for both legs and change the tires or no timing either leg and change the tires. I can average the posted speed easily.

 

[RonaldLarson]

Maybe..

I missed something, but you seem to be guessing at the difference between tire sizes. In order to accurately check mileage, you must know the correct number of turns per mile for each tire, and do a lot of calculating. An easier way (proven by my GPS in speed terms) is to measure the actual percentage difference between the tire sizes, and add the percentage difference to your odo reading. This corrects the mileage driven.

With larger tires, you travel farther than the odo records. My size difference is 6. 7%, from 245/75 to 265/75.

Also interesting to add here, cruise control actually increases mileage, and does not decrease it. It only acclerates when needed, and backs way off throttle when descending hills. Having a set throttle going down hill wastes fuel and attracts badged types.

Ron

 

[LSMITH]

Originally posted by jponder



LSmith> Do you mean you went to a smaller thinner tire and lost 1 MPG? that is what I would have expected but I think Ifflem said that people going to larger tires experienced no gain and that would go counter to what I would expect.

Yes, I went from a 285/75/16 to a 235/85/16. Speedometer error went from 10% to 5%, RPM went up about 125 for any speed. Rolling resistance should have dropped as well as aerodynamic resistance, but I LOST 1 MPG + (+ because I don't know for sure yet, I only have 3K miles on the new tires and like to average over at least 5K).

 

[illflem]

I would think that smaller tires would only lower mpgs in stop and go since they take less energy to get rolling. Bigger tires should get better mpgs on the highway due to fewer engine rpms, but I tend to think it doesn't work this way. The guys with overdrive gearboxes report no better mileage in high gear cruising. It's more a function of mph (aerodynamics) and vehicle weight equals the amount of energy required, rpm makes little difference as long as you are in the engine's power band.



Ron, cruise should get better mpgs but it doesn't in our trucks because it's too sensitive. Try driving barefoot in cruise sometime, the throttle is constantly moving. Diesel engines do their best at a set throttle position, the governor can adjust for the load of a up or downhill. I consistently got 2-3 mpgs less in cruise until I slowed it down by restricting the vacuum hose to the cruise actuator. Now I get the same mpgs in cruise as by foot.

 

[jponder]

Re: Maybe..

Originally posted by ol ron

I missed something, but you seem to be guessing at the difference between tire sizes. In order to accurately check mileage, you must know the correct number of turns per mile for each tire, and do a lot of calculating. An easier way (proven by my GPS in speed terms) is to measure the actual percentage difference between the tire sizes, and add the percentage difference to your odo reading. This corrects the mileage driven.

With larger tires, you travel farther than the odo records. My size difference is 6. 7%, from 245/75 to 265/75.

Also interesting to add here, cruise control actually increases mileage, and does not decrease it. It only acclerates when needed, and backs way off throttle when descending hills. Having a set throttle going down hill wastes fuel and attracts badged types.

Ron

Yes I understand all the calculations. I will be using a GPS. I mean I already did the calcs with rev per mile, but I will use GPS for mileage



So far as cruise. I could not disagree any stronger. Engine braking going down hills and then accelerating going up will give you a nice even MPH but it will kill your mileage. Set throttle is going to smoke a cruise control everytime. The only time cruise would have a chance is on absolute flat terrain and it will loose that fight if it surges at all. People who have driven for mileage and used their foot to hole the throttle constant have seen big gains.

 

[jponder]

Originally posted by illflem

I would think that smaller tires would only lower mpgs in stop and go since they take less energy to get rolling. Bigger tires should get better mpgs on the highway due to fewer engine rpms, but I tend to think it doesn't work this way. The guys with overdrive gearboxes report no better mileage in high gear cruising. It's more a function of mph (aerodynamics) and vehicle weight equals the amount of energy required, rpm makes little difference as long as you are in the engine's power band.



Ron, cruise should get better mpgs but it doesn't in our trucks because it's too sensitive. Try driving barefoot in cruise sometime, the throttle is constantly moving. Diesel engines do their best at a set throttle position, the governor can adjust for the load of a up or downhill. I consistently got 2-3 mpgs less in cruise until I slowed it down by restricting the vacuum hose to the cruise actuator. Now I get the same mpgs in cruise as by foot.

Ifflem, Its more than an over sensitive cruise. admitedly that will hurt you. When i go down a hill I am going from a place of high potential energy to a place of low. Think of a marble, it will convert that potential energy into kinetic energy and it will use that kinetic energy to climb the next. When your engine starts braking you turn that lost kinetic potential into HEAT! You can never get it back. At the bottom of that hill you have lost some of the energy that would carry you up the next hill. You have to use fuel. The motto is BE THE MARBLE. You are doing a couple of thing when you restricted your cruise. One is you are removing surges and second you are making it more like a constant throttle. Put a throttle cable in and see what happens. 12 valvers are a little diff I think in that, dont they try to maintain a set rpm from a set pedal position? That seems like they wouldnt freewheel like ours would on downhills. Anyway I dont have cruise or I would do a test on that. Its Ironic that I didnt order a cruise because I think they are a waste but If I had ordered one I could show hw wastefull they are.



So far as rpms making little difference. I cant say. I would have thought it did. If it doesnt I geuss the only advantage of these big tires is the weight rating and the fact my pistons are doing more work per rpm. Hopefully longer engine life. Have to check back in at 500K to tell

 

[Clinton]

Personally, I like to travel between 70 and 75 mph, so I'm suggesting the test at 70 mph. Hopefully you'll have nearly identical weather and travel at the same time of day in both directions.



About the cruise control... I've learned since owning this truck that cruise isn't my fuel economy friend. That was hard for me to believe at first, but it's true. Illflem, how did you go about restricting the vacuum to the actuator? I'd like to give that a try. Thanks.

 

[Jakebud]

I would have to question the validity of how you are going to conduct the experiment. My biggest concern is that you will not experience the same winds on the same road with the different tires. I have always found the winds to be a huge factor in determining MPG. I really think that said wind variable is too large to make your results reliable. I suspect that you might get something closer to usable if you did a given trip on a number of different occasions in order to let the numbers average out, still knowing that the result will be a guess at best. I have a hunch that many who report MPG on the site do not take winds in to consideration. Just my 2 cents worth.

 

[illflem]

Clinton, to slow down the cruise I just cut the vacuum line to the actuator and inserted a barbed hose coupling full of glue with a pin hole poked though it. It took quite a bit of experimenting to find that the hole needs to be very small, a 1/64'' drilled hole is way too big.

 

[jponder]

Clinton> That is what I will do. I will avg 70 MPH over the 500 miles. What would be interesting is the effect of a 500 mile death wobble on MPG. People might think you were a lowrider and were doing it on purpose.



Jakebud> I dont have alot of options on HOW I am going to do it. I cant control wind unless I got a big wind wall pulled by a support vehicle to drive right in front of me. Actually that would be kinda cool as I am certain I could crack the 30MPG mark with that. I am actually not quite that into it, but give me time who knows. Wind is absolutely the one thing that can make the test worthless. I'll just record the wind at dep and arrival point by weather .com and hope I get lucky. The only thing I can guarantee is that I will produce DATA whetehr or not that Data has any worthwhile meaning is a whole different question. I will produce more data than if I didnt do it!

 

[Pit Bull]

Originally posted by jponder



I dont have the front lift yet so I may just run the back tires back and carry the other tire with me. This will make up some for the 100 lbs or so of the 2 michelins I will leave in La.

Your not saying your going to run the 19. 5's on the rear and 16's on the front??? If you are I don't think that would be a very good idea.

 

[jponder]

I am getting the front lift next Monday but yeas that was my original plan. I mean it would look funny but as long as I didnt use 4 wheel drive, It would be okay

 

[dan_gilson]

Your Speedo will be off by the difference in the diameter of the tires. Unless you know an exact distance between 2 fuel stops it will be hard to get an acurate MPG. A GPS might not help unless you have a straight stretch of road.



I don't know how the 285's will be. I have the 225's on 19. 5" and my milage went down slightly (< 1MPG).



I got an electronic speed correction box. With my old tires the speedo was slow. Now I am 1-3% fast. I have not been on a highway recently that had mile markers to zero it out.

 

[jponder]

Originally posted by dan_gilson

Your Speedo will be off by the difference in the diameter of the tires. Unless you know an exact distance between 2 fuel stops it will be hard to get an acurate MPG. A GPS might not help unless you have a straight stretch of road.



I don't know how the 285's will be. I have the 225's on 19. 5" and my milage went down slightly (< 1MPG).



I got an electronic speed correction box. With my old tires the speedo was slow. Now I am 1-3% fast. I have not been on a highway recently that had mile markers to zero it out.

I'm not going to shoot a vector with the GPS and see my distance from it. I am going to use the trip computer which is more accurate than Mile Markers. The mileage will be very accurate. I wont need to stop for fuel. I have been 400 miles and still not touched the 1/2 mark.

225/70R19. 5 tires are about 32 inches depending on brand. My stock michelins are 31. 8 inches. No change in gear ratio plus added weight would mean less mileage. What tires did you have on before the 19. 5? Looking at the rickson site the max weight of a 225 is 3640. my michelins are 3415. Just curious but why would you go to a 19. 5 for that minimum weight gain. My 285 are rated at 6175. You do have lower sidewalls but just wondering why you went with a small 19. 5

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