Re:Gearing & Milage
 

The tire size difference would seem to explain alot in my case. stk. tires 245x75 tires now 285x75 this calculates out to 7.8% ( from tire calculator) so 337 miles and 23.6 gal = 14.27 presumed mpg but if speeddo is off by 7.8% this means 363 actual miles traveled or 363 miles and 23.6 gal = 15.38 actual mpg. so something to consider. when you change tire it makes the on board calculator pretty useless as anything other than a compass and thermometer I guess the dealer could set tire size and correct speedo ? on my harley I dropped 2 teeth in my front gear and was able to buy an aftermarket correction modual. Is there such an animal for our trucks?

Re:Gearing & Milage
 

Getting the Dealer to do it is waaaaaay too much trouble.
Many have tried and tried, but the techs don't seem to know how to do it, if even possible.
I was under the assumption that anything bigger than 265's wasn't able to be flashed.

The two aftermarket units I am aware of are the Superlift Truespeed (about 170 bucks) and the Abbot ERA (couldn't find price online).

I have the Superlift Truespeed and it has worked flawless for over 7 months.
After initial install problems (wires had to be reversed per what the instructions said), then the connection came loose (now soldered).

I used my GPS for about a couple weeks after getting the new tires prior to getting the truespeed, and checked it against the GPS and is as close to dead on one can get.


phox

Re:Gearing & Milage
 

Thanks Phox after reading the posts and doing the math the difference in speed is enough that it should be corrected a little more than I thought it would be but now that I know I can work on the correction. I was worried that something was wrong with my truck as it is mostly stk and the milage was below what I expected this tire thing explains alot.

Re:Gearing & Milage
 

[quote author=bulabula link=board=8;threadid=21694;start=0#msg204335 date=1067698321]
[quote author=kingofdodge7131 link=board=8;threadid=21694;start=0#msg204284 date=1067661920]
Tall skinny tires and Gear vendors = mileage!
[/quote]King, how much improvement did you realize with the GV overdrive?

I looked into one a few years ago. I don't remember the cost associated with purchasing it and installing it; but lets say it was $1500 (a guess), and the mileage gain was ~2-3mpg, it would take a LOT of miles for it to pay for itself. I wouldn't own the truck long enough to make it viable. And to top it off there aren't any other performance gains to be had from that overdrive unit.

Now, .... if a chip can give you that extra mpg, AND give you a good seat of the pants kick for the asking, now thats something that makes me sit up and pay attention.
[/quote]

I myself never did purchase one, I didnt do enough towing with my truck to make it viable either. At the time when i discovered it my uncle did all the horse trailer pullin. When pullin he pulled off extra 2 mpg, and it was a LOT nicer ride. Just pulled so much nicer. But when empty, he got a 6 mpg with it. now, This is a Chevy gasser. But that gasser rivaled our diesel for mileage at the time. I could only imagne the mileage a diesel could pull off with it. But if your only keep the truck a year or so then no its not a reasonable way to save money i think its like 2500, but If you go to there site, they will give the new wheel power numbers with it. Maybe a cheepre way around a ATS or DTT? I know id like to have 10 forward gears!. ( if i had a 5sd manual) or 12 for a 6spd. It does confuse me however. I always thought the lower your ratio the better your mileage. AKA the Factory .68:1 for overdrive would bring you better empty mileage than their .72 or whatever it is they have. But then i think i remember about having a over over drive? i dunno its been too long to remember haha
Hope that helped ya.

Re:Gearing & Milage
 

A diesel will NOT benefit from a Gear Vendors (or any other) Overdrive as much as a comparable gas engine.

Why?

A gas engine burns fuel at an almost constant ratio-- between 12.7:1 and 14:1.

A diesel engine varies the ratio GREATLY, depending upon engine load. (how much pedal you are using). So, if you reduce RPM by 20% in a gas engine, you are usually close to using 20% less fuel, since the ratio stays almost the same whether under load or not.

But remember that reducing RPM by 20% also means that the engine is under 20% more load. Since a diesel adjust fueling ratio to load, you end up burning more fuel. Maybe not 20% more, but more nonetheless.

The end effect is that the two tend to cancel each other out, especially when towing heavy. The lighter the vehicle, the more there is to be gained with a really tall overdrive, whether gas or diesel.

The real advantage to a GV on a CTD is being able to find just the right ratio for load and speed conditions.

A GV might give you SOME mpg improvement when cruising around, but the impact on a diesel just simply isn't as strong as on a gas engine.


Justin

Re:Gearing & Milage
 

I think that the author of the article in question may have seen this nice mileage towing with the bully dog box..... on the overhead console. Since the console assumes stock fueling levels for a given set of parameters (throttle position and rpm) it will be thrown off by any means of manipulating fueling. (aka fueling box, injectors).

I think that the mileage does depend on speed a whole lot since this is a quadratic equation. Bigger tires will lead to a higher gearing and people will correct for it according to a GPS- and won't take the tolerance of the stock speedo-tire combo into account. A valid comparison would include going the same speed (by GPS or any other precise means of measurement that is not influenced by tires) with different setups under the same environmental conditions and the same fuel. And then calculating the average mileage over 10-15 tanks to be shure that thermal expansion of the fuel in the tank etc doesn't play a role.

Just my 2c

AlpineRAM

how can you tell what gear ratio is in a rear end, some one told me cause it is a dually that it has 410. but i dont think so, cause i pull a 31 ft fifthwheel and i get on a good day about 12 miles to a gallon, and when truck is empty i get almost 550 miles to a tank.

Once you get over around 65 mph rpm makes little difference in mpgs.
Aerodynamics takes over and lowers your mileage

rail roader, i think it says it in the glove box