Gearing & Milage
 

In this article

http://www.off-road.com/dodge/reviews/bullydog/chip/

Guy is talking about the bulldog box in a cummins. Never mind the fact about how much he loves the new power.

The thing in question is when he states

"I was cooking along at about 75 mph and only pushing 1800 rpm."

What kind of gears does he have in his truck???

I have the 4:10 and "cook along" at 65 mph pushing 2000 rpm ??? ???

Is that why he is getting 18 mpg towing 10,000 weekend warrior?

I get 12.5 mpg towing 10,000 5er

what's the deal here ???

Re:Gearing & Milage
 

The size of tire that you run will also affect your rpm's. Running taller tires is like giving your truck higher gears(lower number). I would say that he is running a pretty tall tire. Those mileage numbers seem kind of questionable though. It's hard for me to believe that he was getting 18 mpg pulling 10,000 lbs. My truck does good to get that empty.

Re:Gearing & Milage
 

Super Overdrive :o

Re:Gearing & Milage
 

I run right about that with my 315's and 3.55 gears.

Only averaging 14mpg mixed though.

phox

Re:Gearing & Milage
 

I was wondering about this Mileage vs gears myself. I am running a 4.10 and 285 tires auto trans my milage is about 14 mixed with a semi heavy foot. I just was noticing my rpms on the ride home just under 2 grand at 70mph. I was told by a frind to put a taller tire on but this tire measures 31 how tall is tall?

Re:Gearing & Milage
 

I like 33 X 12.5 on your body style with stock suspension! ;D

35's are too tall for me :'(

Re:Gearing & Milage
 

[quote author=eieio link=board=8;threadid=21694;start=0#msg203256 date=1067473555]
I was wondering about this Mileage vs gears myself. I am running a 4.10 and 285 tires auto trans my milage is about 14 mixed with a semi heavy foot. I just was noticing my rpms on the ride home just under 2 grand at 70mph. I was told by a frind to put a taller tire on but this tire measures 31 how tall is tall?
[/quote]

You could go up to a 305, I think they are right around 33 inches, without any suspension modifactions.

My 315's are 34.75 inches, a 35 by another name would smell the same.

Not sure mileage would increase any, as the tires are as much wider as they are taller, more contact = more friction.

If I had 4.10's (hindsight being 20/20 of course), I'm sure I'd be seing better mileage, 2-3 mpg most likely, and wouldn't have the power loss I do with the 3.55's.

phox

Re:Gearing & Milage
 

I ran 255/85 tires with a 3.55 gear and a 6 spd tranny, pulling 8K lbs. It was sweet to see the tach standing at 1800 rpm's at 70 mph, but the mileage was 13 mpg.

Now I'm running the same specs, but with 265/75 tires, which aren't as tall. The tach stands at 2050 at 70 mph, but the mileage has gone up to 15.5. So that's the sweet spot, I think.

Re:Gearing & Milage
 

[quote author=Carnage link=board=8;threadid=21694;start=0#msg203171 date=1067461825]
In this article

http://www.off-road.com/dodge/reviews/bullydog/chip/

Guy is talking about the bulldog box in a cummins. Never mind the fact about how much he loves the new power. <snip>
[/quote]Hmmm, 18mpg pulling 10k load. This guy is a writer for a mag? Writing about a Box? Box manufacturers pay for advertising? Guy wants to keep a paycheck?

Unless ..... (and I'm fully aware of truth in advertising - I have some close acquaintences in the ad/magazine business in Detroit), but maybe his test was done under controlled conditions, maybe going downhill?

If it is true, heck, I need to think about a bulldog! But there's an old saying that if something sounds too good to be true, it normally is.

I get 13.5 mpg pulling 9k.

Re:Gearing & Milage
 

Tall skinny tires and Gear vendors = mileage!

Re:Gearing & Milage
 

[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.

Re:Gearing & Milage
 

[quote author=Tartarus link=board=8;threadid=21694;start=0#msg203896 date=1067577794]
I ran 255/85 tires with a 3.55 gear and a 6 spd tranny, pulling 8K lbs. It was sweet to see the tach standing at 1800 rpm's at 70 mph, but the mileage was 13 mpg.

Now I'm running the same specs, but with 265/75 tires, which aren't as tall. The tach stands at 2050 at 70 mph, but the mileage has gone up to 15.5. So that's the sweet spot, I think.
[/quote]

I think most folks with non-stock tire diameters are neglecting to adjust for the difference. Here's an example: Difference btwn 245/75's & 255/85's diameters is ~18% so the correction factor would be 1.18. In other words take the 1.18 & multiply it by calculated fuel mileage & indicated speed to arrive at actual fuel mileage & actual speed.

Using the 13 mpg & 70 mph #'s above that translates to:

1.18 x 13 mpg = ~15.34 mpg (wonders of wonders, pretty close to what you're getting with the 265/75's)

1.18 x 70 mph = ~82.6 mph (oops ... going a little faster than you expected. If you don't believe it get/borrow a GPS.)

Yeah ... I know ... I'm such a smartacre!

Re:Gearing & Milage
 

[quote author=Suburbanite link=board=8;threadid=21694;start=0#msg205359 date=1067928001]
I think most folks with non-stock tire diameters are neglecting to adjust for the difference. Here's an example: Difference btwn 245/75's & 255/85's diameters is ~18% so the correction factor would be 1.18.[/quote]

Using 245/75-16 vs 255/80-16 (tire size calculator wouldn't do an 85 series) only yielded a 5.2% difference when I did it, not 18%.

This is an easy way to figure rpm changes with different size tires, but I don't think these numbers translate in a linear fashion into mpg differences.

Re:Gearing & Milage
 

Yeah, I realized I made a mistake today when I was riding with my brother in his truck. He had put 235/85R16's on it vs the 245/75R16's stock size & my in-cab calculations weren't agreeing with the GPS #'s. I keep on making the same mistake! Not using the OVERALL diameter differences. Once I made that correction, my #'s were a few tenths within the GPS #'s. Differences, altho not great, could be attributed to reading, rounding or speedo error.

For the 245/75-16 vs 255/80-16 #'s here's where I went wrong:

Rubber only #'s: 14.4685" & 17.0669"

Percentage difference: 1 + ((17.0669 - 14.4685) / 14.4685) = ~1.18

"Rubber only #'s" should've had the wheel/rim diameter added to them. So ... adding 16" to both #'s & performing the same calculations:

1 + ((33.0669 - 30.4685) / 30.4685) = ~1.09

Using this correction factor instead yields:

1.09 x 13 mpg = ~14.17 mpg (not even close to the original fuel mileage)

1.09 x 70 mph = ~76.3 mph (again, not even close ... )

It's true, that there MAY not strictly be a linear relationship btwn mph & mpg but I made certain, simplifying, assumptions such as assuming that a larger diameter & wider/narrower tire has roughly the same rolling & air resistance & moment of inertia as the smaller diameter & wider/narrower tire. Based on my own (one) experience in changing from 245/75-16's to 235/75-16's the #'s are close enuf (within a few of tenths) to be considered linear. I knew my fuel mileage pretty accurately before the tire change, 16.??mpg, & afterwards, with the MILEAGE correction factor was able to calculate the new fuel mileage, also 16.??mpg. FWIW, stock tire fuel mileage #'s came from ~860 mile trip & local interstate driving & non-stock #'s from ~2600 & ~2200 mile roundtrips.

bulabula, thnx for keeping me honest. BTW, using your #'s I also came up with 5.2% difference.

Re:Gearing & Milage
 

If your speedometer has been corrected there is no further calculation to be made. If it has not the simple method to fugure the difference per mile is by the specs that most tire manufacturers publish which is the rpm per mile of the tire. Once obtaining the percentage of difference you can also use that to figure the difference in speedometer.

I have 32 inch tires, 3.55 gears, and hit 70 right at 2000. Mileage falls off below 2000 and above 2200 or 2300. If you run a lot of 1800 while pulling a load your mileage will suffer even though it will be fine empty. There are a lot of cases where I can pull at part throttle in direct at 2300 or so or in six at full throttle. The better mileage is with the lighter throttle, always. I have tried it extensively both ways.