gear ratios

[1-Up trucker]

ok so like i have been doing some reading up on gear ratios/different rear ends searching for better fuel mileage.

I currently drive an 07 Century condo 22.5 LP 3.46 rear end and a 515hp DD60.

my current understanding of gear ratios is if I have a 3.46 then that means that the drive shaft turns 3.46 rotations for every 1 rotation of my tires.

The question I have is if I went with a lower gear ratio say a 3.36(?) knowing it would take me longer to get up to speed, let alone bog down on the hills, would it improve my fuel mileage?

or should I start looking elsewhere for fuel mileage improvement?

[Mackman]

3.36 would be a higher ratio. Lower the number higher the ratio.

[LBF]

You want the real deal advice from DD as to what RPM that your engine gets the best fuel economy, and then run at that RPM.

Get your engine serial number, year of truck is not enough. There are several different targets, depending on which engine you have.

I expect that it would be cheaper to adjust your speed, than to change from 3.46 to 3.36, as the swap only changes your highway speed by close to 2 mph at 60 mph.

Read all the government fuel economy materials. The driver's behavior is the biggest variable between good and bad fuel economy.

FleetSmart in Canada FleetSmart: Home

Smartway in the US SmartWay | US EPA

[1-Up trucker]

thanks all for the quick replies.

Would a vehicles VIN be helpful to find the correct ratio?

also is it possible to run a truck at 70MPH at around 1300RPMs? I was under the impression this could only be done by adjusting gear ratios/rear end etc.

i run NE-UT alot and LA to dallas alot so the speed limits are set to 75, i usually run them at 70-72 and notice my RPMs are roughly 1500-1550

[1-Up trucker]

3.36 would be a higher ratio. Lower the number higher the ratio.

so then I should be looking for a 3.7 as opposed to a 3.36?

[LBF]

First job is to check with DD about your engine, you may already be there.

Also, you need to be sure of the RPM, the tach in the dash cannot be trusted for this purpose until you check it against the engine computer with a scan tool.

I expect that you will find that there is a little wiggle room in the fuel economy window for that engine to run a little slower and save the fuel of not pushing so much wind.

A 3.46 gear is already a pretty good gear for a DD engine of that vintage.

Doesn't cost you anything to try running a little slower, swapping gears in an attempt at better economy may not realize you the savings to pay for the swap.

When you call DD, also ask them about the pros and cons of de-rating the engine. Do you really need 515 HP?

[1-Up trucker]

First job is to check with DD about your engine, you may already be there.

Also, you need to be sure of the RPM, the tach in the dash cannot be trusted for this purpose until you check it against the engine computer with a scan tool.

I expect that you will find that there is a little wiggle room in the fuel economy window for that engine to run a little slower and save the fuel of not pushing so much wind.

A 3.46 gear is already a pretty good gear for a DD engine of that vintage.

Doesn't cost you anything to try running a little slower, swapping gears in an attempt at better economy may not realize you the savings to pay for the swap.

When you call DD, also ask them about the pros and cons of de-rating the engine. Do you really need 515 HP?

i dont need the full 515.....i mean its nice for hills knowing i have that little extra umph, but not really necessary. if i recall also doesnt having more horsepower affect how well your jake works? Also, would a qualcomm be able to give me the data i need? I have noticed on the sensor tracs it shows what my RPMs are when idling and rolling down the road.
the gear swap thing isnt necessarily for cost over a short term, its meant for more of a long term gain. I want to be able to run at 70MPH at a lower RPM which somehow in my head equals less fuel consumed each driving shift..... or am i overlooking something?

[LBF]

If you want to run 70 mph, you need to gear for the fuel economy sweet spot for your engine, which may or may not be any slower rpm than what you run now.

It all depends on the engine. Contact DD for the info for your engine.

[Heavy Duty]

ECM tunning would probably give you better fuel mileage for less cost, try PDI or Bully Dog. You can have power and good fuel mileage. DD will screw up you fuel mileage with their LoNox tune.

[Maniac]

To change the ratio will cost roughly $4000.00 figure out how many tenths of a mile increase will be needed to pay for it.

[Copperhead]

3.46 ratio? Not ever seen that one. I got 3.42 in my truck and am turning about 1500 at 70mph on low pro rubber.

Seen 3.21, 3.36, 3.42, 3.55, 3.58, and 3.70 but never a 3.46. Guess one can learn something new every day.

Either way, if you want to run 70-75 mph and "try" to get mpg you can live with, I would consider going to 3.36 or even 3.21. Most engines (at least emissions equipped ones) seem to get their best mpg with the engine operating around 1300-1400. The DDEC 3 liked it higher, but the DDEC 4 and 5 do pretty decent between 1300 and 1400. Can't really comment on the new DD15. I would agree also, with others, that having an ECM tune could be helpful overall.

If one was going to change gearing, an option worth considering is going to one drive axle with a pusher or tag axle. You could put lockers in the drive axle for added traction. A shop would give you credit for the drive axle and power divider you take off and you will reduce your overall weight by 400 lbs, more or less. Add to that you will decrease your rolling resistance and pick up a tenth or two mpg minimum. I can get this done at a shop near me for not much more cost than redoing the gears and keeping the same setup.

[Mackman]

To change the ratio will cost roughly $4000.00 figure out how many tenths of a mile increase will be needed to pay for it.

I didnt know ti was that much.

And tracer changes them once every 6 months lol

[1-Up trucker]

ty for the helpful advice everyone.

gonna go do some more homework

[tracer]

...And tracer changes them once every 6 months lol

well, it's a business. you try something, if it doesn't work - you try something else. I thought 3.73 was too slow, so I got a 3.42 and turned out to be too fast FOR MY PARTICULAR OPERATION. I compromised by taking a step back and getting a 3.58. I drive at 57 MPH now and my 15 L CAT spins at 1,270 RPM and I'm laughing all the way to the bank. Don't listen to people who tell you it costs 4 grand to change the gears. The gear shop where I did my axle ratio job charged me $2,500 first time and the second time I think it cost me 50% less. All you have to do is find the right shop. By the way when I park my truck and get into the Cobalt Sport 2.4 I mostly drive at 75 MPH I like driving fast especially when the PASSENGER vehicle I"m driving gets 34 MPG!