Changing Axle Ratios to Boost MPG

[ATeam]

No special points made here, just thinking out loud.

An acquaintance recently told me he replaced the ring gear in one of his trucks to change his rear axle ratio. The change was intended to improve the truck's fuel economy. This was a recent change and he had no new mpg numbers to report. He did say gradability (hill climbing power) was not significantly affected, though he had not run the truck in mountains yet.

My dealer tells me a ratio change on our truck would cost about $3,000 if remanufactured ring gears are used (not junk yard, not new).

At present, we are running a 3.42 rear axle ratio, which the computer says provides a fuel economy sweet spot range of 61.0-70.2 mph, and 1,482 rpm at 65 mph. On the road, that rpm number is pretty much what the computer says it is. At 65 mpg we are running just a tad under 1,500 rpm.

The next step down for us is ring gears (tandem drive axles) that would provide a 3.21 rear axle ratio, with a sweet spot range of 65.0-74.0 mpg and 1,391 rpm at 65 mph.

Depending on wind and terrain, we get between 9.0 and 9.5 mpg at 65 mph now (worse when wind and terrain are extreme but in that range over long distances).

In repsonse to fuel at $4.00+, we have changed our driving practices and improved our mpg by 0.5. Part of that is reducing our over the road driving speed from 65 mph to 60 mph.

Thinking out loud, I am wondering, what good if any it might do to change our rear axle ratio from 3.42 to 3.21?

[Moot]

Thinking out loud, I am wondering, what good if any it might do to change our rear axle ratio from 3.42 to 3.21?

It most certainly would lighten your wallet. I believe you have some type of automatic transmission. Will that handle a higher gear ratio without hunting and pecking for a lower gear in headwinds or steep hills?

Keep your money and lower your speed.

[gojack]

Spending 3K to save fuel;

1) X-Ones + 9.7%

2) Freightwing skirts + 7.2%

3) TrailerTail + 5.1%

Source: energo2007

PS ...Your dealer is way overpriced

[greg334]

I agree with Moot, the ROI for a gear change to 3.21 is iffy at best with a 3.42 in the axle. I say this because i know of one person who changed to 3.23 and he regrets it, changing back to the original 3.45 gear.

As Moot pointed out the hunting and pecking the autoshift trans may be a factor when it comes to balancing power against load and wind resistance, even if you have the system reprogrammed, then I think it would have to go through the learning process all over again. If you had, say a 4.12 or even 3.73, I would have a serious look at it but 3.42 to 3.21 may put you into a worst situation.

In my case, it may be more advantageous to get an OD trans, I have a direct drive with 3.21 gears in the back. I run at 65 at 1700, a bit too high for me but if I slow down to 62, it preforms rather well and returns 10.5.

Yea, I didn't see that GoJack, yep there are savings there, especially skirting.

[LDB]

Slowing down is the equivalent of changing gear ratio and adds the benefit of lower wind resistance as well. I agree with the others that you may find slower speed combined with higher gear ratio to be less efficient than your current setup.

[davekc]

Could be disasterous for pretty much the reasons listed above.
The only realistic data on that change with your configurations would have to come from Volvo engineering.

We several years did this on a Freightliner, and the folks in Oregon (Freightliner engineering) did most of the work.
Worked for us but with a different engine, transmission, and rear end than what you have.

[ATeam]

We several years did this on a Freightliner, and the folks in Oregon (Freightliner engineering) did most of the work. Worked for us but with a different engine, transmission, and rear end than what you have.

What were the results?

[dhalltoyo]

For anyone considering this type of change, and you have an automatic transmission, you better get it in writing that the ECM can be reflashed to recognize and accept the new ratios.

Otherwise, the torque converter will read the shift points incorrectly and your tranny will go into limp mode.

A shop tech at a drivetrain specialty center told me of the countless number of trucks he has had to restore back to the original factory differential ratio.

[davekc]

What were the results?

For us it equalled about one mile per gallon. Wasn't a huge deal then as fuel was cheaper but you want to make sure you know exactly what your doing when doing this.
We had no clue, thus the reason for going through there engineering. Also acts as a safeguard to preserve manufacturers warranties.
Don't expect a quick answer. It took Freightliner 2 months to have all the information. Not sure on how fast Volvo would be.

[pelicn]

Dave,
Where do you look to find out what the gear ratio is?

[davekc]

It will be listed on the original purchase order. Or, call the truck dealer with your serial number and they can tell you what it is.

[Dynamite 1]

alot of this also has to do with the final drive ratios in each gear and the stepping of the trans. or the way the trans is geared. matching the trans gearing and the rearend gearing is very important. if i am rite phil you have the 10 spd ultrashift or the equivalent meritor and a 3.42 rear and it turns 1482 @ 65 on the otherhand i have the allison 6 spd. auto with a 3.73 rear and i turn an even 1300 @ 66 almost 200 rpm less than you and you have 4 more gears. there are lots of variables to look at between trucks. where the engines are designed to run at, the trans gearing and the rear gear as well as the overall final drive. as stated i dont think that a gear change would be beneficial all on its own. if you p/u 5/10 of a mpg just slowing down a little i would continue on that path. fuel mileage is only going to get so good and after that all else is a waste of money. some of the other fuel saving methods would recoup your investment much quicker. i to have slowed to 65 in the big speed states from the 69 to 70 we use to run and have had the same improvement that you have seen. i have been very interested in airtabs but more buy the idea of skirting a straight truck. just have not heard of any really good info on anyone doing this.

[pelicn]

Thanks Dave

[Scott777]

I am a firm believer in micro managing RPM.. If you go to www.powerspec.cummins.com you can check ratios and tire size and top gear ratios.. A Meritor 12 spd has a .78 top ratio as compared to most class8 trans. that have a .73 or .74.. I once had a Volvo with meritor freedomline trans and a Cummins ISX and 3.42, Bridgestone LP22.5 726's,1465RPM @65, My previous truck (KW) had ISX with 13 spd and 3.55, Bridgestone LP22.5 726's, 1423RPM @65.. If you go to 3.21 IMO you would need to go to Michelin 275/80R/22.5 XZA2 518 revs/mile, your peak torque on your engine is at 1200rpm, if you ran in top gear at 56mph you would be turning 1210rpm, at 58mph would be 1253RPM which to me is ideal, 1405RPM at 65mph.. My SST100 that I had had 3.21, Mercedes MBE4000 410hp 1450lb.ft.@1100,10 spd Ultrasift, Bridgestone 295/75/22.5 726 507rev/mile, 1305RPM@ 65mph, 1104RPM@55mph.. This truck averaged over long term was 10-10.5mpg.MT wt. full of fuel, 2drivers,no belongings, was 27,600lbs.. You may not get the money back in the short term for the gear change but you will cruise alot easier at 70mph, 1513rpm@ 70mph.. To micro manage the rpm you need to know the speed that you will do most of your cruising at, I have always tried to use 65mph for that reason.. I am a FAA certified A&P mechanic so I am also a nut on sythetic oils in the engine.. Royal Purple LongRider for me..