Gearing For Fuel Mileage?

BlackThunder · May 7, 2008

Hi folks...there was an article recently in Truckin magazine about installing a higher gear ratio in rear differential in order to achieve better fuel mileage. If I remember correctly, they tested with 4.88 gears and noted something like a 2-3% increase in fuel economy. This sounds almost too good to believe...the concept was that it put the RPMs of the engine in a more effective range...truck lugged less when trying to accelerate and therefore was more efficient.

Has anyone else tried this or have any commentary?

Draenor · May 7, 2008

It has been discussed numerous times. While 4.88 will accelerate the vehicle quicker, you suffer on the top end. IMO, 4.88 is way to low, you would definitely suffer at highway speeds. The gearing you have was already designed to keep engine in its power band.

C & A s Dad · May 7, 2008

Chris there is some merit to what they are saying. I would agree with John though, 4.88s would be too low unless you were running some big tires.

I have 4.10s and avg about 18 from a Yukon XL. This is mostly rural 2 lane driving with some city mixed in. It will get 20 to 21 in it is all hwy driving at 65 MPH or less. Take it up to 75 to 80 on the interstate and I will pay for that. The 4.10s take the rpms past the range of efficiency.

I belived 3.73 are great gears if you do some interstate and some rural driving with moderate towing. If you tow a heavy camper like I do and most of your driving is rural at 65 or less, the 4.10 s are hard to beat.

BlackThunder · May 7, 2008

I personally would have though the 4.88's would be too low as well, especially for the highway. My lease is up in February and if I get another truck, I'm wondering what the best option would be. I believe my truck currently has 3.42's with a 4.8L. If I get a 5.3L, I'm wondering if the 4.10 is a good option, or should I stick with 3.73's? Most of my driving is city with short 80-90km/h (55MPH) drives for about 15 minutes at a time.

C & A s Dad · May 7, 2008

Are you towing anything of any significance? If not and you were absolutely sure you were not going to begin towing something like a camper or large boat, I would stick with the 3.73s. On the other hand, if you are planning to tow, those 4.10s will keep that little 5.3 right in it's sweet spot.

BigJayDogg3 · May 7, 2008

Are you towing anything of any significance? If not and you were absolutely sure you were not going to begin towing something like a camper or large boat, I would stick with the 3.73s. On the other hand, if you are planning to tow, those 4.10s will keep that little 5.3 right in it's sweet spot.

Just asking. Don't take my advise.

If his driving is mostly city, wouldn't he be better off with 4.10s? The engine wouldn't have to wind up as much.

May 8, 2008

if you would drive in the city all the time, get something like 4.56's. but then again if you always drive in the city you dont need a truck . well maybe idk

iminaquagmire · May 8, 2008

Go a minimum of 4.10's if you do less than 40% highway driving IMO.

I have 3.42's in my truck now. At least 80% of my driving is in the city and what isn't is stop and go rural driving. My truck winds up too much off the line and I know 3.73's aren't going to help that much. My gears are killing my mileage and I plan to go to 4.56's and maybe 4.88's since I plan on a lift in the future.

C & A s Dad · May 8, 2008

Are you towing anything of any significance? If not and you were absolutely sure you were not going to begin towing something like a camper or large boat, I would stick with the 3.73s. On the other hand, if you are planning to tow, those 4.10s will keep that little 5.3 right in it's sweet spot.

Just asking. Don't take my advise.

If his driving is mostly city, wouldn't he be better off with 4.10s? The engine wouldn't have to wind up as much.

I would agree. I did not realize he was almost all city driving.

05silvycrew · May 8, 2008

IMHO, with a stock vehicle, lower gears (ie 4.10's) will hurt your city mileage. Now if you're talking about re-gearing to compensate for bigger tires (and effectively give you the factory drive ratio) that's different.

If two identical trucks (save for gears) take off at a stop light and both shift into second gear at 20 MPH, the truck with a higher gear ratio (lower number, ie 3.42s) will shift at about 2220 RPMs. the truck with a lower gear ratio (higher number, ie 4.10's) will shift at about 2650 RPMs. Thus the truck with lower gears spends more time at a higher RPM range, which should equal more gas burned.

when they go into second gear at 20 MPH the RPMs of the 3.42 truck will drop to around 1200, while the 4.10 truck will be around 1500 RPMs.

I've had both 3.42s and 4.10s and I always liked a 3.42 gear ratio because I could run out the gear, without running up the RPMs.

take it like a grain of salt, just throwing my $.02 in

hubbycap · May 8, 2008

IMHO, with a stock vehicle, lower gears (ie 4.10's) will hurt your city mileage. Now if you're talking about re-gearing to compensate for bigger tires (and effectively give you the factory drive ratio) that's different.

If two identical trucks (save for gears) take off at a stop light and both shift into second gear at 20 MPH, the truck with a higher gear ratio (lower number, ie 3.42s) will shift at about 2220 RPMs. the truck with a lower gear ratio (higher number, ie 4.10's) will shift at about 2650 RPMs. Thus the truck with lower gears spends more time at a higher RPM range, which should equal more gas burned.

when they go into second gear at 20 MPH the RPMs of the 3.42 truck will drop to around 1200, while the 4.10 truck will be around 1500 RPMs.

I've had both 3.42s and 4.10s and I always liked a 3.42 gear ratio because I could run out the gear, without running up the RPMs.

take it like a grain of salt, just throwing my $.02 in

Thanks Ben, I have the 3.42s and have been pleased and felt I was getting pretty good mileage. The last several posts had gotten me to wondering about whether maybe the 3.73s would be a more appropriate ratio. I'm about 50/50 city/highway. Though the logic of "winding up too high" with 3.42s seemed convoluted at second glance. 70mph puts me right at 2000 RPM, 4.10s would be several hundred RPMs higher. Logic would seem to dictate that a slower revolving engine would sip less fuel.

mi.bowtie · May 8, 2008

3.73 is always the the best all around ratio for a 5.3. Unless you CONSTANTLY tow a large trailer.

Ranger · May 8, 2008

I pull a fairly loaded trailer at times, have had about 5-6,000 lbs on it easy. I also had about 2,300 lbs in the bed and I had NO lack of power getting up to speed or passing. I am considering going up to 4.10s since I upsized the tires and may go up to 35s in the future.

Mikeee · May 9, 2008

It has been mentioned (here and other places) about the "sweet spot" of the engine.

And just where is that ... say on/in a 5.3?

Lower RPM does not always mean more MPG.

Higher RPM does not always mean less MPG.

Yes, I know, there are lots of variables i.e. weight, frontal area, slip stream, right foot to just name a few.

I had a '80 Caprice with a semi-custom built 350 (and stock 350 trans) that at WOT would shift out of 2nd gear somewhere after the speedo pegged at 80MPH.

C & A s Dad · May 9, 2008

Mikee, I would agree with this completely. I am sure there is more detailed research and info somewhere but every engine does have a spot where is runs most effieciently. An engine straining can burn more fuel than one operating in it's sweet spot. For towing my 5.3 likes to sit around 2200 to 2500 and at this point it is always ready to go when a hill comes. I cannot say where it runs most efficiently but I know running at 65 MPH or less on the highway it wll get 20 to 21 MPG. If I take it over 70, the MPG drops to approx 18. There are other factors involved other than engine speed and I think the biggest is wind resistance, especially with a truck.

I would not want 4.10s if I spent the majority of my time on the interstate at 75 +, however, I think they are great for the rural and around town driving we do.