Rear axle ratio/change

[ricnor]

I posted a thread about possible engine swap for my 454 but the results are making me think that to get better gas mileage an engine swap maybe more trouble and expense than it is really worth to do, so I have had another thought and that is what about changing the rear axle gearing.
I don't do a lot of towing, possibly twice a year, I do need 4x4 as I live on a farm. My current rear axle is of unknown gearing at the moment but is a locker diff. Cruising at 65mph the rev's of the engine is 2000.
Even if I can get a couple of more miles to the gallon it would be worth the hassle if I can get the parts easy enough.

One thing I noticed when I first got the vehicle was that the rear axle appeared to be narrow, I added 2" spacers on each side to get the rear wheels to come in line with the front set and the rear bodywork.
I have the original bill of sale somewhere but all I can remember from it at present was that everything was ordered as heavy duty.

Any thoughts?

[777stickman]

You don't say what year your Sub is. I would say by your speed and rpm you're running 3:73 gears which is what my '98 K2500 7.4 has. Look in the glove box door to confirm. Because it's a 4x4 you'd need to change both front and rear. To have that done would cost $2-3,000 maybe. I did a little calculation based on $3 a gallon. If you geared up and went from 12 mpg to 13 mpg you'd save $960 over a 50,000 mile distance. So, is it worth it??? Only you can decide..................Steve

[1990 Suburban]

Please learn from my mistake and leave ratio alone. I have a 1990 R1500 Suburban(2-wheel drive) that came with a 3:73 ratio. I was getting an average 17 mpg and got greedy thinking I would squeek out a little more when my rearend blew. I opted for a 3:42 gearset and my mileage immediatly suffered to an average of 13 mpg ever since. It was the only change made. I know the math works out on paper, but it seems that the slight change in power band of the cam being shifted is all it took to loss instead of gain. My motor is a fresh 350 with 9000k on it with the 700R4 tranny. I had a range of 600 miles with the 3:73 on now I can only travel 440 miles per tank (and that's taking into consideration the 10% difference not registering on my tripometer). I hope this helps.

[corning_d3]

Yup, taller gears aren't always better. It sounds like you already have a good combination. I'd spend that dough on some tune-up parts and some mileage modifications. I went from 25mpg to 31mpg with exhaust, intake, throttle body and a dremel(gasket matching) on a 3100 monte carlo...

[ricnor]

Wow talk about scary stories. The burb is a 97 454 4x4 K2500 with auto box and has done 92000 miles. Forgot to say that at the begining. I didn't think I would need to change both sets of gears so that is a shock. The cost of fuel over here is $8 a gallon and at less than 10 mpg is a bit expensive so was looking for anything that could be done to get a bit more out of it.
I appreciate all your imputs so far but would like any information on what I can do to get a bit more mileage if there is anything.
Thanks.

[1990 Suburban]

Ricnor,
Borrow a set of taller tires on rims that fit your Burb from a buddy just to get a feel for the difference a front/rear ratio change would do for ya. Same difference and cheaper too! Be understanding of this fact though....if you change to a larger tire that gives about, say, 10% change in the speedometer, than your tripometer will only register 9/10ths of a mile for every mile marker you pass. This means that you'll be traveling the same distance but your tripometer/odometer will be missing 10%. So when you calculate your mileage, add 10% more miles to the tripometers reading before you divide by your gallons used. You can use the mile markers to help figure the percentage difference, so long as your exactly mile-for-mile be fore the change. Did you understand all that, because I didn't??!!!
Peace

[ricnor]

Yes I understood what you mean (rolling radius and all that). Now here is the sob story bit I'm the only one with a Burb in my area, the last owner of a Burb I spoke to had a K2500 but it was a diesel unit and had six lug wheels (I noticed because they were alloys and looked great) Mine has eight lug wheels.
American motors over here are not common and all my friends with american tin have either hot rods, caddy's, mustangs, camaro's or Vettes.
I have seen a few dodge rams and Ford pickup's around but all of them have six lug fixings from what I have seen.
The largest tyre size I could get at a reasonable price (equivalent to about $250 each) were 235/85/16 and they are pumped up to 80 psi in the rear.
I do appreciate all the informtion you are trying to help me with and I don't want to sound negative all the time but over here in England things are not as easy as in the states. They are now even trying to make us pay for each mile we travel by having trackers in our vehicles, they have had such a backlash from it that they say they are not going to do it just yet but everyone knows how poloticians work so expect to see the United Kingdom drivers paying for every drive they do at some point in the future.