Chevy & GMC Vans

I have a 98 Astro with 168,000 miles, with using syntheic fluids, keep the OEM air filter changed every 10,000, fuel filter every 30,000, have my tranny flushed every 50,000 anc change my diff fluid every 50,000, keep tires at 35 lbs with valve tattle tales, I get 18 mpg in town and 20 to 21 on the road, I drive about 1500 miles a week, checking my millage after every fill up, once a week, once a month, quarterly and yearly. So I know every little drop or gain in millage.
There has to be more millage there.
So looking for any ideas!
I was wondering what does a 98 Astro have for numbers in the diff. I have read anywhere from 3.23s to 4.10s. How do you tell? Now I think they are 4.10s and only because when I pull my enclosed motorcycle trailer you wouldn't even know it was back there. Van weighing in at 4500#, trailer (all aluminum) 1500#, motorcycle 850#.
So if it is 4.10s and I was to change it to lets say 3.23s, what would I gain?

Looking for ideas

Dan

To tell your rear end ratio - look up the RPO codes on the door and then find the lists of RPO codes on this site and cross reference.... simple as that - as for more mileage - if someone has some secrets.... SHARE! - I get worse mileage with my AWD.....

Sounds in line with what Green gets. I've heard the underdrive pulleys are good for about 1 mpg gain.

I've thought about making some sort of retractable ground effects, supposably less air flow under the vehicle the better mpg.

That sounds like one the best mileage vans here! And you most likely have 3.73's or 3.42's as 4.10's were last offered in 1990 or 1991 I believe.

Dan,
I think the only thing you would gain from 3.23 gears is aggrevation. My '89 came stock with 3.23's. I drive a lot of stop and go traffic and wish every day I had 3.73 gears. (one of these days)
Lugs something awful in the bottom end of 4th gear.

kings-x59 wrote:Dan,
I think the only thing you would gain from 3.23 gears is aggrevation. My '89 came stock with 3.23's. I drive a lot of stop and go traffic and wish every day I had 3.73 gears. (one of these days)
Lugs something awful in the bottom end of 4th gear.

I agree. I've had a few vans with 3.42s, (my AWD has them), and feel that they are the highest you should consider. I think they are almost too high for a heavy AWD conversion van, even on the highway, because it has to shift down on hills to hold speed.

I had a '94 cagro at work that had 3.23s, and it was OK unloaded. I also had a '95 cargo with 3.42 that was all around great, unless you loaded it way down.

My 4x4 and the v8 have 3.73s. They are great. 4.10 is even better. If I ever build a new rear for the V8, it will be 4.10. That is good for me cause I run back roads a lot and rarely hit 60 for any amount of time.

Higher gears don't always mean better mileage. You need to run where your van is efficient, which is different for every combination of van, load, driver, speed and road condition. There is no majic gear that gives the best mileage. V6 vs V8 is another matter. My RWD V8 got better numbers than my 4x4 (in 2wd) v6 with the same gears.

my 2003 AWD with 225/70-16 BFG TA KO load range C tires , get 16 or 17 freeway and about 13 or 14 town. it has 3.42 stock gears. the heavy tires hurt mileage. the load range C tire weighs about 3 lb more than P rated tire.

If you calculate the anual savings in $ for the slight mileage. it would take many years to amortize the investment to change and axle. several hundred dollars.

In Engineering, the profound summary statement is Size, Weight and power, is relative to cost. but heck if you loaded down a Suburu wagon with that kinda weight its mileage would drop also.

don't worry too much about mileage .... calculate the total cost of ownership...gas plus modifications plus labor to install parts etc. don't worry be Astro happy

Astrophysics.

i just dorve 500 miles yesterday and got 14.7 mpg the whole way i would be so happy with 19 + how do you do it.

I regularly get 18-19 mpg with my city/highway mix. Green has the 3.23 rear end and I've try to use some hyper mileage techniques. Right lane w/ cruise set at 65, reading traffic ahead of me to avoid touching the brakes, letting the van coast to stop signs and lights. People in a hurry hate me sometimes.

If your getting that kinda mileage,and your happy with how it tows then do not change anything.

Try walking....

One thing you can do is over pressure the tires. Alot of the loss is rolling resistance as the tire changes shape to create the contact patch. Just remember that if you drive in the snow / ice this would be detrimental. Also over pressure to much and when that tire gets hot it'll be way over pressured. Personally I run my tires rated for 35PSI (when cold) at 37-8 during the summer months when I'm about to go on a long drive, Usually gets me a few more miles before I have to fill up.

Edit: It will also harshen the ride as the tire won't absorb as much of the road irregularities

I use synthetic fluids, change fuel & air filter frequently run 36# air presure in tires and I never get less then 18 but on the high I get 20 to 21.

Now I just tried something on my run from Des Moines to Omaha today, I put 4 oz of acetone in my fuel, crazy or not i tried it, I felt better performance, I know I didn't get any less millage then usual. Now tomorrow I will fill up and see what I got, I think I did a little better as the round trip is 323 miles, when I filled up today I could only get 16 gal's of fuel in it. And according to my fuel gauge it is higher then usual. Probably should have filled up when I got back this evening as I won't sleep better wondering. lol

Has anyone ever tried the acetone before?

I found the plate on my door jam, but what is it that I'm looking for so I can tell what my rear end gear is?

Dan

The acetone trick was discussed at the old board, IIRC. I think the verdict was that there was a difference, but it was easily out weighed by the expense of the acetone.

I don't always remember things exactly right, though.

I read a while back about using acetone to de-varnish your gas tank. I assume it would do the same thing to the entire fuel system.