Any Ideas on which Differential ratio 4.30 or 4.88

[Als69GT]

My current service truck with a 200" WB is app 18,000 lbs.
At freeway speeds ( 70'ish) I believe I am turning 2600 Rpm.
I usually get App 7 to 8 MPG.

I have dont the math and for simplification I used 2600 rpm and 1 to 1 for the output of the tranny.

2600 / 4.30 = 604
2600 / 4.88 = 532
604 / 532 =1.13

app a 13% Difference. Does anyone know what the difference may make in mileage.

Secondly since my truck is so heavy should I even consider the 4.30 diff ratio.

Has anyone seen a guide of Torque vs weight vs acceleration and mileage?

Al [img]/ubbthreads/images/graemlins/shrug.gif[/img]

[jonrjen]

I have 4:30s on my 03 F-450. Curb weight around 11K. If I hook up and tow an additional 6K loaded hay trailer (which puts us at about the same total weight) I find no need for a lower geared rear end. Unless you are towing a great amount of trailer weight behind your rig the 4:30 ratio should be fine.
As for mileage, there would be a difference between the two, but all things taken into account, if you are driving a 450 loaded that heavy, MPG will more be decided on how and where you are driving. On the highway the 4:30 may give maybe an extra 1 MPG. If doing a lot of in city stop and go the 4:88 would put the TQ to the road quicker and get the weight moving with less fuel being used.
Now one thing I did note from your post is your curb weight. You must be at or over the GVWR of your truck. My 450 4x4 6.0 per the sticker on the door post lists a GVWR of 15K. You may want to upgrade to the max GVWR 450 or step on up to the 550 for more GVWR.
Hope this helps a little, and did not confuse the matter any.

[Als69GT]

Thanks for the Info. I tend to do a lot of both long and short hauls. I go on service call as far as 100 miles. In between those calls, I usually am in a 25 mile radius 2 to 4 different stops a day. Someone else usually gets my parts so I don't often have much city driving. (except for the two or three stops at the pump)This truck had the upgraded payload package of 17000 lbs and I put on larger springs to compensate for the crane of 2000 lbs at the rear corner.

On my 2001 I put on the stage 3 programmer and never took it off. (In my younger days I knew the traffic judge by his first name). I really like the power and take off ability. Here in SoCal you need all the Pickup you can get. Not many people give you a break. They allways want to catch that last bumper and you better not get in the way.

I'm still undecided on the diff ratio. I am gettting the F-550 with the Payload upgrade to 19,000 Lbs

Al

[SmokeyWren]

[ QUOTE ]
I'm still undecided on the diff ratio. I am gettting the F-550 with the Payload upgrade to 19,000 Lbs

[/ QUOTE ]

The payload upgrade pkg is available only with the 4.88 rear end ratio.

So you have to compromise:

17,500 GVWR with 4.30 ratio, or

19,000 GVWR with 4.88 ratio

If your truck weighs 18k without a trailer, and you don't usually tow a trailer, then I'd probably go with the 4.30 rear end and without the ungraded payload pkg.

500 pounds over the GVWR is no sin. And the 4.30 ratio would be better except in steep hills.