Okay, here's my question:
My bus is not really great on mileage (10/13 at best) 4.10 rear. 8000# empty. Wondering if I could get away with a 3.73 rear or not? If so, how hard to do? Gear set only, or what?

Now, one other related question here. I have a 94 Ford E-150 gasser. 134k miles on it. Inline 6 4.xL engine. Last year at 120k miles would burn about 1 qt of oil per 5000 miles. Now burning 1 qt every 2500 miles. Oil pressure reading lower on dash too. Has overdrive, and it's my service truck with too much stuff on it.

My thoughts are to run without overdrive to preserve engine. More RPM's, but less loading on bearing surfaces. Correct in my thinking? So if I carry this thought pattern back over to my bus, will 3.73's decrease my engine life? Or am I just all cracked up?

I've towed up to 24K GVRW with my 3.73 set up, how often towing that weight and how far, I had HD clutches installed in my trans plus aux cooler with electric thermostat controlled fan.

I'm sure that it will work with 3.73 gearing, I was just wondering what I'm doing to it in the long run. I'm more of the longevety mindset, moreso than power hungry.

I'm kinda asking two different questions about two different vehicles at the same time, yet inter-related.

The bus will take off better with 4.10 and get better milage with 3.73.

The ford has valve wear or similiar.

Not quite an answer to your question... more of a suggestion. Something to look into anyways...

Might be a lot cheaper to swap out the whole rear end with another that's already set up with 3.73's. It's definitely "easier" to do.

10/13 appears too low even with 4.10s. Worst I ever had was about 11.5 when towing my camping trailer with the broken vacuum pump and thus no boost. Towing at about 96kph through a mountaneous area, I got about 14. Normal driving usually gets me around 20 unless it's cold like now. (~1500 RPM in OD at 80kph is 3.73? Or 3.42?)

PS: Generally, lower RPMs mean less stress on the engine (and give better fuel economy).

SHOOT, 13-15 is all I get even on the highway BUT I run between 2400-2800 RPM at highway speeds so I know that's the reason. I will make a tire size adjust with my next tire swap to try to get those down a little.

This freakin' thing is, I dont know, 8 ft high, or something crazy like that. It pushes a bunch of air.

Lower RPM's means more torque, and more stress on the engine, at least that's my understandin'. It's like riding an 18 speed bike. Put 'er in 18th gear, and see how hard you've got to pedal to get going. Sure, you're only making about 1/4 the revolutions, but much more torque required to move your fannie down the street. I was just throwing this idea of 4.10's vs 3.73's out there for a general idea of what should I expect as far as the engine life is concerned. 4.10's sounds like it's really babied, but might actually be needed with this 'bus' application.

I'm not sure that you will see enough difference to pay for the swap my self. 3.73 would work ok just lose some bottom end power, slower takeoffs, but would be a good gear just I would do it JUST for the fuel saving. Not sure but I think it would take alot of miles to pay for it's self.
look at this site and see what you think
http://www.f-body.org/gears/

Would I have to swap out the entire rear, or can you just change the gear sets inside?

If the van body is high up you need a shaped air spoiler on it to reduce drag.

I just did 500 mi run light loaded avg 16 mph running 75+ mph with 500lbs in the bed with my 3.73 running 75+ mph, when I tow I have plenty of power depending on load 12-13 mpg running 65-70 mph

70 mph on my truck is 2500 RPM +/- It runs about 2800 at 80 mph and gets about 13 MPG

Doing some more study but pulling the MAF seems to have made my numbers go up, too soon yet to tell 100%, yesterday bursting to 90-& 100 mph 2900-3200 rpm I still was getting 16 mpg. I have some measured distances I run regularly so now that I'm in my back yard I'll be able to know if I'm getting actual mpg increase or a percieved one. But 2 fill ups yesterday looked promising, I also run 6 oz FPPF total power to a tank that may be adding some mpg, but I run that with every tank anyway so nothing new there, only change is L65 reflash PCM and removing MAF; I went over to Houston as a reflashed L56, came back as a 65, just got it in the mail day before leaving for Houston.

Would I have to swap out the entire rear, or can you just change the gear sets inside?
You can change the gears if you want, but it's very tedious and expensive to have someone setup the gears for you.

That's why I said you might be better off swapping the entire rear end. Four nuts on each spring perch, disconnect the drive shaft, and your most of the way there.

YOu could find a used rear end in a salvage yard. Check the gearing by spinning the hubs and counting the rotations. Someone here should be able to give details on how to do this.

Go to the glove box of the donor vehicle gear ratio should be there, in RPO code

GT4 -- AXLE REAR, 3.73 RATIO
GT5 -- AXLE REAR, 4.10 RATIO
GU4 -- AXLE REAR, 3.08 RATIO
GU5 -- AXLE REAR, 3.23 RATIO
GU6 -- AXLE REAR, 3.42 RATIO
G80 -- AXLE REAR, LIMITED SLIP (POSITRACTION)
HC4 -- AXLE REAR, 4.56 RATIO

also on some rears a metal tag is present that shows gear ratio, junkyard Hollander book should be able to tell you which pn on tag = which gear ratio

Yes sir can cost some money to swap gears back there BUT every GM 14 bolt that I have found for sale in salvaga yards they want over 500.00 for them. I 'm getting my gears swapped in my COP car and all new bearings, which you wouldn't have with a axle swap, for less than that much. I also haven't seen many of those rearend with 3.73 in them most are 4.10's that I've seen. Just something else to think about, thats why I'm not sure you will recover cost in fuel saving any time soon.

I was considering a 3500 single wheel axle swap in mine but the PITA factor and cost was the deciding factor for me.

Yes sir can cost some money to swap gears back there BUT every GM 14 bolt that I have found for sale in salvaga yards they want over 500.00 for them. I 'm getting my gears swapped in my COP car and all new bearings, which you wouldn't have with a axle swap, for less than that much. I also haven't seen many of those rearend with 3.73 in them most are 4.10's that I've seen. Just something else to think about, thats why I'm not sure you will recover cost in fuel saving any time soon.
I think full-floating 14 bolt availability and price vary considerably from place to place.

And there's a considerable variety in the quality/$ of different gears.

At any rate, Bowtie's right. It'll take a lot of miles to recover the cost of either in mileage increase.

I had a '96 GMC 6.5td 3500 DRW 4x4 with 4.10 gears. Went from the stock tires to a 235/85R16. Lowered rpm's at hiway speed and increased mileage. I also used the truck for hauling my hay trailer with #12,000 of hay. I did also have the boost increased to a max of 11psi with a larger exhaust. Changed the anti lock break calibration for the taller tires. Tried setting the speedo to the correct tire size and it started shifting like I was back on the stock tires.

you went to 235/85/16 from stock? how much smaller were the stock tires. I run 265/70/16 now and looking to go to 285 or 305's myself for the same purpose.

Gee, what a shame, I just got done doing the tires just after I got this rig. Only 5000 miles on the tread. Don't want to have to replace the rubber again to pick up mileage, seems kinda wasteful. Maybe next go-around I'll up the size and squash the idea of axel/ratio change, not lookin' like much fun.

you went to 235/85/16 from stock? how much smaller were the stock tires. I run 265/70/16 now and looking to go to 285 or 305's myself for the same purpose.
Will it take any lift to squeeze 285's under a 3/4tn burb? I wouldn't mind having to trim the fenders front plastic a little, but I don't wanna go lifting it.

Will it take any lift to squeeze 285's under a 3/4tn burb? I wouldn't mind having to trim the fenders front plastic a little, but I don't wanna go lifting it.1/2 ton or 3/4 ton ?
2 or 4 wheel drive?
what tire you got now ?

Specs are in my sig. I'm currently running 265's. Looks like the 285's will add 1.2" diameter.

Wade how wide are yor rims, I lowered my height a little when swapping to 8" from 7" when I needed a rim with more weight carry cap, I think with extra room that gave me I could fit a 285 on my LD frame, your HD frame should also give you a little more height, probably only way to know for sure is mount a tire, maybe you can find a tire shop or off road shop that will mount a used 285 on one of your rims for test, I don't know of anbody that will mount a new tire on a rim and test fit it and let you turn it back in if it does not fit.

The 285 should fit without an lift kit. Or you looking at the 70 or 60 size or something else?
Here is a tool I find usefully at times too.
http://www.miata.net/garage/tirecalc.html

Neat tool

and here another one thats handy at times.

http://www.f-body.org/gears/

The stock tires were 225/75R16 or 29.5 inches tall vs. the 235/85R16 measure 31.9 if my cal's are right. That gave me a 7.5% increase in height and improved the rolling distance. The engine rpm and transmission shift points were very similar to my '93 2500 6.5td that had 3.73 gears on the factory rims and tires.

Your tires are narrower than miine. I run 265/70/16 which work out to be about the same height as yours. If you wanted to go taller then someone might have to work that around the width area. So to answer the earlier question about 285's, I guess it would depend on the width you choice. 285/60/16 are shorter than 235/85/16 so it depends or how wide you want to go. And then there the question of rim width but thats another can or worms.

The 285 should fit without an lift kit. Or you looking at the 70 or 60 size or something else?
Here is a tool I find usefully at times too.
http://www.miata.net/garage/tirecalc.html
I'm looking at mounting them on my stock rims, so whatever width they are. They're the ones with a chrome covered plastic center cap, plastic lug covers, and beauty rings on the rim edge.

I'd like to tuck some 285/75/16's.

265/75/16s are stock for my Z71, 285/75s fit nicely and didn't affect mileage at all.

Back to oringal question, what's better on longevity: more rpm's or less over the course of time? Hanging in the balance of course is how hard the engine is working at those rpms.

Diesels are driven on torque rather than on power. They give you great torque at low RPMs, but less power than gassers. They don't need as much power because they have much more torque. Being underpowered but having much torque is what makes Diesels last longer than gassers (besides a somewhat more sturdy design). If you drive a Diesel against the way it is designed, it won't last that long. Just think about things like higher compression, friction, flowing more air and such ...

Running the 6.5 above 2500 RPM imho doesn't do good on it. It even shuts off shortly after 3400, thus, at 2500, you're already quite close to max RPM.

PS: High RPMs put more stress on gassers, too. The small gassers that are common in Europe last about 180kkm, if not less, because they're running high RPMs. But decent engines like the LT1 --- or the 6.5 :) --- are just brought in at 180kkm and last much longer because they give you plenty of power at low RPMs. Think about the Vortecs in the trucks, seems they don't last as long as one would expect because they are lacking torque and need to be driven on power. Put them into something they can move on torque, and they'll last. And it's torque what you need for towing ...

BTW, you could try staying below 120kph to get better mileage, at about 120kph the wind drag greatly increases.

Will it take any lift to squeeze 285's under a 3/4tn burb? I wouldn't mind having to trim the fenders front plastic a little, but I don't wanna go lifting it. 285's will fit perfect. They look much better than stock. I have a 96 2/4 burb and moved from 245's to 285's. It results in a net 10% difference in speedo so if your speedo says 60 you are going 66, etc. It did reduce my highway engine speeds but my mileage stayed the same, still 14.5-16. I have never had them rub and the ride is a little softer now not so rough. I can send you a pic but I don't know how to insert it into this message. Also I also run 4.10 limited slip.

Cool. That's what I'm going with then. Might do a 255/85 instead (same diameter as a 285/75), but availability is a killer on those.

I might even give in and spend the $ for the 285/75/16 Revo's.

I have a 3.73 14BoltFF sitting in the garage unused. If you are near Indiana I am willing to sell it.

Actually, Lee,
The '94 will go to 3500 rpm at the top of shifts under WOT. I'll agree it shortens the life of the engine especially this LD diesel to rev it so high, but then I think of bill heaths TT 6.2L that could double that at 7000+ rpm and I push mine harder to experience the top rpm output that wasn't there before advancing the timing and getting full boost. one thing leads to another.

QM, I should have mentioned that I'm still with the stock exhaust, boost and
timing. But even with improvements of these things, I'd go for longivety
rather than for max power.

It doesn't pay out to run things on the edge of their capabilities ---
that's true for computer monitors, for example, as well as for cars/engines
or anything else. If you run things on the edge of their performance, they
will wear surprisingly quickly and fail very soon.

If I need something that's up to a proposed job and I can't tell exactly in
advance what the job will demand, I tend to get something with capabilities
that, reasonably, go a bit beyond of what I think the demands will be. That
appears more expensive, and it usually is in the beginning, but on the long
run, it pays out in both money and reliability, and in longivety; and often
enough, I find myselfe very lucky that I've got something that is capable to
an extend I never thought of when I aquired it. Otherwise, I end up in
either remaining vaxed about insufficient equipment or/and in having to get
a sufficient replacement and thereby having to spend twice --- or more than twice --- the money
I otherwise would have had to.

Of course, there are a lot of things not worthwhile to this kind of
'oversizement', but at last those that must not fail are worth it.

.

It doesn't pay out to run things on the edge of their capabilities ---
that's true for computer monitors, for example, as well as for cars/engines
or anything else. If you run things on the edge of their performance, they
will wear surprisingly quickly and fail very soon.


Lee; All mods in my sig do allow me to run to max design limit but consulting with shoulder mounted computer I keep with me to control application of right foot keeps me from doing harm or degrading longevity; but it is a struggle at times to adhere to it's recommendations ):h . Actually the stuff I've added is pretty much stuff GM does stock to DMAX, they could have done it as well but market forces at the time they did not see the return on investment to upgrade these trucks to their full potential.

Ok, then I should add that you can get max performance from things without destroying them soon, provided that you very carefully monitor how far you're going and how close to the edges you're getting so that you don't overdo it :)

But who really does monitor the engine with a shoulder mounted computer and such all the time? :) And still, I still think that running higher RPMs will wear the engine more than lower RPMs and making use of torque rather than of power. Or is that not true for the 6.5?

you can get a ring and pinion set from Reider Racing, it will probably cost you $300 with the install kit. -that's if you can use the same carrier. It's alot of work to swap the gears and you need a dial gauge indicator to set up the back lash. Plan a LONG day to swap the gears if it's the first one you've done.

Swapping out the entire rear is faster and easier. If you buy from a junk yard, you may even be able to trade in the your old rear. If you're looking for fuel economy, go with 3.55:1 instead of the 3.73:1 ratio. If you're not doing a lot of HEAVY towing, your van will do fine with the higher ratio.

I swapped out the rear end on my ford f250. It took me 4 hours to complete. Last year I decided I wanted even 5.13:1 gears to turn the 44" tires. It took me a Saturday AND a Sunday to complete (first one I did).

Hope that helps.

diesels operate more effiecently when they are hot and driven hard.

Running a truck camper, Swapped from 4.10 to 3.73,


Lost about 1 mpg when loaded. At 60 , keeps shifting on hills.

Did gain a little unloaded, But I would not do it again....

So I've been thinking about going to 4.10's myself. I currently have 3.73's and am running a 35" tire. The reason I would like to use the 4.10's is for the trails. My 4HI is kind of worthless, it get's me around, but when I encounter an obstical, it'll stop me dead in my tracks. Have to switch over to 4LO, which is awesome, especially on the steep hills. I would like to keep it in 4HI on the moderate stuff and use 4LO only on the steep stuff. Bad thing is, because I use my 4 wheel drive, I have to get both done.

So back to the original question, changing out the gears will depend on what you use your truck for. For me and my off-roading beast, I need the 4.10's.

JMK,
I think you would be impressed with bang for buck improvement of an opened up exhaust system GM's down pipe is a real power robber I'd invest in that at a minimum for mods, and free airbox mods, check Kennedy Diesel website FAQs, http://www.kennedydiesel.com/ I move my GN trailer loaded to 18K with my 3:73, moving 35" tires is a tall :D order also so you may still need the 4:10, but the exhaust will benefit you regardless what gear you run.

Yeah, I agree. I plan on getting the exhaust done sometime in the future. This past week, I did the snorkle removal and airbox mods and got rid of the webbing in the intake. I know what I want for my exhaust, the problem I'm having is geting the 2 or 3 bills out of the bank without pissing the wife off too bad. It'll come eventually.....

Dude you got to get creative, tell her of the mpg increase with the new exhaust it will pay for itself in no time, that is what works for me when I get permission to mod mine some more.