Re-Gearing 7.5 LSD from 3.73 to 3.08, manufacturer recommendations for Ring and Pinion Gear set.

[sgtsandman]

Doing a bit of reading today, I've struggled to find the 3.08 gear set being used with an LSD carrier in the 7.5. Short of having all the parts in my hand to compare, does anyone know if the LSD carrier will even work a 3.08 gear set?

I seem to remember SN95 mustangs having LSDs and 3.08 gear sets, but I'm having trouble confirming.

As far as I know, the only difference in an open carrier and an LSD is the guts in the carrier. The part where the ring gear bolts to should be the same. At least it was on my '98 with the 7.5. Since you already have the carrier, clearance for the tone ring (if you have one) will be a nonissue since the carrier will already have the relief for it.

 

[bhgl]

As far as I know, the only difference in an open carrier and an LSD is the guts in the carrier. The part where the ring gear bolts to should be the same. At least it was on my '98 with the 7.5. Since you already have the carrier, clearance for the tone ring (if you have one) will be a nonissue since the carrier will already have the relief for it.

I think you're right, I've hesitated on pulling the trigger on the gears just a moment after reading about issues some folks have had in re-gearing their 2.73s up to 3.73 or higher. I emailed the folks at Yukon Gear to get a solid answer, hopefully they'll be able to confirm.

 

[bhgl]

The official answer from Yukon Gear:

Alrighty folks wish me luck.

Also, would anyone on this forum shoot me dead if I went to a 2.73 gear set?

 

[fastpakr]

I don't know about shooting you, but you'll get a lot of feedback that it's not a great idea.

 

[lowspeedpursuit]

You can't freely trade power for MPG (or vice versa) all up and down the scale of all possible gear ratios.

If you burn 5% less gas at cruise, but have to spend 2x as long on the gas every time you accelerate, maybe MPG evens out in mixed driving.

You've said you're highway-dominant, but you'll still reach a point of being undergeared where you can't actually maintain the speed you want in the gear you want, and have to downshift, at which point you should have gone with deeper gears.

Manufacturers care about mileage as well. I would be wary of gearing so as to drop your RPMs too much below the lowest offered by a factory gear-and-tire combination.

 

[Uncle Gump]

WWBD

 

[19Walt93]

Don't do it unless you live in flat country. 3.73's act like 2.61 gears when the trans is in overdrive and 3.08's would act like 2.16's (assuming a .70 overdrive ratio which is probably close) and the transmission will be constantly downshifting.
If you choose to go ahead, I've had good luck with Richmond and Motive Gear. I tried Summit gears once and they whined but that might not be a concern in a pickup. Whatever you install needs to be properly set up or you'll end up with a lot of noise. I've built dozens of rear ends and never had one that reused all the shims for the set up.

 

[bhgl]

Don't do it unless you live in flat country. 3.73's act like 2.61 gears when the trans is in overdrive and 3.08's would act like 2.16's (assuming a .70 overdrive ratio which is probably close) and the transmission will be constantly downshifting.
If you choose to go ahead, I've had good luck with Richmond and Motive Gear. I tried Summit gears once and they whined but that might not be a concern in a pickup. Whatever you install needs to be properly set up or you'll end up with a lot of noise. I've built dozens of rear ends and never had one that reused all the shims for the set up.

The overdrive ratio is 0.75, and I'm running a 27.3 inch tire, which is something like 0.4% smaller in diameter than the stock 225s. So frankly not much of a difference overall.

I've done some crunching with 3.08 gears and I SHOULD be alright in my use case. At my regular highway speeds I'll be smack in the middle of the power band pretty consistently, and I'll be in a much better RPM range for fuel economy.

Some weight reduction, aero, and power mods are on the way as well which should hopefully ensure that I won't be dropping out of overdrive on the highway constantly.

 

[bhgl]

You can't freely trade power for MPG (or vice versa) all up and down the scale of all possible gear ratios.

If you burn 5% less gas at cruise, but have to spend 2x as long on the gas every time you accelerate, maybe MPG evens out in mixed driving.

You've said you're highway-dominant, but you'll still reach a point of being undergeared where you can't actually maintain the speed you want in the gear you want, and have to downshift, at which point you should have gone with deeper gears.

Manufacturers care about mileage as well. I would be wary of gearing so as to drop your RPMs too much below the lowest offered by a factory gear-and-tire combination.

Very cognizant of this, my use case effectively involves long stretches at over 100 KpH, my last 1000+ KM trip had an average speed at just over 110 KpH, for much of the trip I was over 3000 RPM for long stretches, particularly on a high speed section where the truck sat at just around 140 KpH for a little over an hour and a half.

I think a drop down to 2.73 is probably excessive, although it may be useful down the road, but I think dropping down to 3.08's will significantly improve the truck's highway performance.

I have some weight reduction, aero, and modifications to improve available torque and horsepower on the docket as well.

 

[Blmpkn]

Personally.. I think even going to 3.08s is excessive.. all highway driving or not. It's just a HUGE jump.. 3.55s or 3.27s would be a better route to go IMO. Especially if it's an auto trans.

When I went from 3.73 to 4.10s in my old truck it made about 'a gears worth' of difference.. as in.. turning onto & driving down the last road to my house with the 3.73s was most comfortably & reasonably done in 2nd gear.. once the 4.10s went in I could do the last stretch in 3rd.. and thats barely a .4 difference in ratio..

 

[pjtoledo]

get a vacuum gauge, hook it up to the manifold and run the hose into the cab.
try driving in various scenarios taking note of the vacuum.
when the vacuum is highest the engine is running at it's most efficient relative to power vs load.
take note of the rpms and calculate the gear ratio.
that may help show if a drastic change would do you any good.

 

[Blmpkn]

get a vacuum gauge, hook it up to the manifold and run the hose into the cab.
try driving in various scenarios taking note of the vacuum.
when the vacuum is highest the engine is running at it's most efficient relative to power vs load.
take note of the rpms and calculate the gear ratio.
that may help show if a drastic change would do you any good.

This is good advice.. efficiency is a delicate balance when it comes to rpm vs load. Higher rpm/low load has a good chance of being more fuel friendly than lower rpm & higher load.

Mabey just slow down a bit too? Aerodynamic drag does not increase in a linear fashion as speed increases.. it grows exponentially. 50-60 mph is generally the most efficient speed for a vehicle.

Sort of apples/oranges but with my bronco.. on the highway.. I can achieve 23 mpg if I stick to 65. If I wanna do 80 though.. 13mpg with a tailwind on a flat stretch.

 

[bhgl]

This is good advice.. efficiency is a delicate balance when it comes to rpm vs load. Higher rpm/low load has a good chance of being more fuel friendly than lower rpm & higher load.

Mabey just slow down a bit too? Aerodynamic drag does not increase in a linear fashion as speed increases.. it grows exponentially. 50-60 mph is generally the most efficient speed for a vehicle.

Sort of apples/oranges but with my bronco.. on the highway.. I can achieve 23 mpg if I stick to 65. If I wanna do 80 though.. 13mpg with a tailwind on a flat stretch.

I do have a vacuum gauge actually! It's going in fairly soon alongside a Scangauge 2 or E to track economy.

Slowing down is the easiest way to improve efficiency no doubt, part of the equation for my use case is however time over distance. given 8-12 hours of driving in a day, a higher average speed does actually get me to my destination sooner by a worthy margin.

The first long distance test with MAB3L, the truck in question, we achieved a pretty striking average of just about 23 MPG highway, that's with a 400-600 ish lbs payload, an average speed of a little over 115 KpH, and cold/poor weather.

I know power dyno graphs aren't exactly the efficiency calculation we're looking for, but you may be right in saying that I'm already in or near the ideal RPM range. Given that the truck makes most of it's power between 2500-3500 RPM.

These graphs were posted previously on the forum, and do not super accurately represent the 3.0 Vulcan as it's used in the Ford Ranger, but definitely food for thought:

This is the Vulcan in a 2005 Taurus:

And this is the Vulcan with roller rockers installed

EDIT: Included images.

 

[bhgl]

What's interesting is that the air/fuel ratio really does lean out on the one test after 70 MpH, I've got no information as to what transmission is being used, but it ought to be running in it's final drive ratio for a dyno-test.

That could explain why I was able to achieve pretty good fuel economy despite the high RPM. I should definitely consider dropping down to a 3.45/3.55 instead after I do some more number crunching.

 

[superj]

i htink you should make one of those cool aero bed covers that slopes down from the top of the cab and sticks out the back a bit to come to a point. i saw on some fuel efficiency forums where people made their own aero camper shells and they looked super slick.