Ambulance 2wd regear for lower noise and better fuel economy?

[iggi]

Hey folks,

Back from another great camping and climbing weekend.

Now that I've got a couple thousand highway miles on the rig and about 10 hours work in soundproofing it, I'm wondering about just going with a higher gear (lower numerically) in the diff.
I'm running about 2200 rpm for Canadian highway speeds and the 6.0 is kinda noisy. Driving on the secondary highways where the engine sits at about 1800-1900 is definitely easier on the ears.

Running a 4.11 LSD in a Dana 70 HD axle.

Currently it takes a pretty steep hill before it even comes out of overdrive so it seems I could either use another level of over drive or a taller rear gear and the taller rear gear is a lot cheaper!
Looks like a 3.55 or 3.73 are both easily available.

Thoughts? Anyone else gone this route?

 

[vintageracer]

3.73's would work very well in your rig. 3.55's will certainly lower your RPM and maybe give a little better fuel mileage over 3.73's however I would be hesitant to use 3.55's due to the overall weight of your rig itself without a trailer or load.

My 1995 Powerstroke 2WD F350 with 4.10's runs about 2400 RPM at 75 MPH with 235/80-16 tires. The 235/85-16 inch tires are 31 3/4 inches tall and are 1 size larger than the stock tire recommendation for this truck.

I have 3.55's in my Dodge 2WD SRW 2500 Cummins diesel powered PU. Does great on the road and has plenty of power pulling 8K-10K loads with ease. I'm running 33 inch tall tires on 18 inch late model factory Dodge aluminum wheels which are taller than the stock original 235/85-16 wheel/tire combination that was 31 3/4 inches tall.

Last week picked up in Dallas TX and pulled back to Nashville with the Dodge a 7K-8K lb Malibu WakeRunner boat/trailer combination doin 80 MPH the whole way back. Right at 21 MPG empty goin to Texas without a trailer and 18 MPG cummin back home to Tennessee with the boat/trailer in tow. 80 MPH is about 2000 RPM with this Dodge. Remember my Dodge 2500 PU is certainly lighter than your Ambo rig.

In Normal city/street driving in the Dodge without a trailer and with 3.55's it's at least 45-50 MPH before shifting into overdrive. I never use overdrive when towing in a city traffic situation as it just lugs the engine way to much and is hard on the automatic transmission.

 

[iggi]

Thanks @vintageracer Appreciate your input!

 

[Bikersmurf]

3.73 will lower your rpm by about 10%

I been told some of the Crestline rigs came with highway gears and others with lower ones. I’m really happy with 4.10s, but I did find towing a trailer on the highway it lost it’s snappiness. If you’re interested in the towing I’d not go taller... but certainly could go taller on the street.

 

[iggi]

Pretty unlikely that I'll tow anything.

I ran a gear calculator:

Still waiting to see how the next round of sound insulation goes first though.

3.73 will lower your rpm by about 10%

I been told some of the Crestline rigs came with highway gears and others with lower ones. I’m really happy with 4.10s, but I did find towing a trailer on the highway it lost it’s snappiness. If you’re interested in the towing I’d not go taller... but certainly could go taller on the street.

 

[TomsBeast]

I've played with gearing on several truck camper combinations, gear calculators will get you very close, but the proof is in the driving. I have gone so far as to buy and fit a used a pair of undersize (diameter) rear tires, to simulate my proposed ratio change. It really gave me a good 'feel' for it before taking the plunge of a R&P swap. Something to consider.

 

[CaptainBuilt]

I have 3.55 with 33’s and they suck. I’m about to go to 4.10. But 3.73 would seem like a good middle ground.

 

[ujoint]

Going to 3.73 or 3.55 will increase stress on the drivetrain getting up to speed & raise EGTs. The most stress the drivetrain sees is at takeoff and you have a 5 speed with a "high" revving diesel so I'd leave the gears as is.

 

[Len.Barron]

slow down to 64mph and save yourself both the time/expense of a gear swap and get better mpgs..

 

[iggi]

Slow down?? Now that's just crazy talk.

slow down to 64mph and save yourself both the time/expense of a gear swap and get better mpgs..

 

[iggi]

My ambulance is running 29's. A 3.55 diff on 33 inch tires is pretty tall.

My landcruiser is running 3.73 gear with 33 inch tires and 4.10 would be better. It's a bit gutless though.

I have 3.55 with 33’s and they suck. I’m about to go to 4.10. But 3.73 would seem like a good middle ground.

 

[CaptainBuilt]

My ambulance is running 29's. A 3.55 diff on 33 inch tires is pretty tall.

My landcruiser is running 3.73 gear with 33 inch tires and 4.10 would be better. It's a bit gutless though.

Yea, my transmission won't shift down into a lower gear when climbing mountains. So I have to hit the OD off button to make it climb.

I have a set of 4.10's and a locker I just need to install them.

 

[Bbasso]

As mentioned I'd be worried about EGTs with different gears, especially worried about TFT too.

 

[Mwilliamshs]

As mentioned I'd be worried about EGTs with different gears, especially worried about TFT too.

If you're worried, add gauges.

 

[iggi]

Thanks for the feedback. I'm no expert... on gearing or the Ford 6.0 but... I am a bit confused over the concern about EGT's and such on a relatively minor gear change.
3.73 was a factory option in the F350 with the 6.0 so I'm not sure how it would be a risky choice. What am I missing?

As mentioned I'd be worried about EGTs with different gears, especially worried about TFT too.