7.3 diesel with 355 gears

[djg]

Has anyone had 355 gears with a 7.3 diesel was thinking about changing to them from 410's but want to know if it will be geared too low, also what rpm will 65mph, be at 410's at 1900 is about 56mph

[djg]

Grit dog wrote:

djg wrote:
Grit dog it has an intercooler on it and that was what I was thinking about the gears going to 355's too much loss, I was also thinking about a tire change next time though as I only have 16's on it now but won't the increased tire height loose fuel milage I hear 1 MPG per 1" of lift can't say for sure though just what I have heard

Tire size = increase or decrease in mileage, like others said, there's a lot of variables there. Increasing rolling diameter in and of itself theoretically decreases rolling resistance, but increase in moment of interia by itself decreases efficiency.
for best mileage, keep highway tread on it. Going from a HT to a AT decreases mileage a bit and a MT a bit more.
But if your intended speed is not in the sweet spot for mileage, a tire diameter change could actually help you.
Throw ALL the variables in a blender, including weather, topography and your right foot and what comes out is.....
BTW 16s is your rim diameter and not indicative of tire diameter.


I didn't see where you mentioned if it's 2 or 4wd, but 4wd is a total non starter for gears IMO. That's putting 2 grand into an old truck for not "a lot" of gain.
Again, you could very well see some gains with higher gears like you're thinkin about but it might be a push. If your sweet spot for speed is 60 (assuming, since you didn't say) then dropping that much gear ratio will lower your rpms too much at that speed based on 2000rpms now and raise them too much if you shift out of OD.
JMO

Edit:Marcela said what I was trying to explain with the gear splits.

It is 2 wheel drive and mileage is about 13.5 right now I wasn't too worried about really getting better fuel mileage as not wanting to loose too much and just get going a bit faster as 55 is just too slow, I drove tractor trailer for most of my life and know all about wind resistance and how it affects fuel mileage, on the hwy I used to not go over 60 as that is the sweet spot for fuel and resistance. After reading these posts I think I will increase to 60 and see how that will work, no investment but a little more out of pocket for fuel expense. I drive a hybrid all year for a daily driver so I guess I can part with the little more fuel for the adventures, just retired and am going to sell 1 of the cars so less expense that way to begin with and as for the truck it is in too good of shape to part with and have too much money into it so I'll see what happens and thanks everyone for all the posts and info.

Dave

[djg]

Kayteg1 wrote:

Marcela wrote:
Pulling slide in would get around 14 mpg, unloaded 19-20. This is with tall 85 series tires. Cruising at 1900 rpm would be around 65-67 mph.

Had a 2000 7.3 with 2wd drw cc auto and 4.10, pulling slide in it would be 2 mpg less then the above but can't remember at what mph or rpm..

So you are saying that 2000 7.3 dually was making 17-19 mpg? At what speeds?
I had one of those with flatbed configuration and was happy getting 10 empty, when engine was whining high at 65 mph. Flatbed with tall sideboard adds a lot to air drag, but way less than TC.

I think he said 2mpg less would make it 12mpg

[Kayteg1]

Marcela wrote:
Pulling slide in would get around 14 mpg, unloaded 19-20. This is with tall 85 series tires. Cruising at 1900 rpm would be around 65-67 mph.

Had a 2000 7.3 with 2wd drw cc auto and 4.10, pulling slide in it would be 2 mpg less then the above but can't remember at what mph or rpm..

So you are saying that 2000 7.3 dually was making 17-19 mpg? At what speeds?
I had one of those with flatbed configuration and was happy getting 10 empty, when engine was whining high at 65 mph. Flatbed with tall sideboard adds a lot to air drag, but way less than TC.

[Grit_dog]

djg wrote:
Grit dog it has an intercooler on it and that was what I was thinking about the gears going to 355's too much loss, I was also thinking about a tire change next time though as I only have 16's on it now but won't the increased tire height loose fuel milage I hear 1 MPG per 1" of lift can't say for sure though just what I have heard

Tire size = increase or decrease in mileage, like others said, there's a lot of variables there. Increasing rolling diameter in and of itself theoretically decreases rolling resistance, but increase in moment of interia by itself decreases efficiency.
for best mileage, keep highway tread on it. Going from a HT to a AT decreases mileage a bit and a MT a bit more.
But if your intended speed is not in the sweet spot for mileage, a tire diameter change could actually help you.
Throw ALL the variables in a blender, including weather, topography and your right foot and what comes out is.....
BTW 16s is your rim diameter and not indicative of tire diameter.


I didn't see where you mentioned if it's 2 or 4wd, but 4wd is a total non starter for gears IMO. That's putting 2 grand into an old truck for not "a lot" of gain.
Again, you could very well see some gains with higher gears like you're thinkin about but it might be a push. If your sweet spot for speed is 60 (assuming, since you didn't say) then dropping that much gear ratio will lower your rpms too much at that speed based on 2000rpms now and raise them too much if you shift out of OD.
JMO

Edit:Marcela said what I was trying to explain with the gear splits.

[Marcela]

I had a 99 7.3 2wd srw rc auto with 3.73 gears and pulled some monster loads with a gooseneck and also truck camper and everything in between. Keep in mind the gear spacing between 3 and 4 is pretty large, jumping to larger rear gear makes it larger. I always thought about going to something like 3.27 for mileage but never did. Pulling slide in would get around 14 mpg, unloaded 19-20. This is with tall 85 series tires. Cruising at 1900 rpm would be around 65-67 mph.

Had a 2000 7.3 with 2wd drw cc auto and 4.10, pulling slide in it would be 2 mpg less then the above but can't remember at what mph or rpm.

Don't think you want the 3.55 although not much different then the 3.73, the 3-4 gear jump with an automatic and cruising at 65 mph and 1900 rpm is about right with the 3.73. Wind resistance is squared as the speed increases and a lot of that drag is off the back so if you tow or have something to smooth the air flow off the back helps significantly.

[George_H]

With 7.3 and 4:10 gears I run around 2100 RPM on highway at 60+ MPH. With Superchips tuner my peak power is between 1900 and 2100 RPM. Getting bogged down to under 1700, must downshift on any hint of an uphill grade.
Keep the 4:10s.

[VTLee]

I have the 7.3 with 355 gears and 5 speed stick shift. Been all over the USA hauling a 9.5 ft. camper with no problems. My previous truck had the 410 gearing and when I ordered the 7.3 I went with the 355 gearing for a quieter ride. Unless you are pulling a big 5er you don't need the 410.

[burningman]

Well the Gear Vendors would give you both worlds. Pulling gears and freeway gears.
There’s no hard rule that x tire size will change your mileage y amount.
There are too many variables.
If you lack power, that can be fixed. The 7.3 is a great motor and it’s not hard to bump it up a little to a more acceptable level without going too wild. I’ve driven some slightly modded 7.3s that were absolute stompers, and reliable everyday.

I also believe the better MPG at the slower speed has everything to do with drag and almost nothing to do with the slight engine RPM change.
It’s pretty well known that trucks with campers will get a lot better mileage if you slow down somewhere in the 50s for MPH vs. 60+.

[djg]

Grit dog it has an intercooler on it and that was what I was thinking about the gears going to 355's too much loss, I was also thinking about a tire change next time though as I only have 16's on it now but won't the increased tire height loose fuel milage I hear 1 MPG per 1" of lift can't say for sure though just what I have heard

[Grit_dog]

djg wrote:
My 7.3 is stock tune with a banks on it, runs exceptional just tired of 55 to 57mph to keep fuel mileage decent at 60mph I'm running 2000rpm and fuel milage drops so that has been my thinking on changing gears

Stock fueling and a Banks what? Exhaust, turbo, intercooler, bumper sticker?

Question is power and speed vs mileage. Gearing helps to an extent, but IMO not as much as you think. Now, are you running out of comfortable rpms at your intended speed? Do you want to just drive 60 mph giver take? If so keep the gears and the power they afford you.
You won't pay back mileage increase due to gears in a lot of miles. That would be the last reason to re gear IMO.

If you want a little rpm decrease, upsize the tires next time around.
You need to balance how much power to spare you have now with cutting the gearing by that much. And does your Trans have enough gears and the right ratio to support those new higher gears at your intended speed?
2000rpms isn't _____. Sounds like the low end of operating range for a 7.3 if you're not just empty cruising. What will that do to your (4 or 5 speed) trans?

[jplante4]

Isn't that drop in mileage more a result of increased wind resistance which increases as the square of the velocity? If so, then it doesn't matter what gears you have. You'll need to add the same amount of power to overcome the resistance at 60 mph.

The only reason I mention this is due to an experiment that was forced upon me. I had belt sep in the 2 rear tires on one of my cars. I had to wait 4 days to get new tires and in that 4 days I was driving about 55 MPH everywhere (instead of my usual 9 over the speed limit lol). I noticed that average gas mileage took a 25% jump. Nothing else changed except velocity.

Just something to ponder.

[djg]

My 7.3 is stock tune with a banks on it, runs exceptional just tired of 55 to 57mph to keep fuel mileage decent at 60mph I'm running 2000rpm and fuel milage drops so that has been my thinking on changing gears

[rhagfo]

djg wrote:
Has anyone had 355 gears with a 7.3 diesel was thinking about changing to them from 410's but want to know if it will be geared too low, also what rpm will 65mph, be at 410's at 1900 is about 56mph

Is your 7.3 stock or chipped at all? We pull a 12,000# 5er with a CTD with RV275 injectors and a small 50 HP chipfigure about 300 HP and 610 TQ.

Your total weight should not exceed about 14K we tow at about 20K.

[Grit_dog]

Contrary to one of the replies here, if you're talking about a healthy 95 7.3 diesel, it's anemic in stock form. It should have just enough to haul a TC ok with higher gears, but it will be a bigger dog than it is now.
I'd keep the 4.10s unless you're looking for empty freeway driving manners to be better.
For future reference, don't believe any claims of towing 15klbs in overdrive with your truck, except maybe downhill.
Depends how much you want to sink into a truck compared to what its worth.