37’s with 3.73

[Amd241997]

Got a 2004.5 5.9 with 3.73 gears and running 35’s now. At 70mph I’m right at 2000 RPM and rarely go above 70 except when passing since I’ve noticed my fuel eco drops. I have an Edge juice monitor/programmer that’s telling me when running the hot tune and cruising at 70 for a while it’s been telling me I’m getting a little over 30mpg (don’t know if there’s any truth behind that). I’ve realized my RPM’s would drop running 3.73 and 37’s but it shouldn’t drop too much maybe 300 RPM. Is that lugging the motor too much? I don’t tow or do anything hard with it but I’m in need for new tires soon and it’s only about $200 more for the 37x13.5 vs 35x12.5

Any input would be great. Thanks!


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[RamIt]

This is a VERY generic chart. However, in a 1:1 ratio, the RPM drop is only 125.

https://docs.google.com/spreadsheets...eNI/edit#gid=0
According to this spreadsheet (I did not make it), your RPM drop is closer to 100.

This is only the drop between 35-37, you need to look at the drop from OE size to 37 and idk what the OE size for your truck is.

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[Amd241997]

This is a VERY generic chart. However, in a 1:1 ratio, the RPM drop is only 125.

https://docs.google.com/spreadsheets...eNI/edit#gid=0
According to this spreadsheet (I did not make it), your RPM drop is closer to 100.

This is only the drop between 35-37, you need to look at the drop from OE size to 37 and idk what the OE size for your truck is.

http://www.anycalculator.com/tireandgear.jpg

According to that chart I’m looking at a 500 drop from oe to 37’s (currently a 400 drop). Oe’s should be 30’s. My concern is that even with the 35’s at 70 which is highway speed, I can’t go any faster or I risk jumping to high in the rpm’s. I forgot to add that I also have a non billet 48re with the tc kicking in at 50mph. So with the 37’s I should stay pretty close to my current RPM’s anyways. Anyone else care to chime in?


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[RamIt]

Oh you have a 4 speed....my bad.

That's my fault, I'm thinking 68, not 48. Regardless, if the drop is only 100rpm across the board, then you shouldn't have much to worry about.

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[Amd241997]

Oh you have a 4 speed....my bad.

That's my fault, I'm thinking 68, not 48. Regardless, if the drop is only 100rpm across the board, then you shouldn't have much to worry about.

That’s what I’ve figured too. My main concern was dropping high speed rpm’s so maybe I could go a little faster and not waste so much fuel but also be able to drive around town at lower rpms without lugging it too much. Been told it could have a possible negative effect on the tranny but in what way I don’t know. Fitment with 37’s is fine since I have a huge gap with the 35’s, just wondering about drivetrain longevity issues even if the gap in rpm loss is that small. If it was like a 500 drop then maybe that’s a bad thing but even still.


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[Polaraco]

This is a VERY generic chart. However, in a 1:1 ratio, the RPM drop is only 125.

https://docs.google.com/spreadsheets...eNI/edit#gid=0
According to this spreadsheet (I did not make it), your RPM drop is closer to 100.

This is only the drop between 35-37, you need to look at the drop from OE size to 37 and idk what the OE size for your truck is.

http://www.anycalculator.com/tireandgear.jpg

That’s what I’ve figured too. My main concern was dropping high speed rpm’s so maybe I could go a little faster and not waste so much fuel but also be able to drive around town at lower rpms without lugging it too much. Been told it could have a possible negative effect on the tranny but in what way I don’t know. Fitment with 37’s is fine since I have a huge gap with the 35’s, just wondering about drivetrain longevity issues even if the gap in rpm loss is that small. If it was like a 500 drop then maybe that’s a bad thing but even still.


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I have 342's in my 03 with 34" tires. I'm running lower than the chart says. 70 MPH, I'm around 1900. MMMMM EGT's are very low too. One of the problems with the 3rd gens is they have the aerodynamics of a parachute. There isn't much you can do but here's a couple of trick I used. 4th Gens are 24% more slipperier er er er.

I retrofit a 2nd gen air dam under the factory. You'd never know it's there to look at it. Reduces the air from going under the truck crating friction on all that hardware.

I bushed out the gasket between the door and windshield. It stops the air from being forced into the door frame, causing less drag. It makes the cab quieter too. That's a good sign.

Run weatherstriping around the doors where ever there is a gap air can get into. Especially the front.

I have a roof high cap on the bed. I put some air tabs around the back of the cap breaking off the vacuum on the back window. They work!

I think, the combo of all that is reducing my friction. I'm within 100 RPM of @F350Cummins truck now. I just recently did the door sealing so I haven't checked the RPMs again. I just know the cab is quieter now which tells me I hit a nerve.

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[Amd241997]

I have 342's in my 03 with 34" tires. I'm running lower than the chart says. 70 MPH, I'm around 1900. MMMMM EGT's are very low too. One of the problems with the 3rd gens is they have the aerodynamics of a parachute. There isn't much you can do but here's a couple of trick I used. 4th Gens are 24% more slipperier er er er.

I retrofit a 2nd gen air dam under the factory. You'd never know it's there to look at it. Reduces the air from going under the truck crating friction on all that hardware.

I bushed out the gasket between the door and windshield. It stops the air from being forced into the door frame, causing less drag. It makes the cab quieter too. That's a good sign.

Run weatherstriping around the doors where ever there is a gap air can get into. Especially the front.

I have a roof high cap on the bed. I put some air tabs around the back of the cap breaking off the vacuum on the back window. They work!

I think, the combo of all that is reducing my friction. I'm within 100 RPM of @F350Cummins truck now. I just recently did the door sealing so I haven't checked the RPMs again. I just know the cab is quieter now which tells me I hit a nerve.

So essentially going from a 35 to 37 with my gearing wouldn’t be detrimental? I’d probably in all reality lose 100 RPM anyways. I’ve also realized that it is like driving a tank but I haven’t done much to alter aerodynamics. Maybe I’ll go a bed cover but not a topper. I’ve got dirt bikes so it’s a bit of a pain to have to remove the topper every summer (had the same issue running a cap on my old f-150). Thanks for the input though!


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[Polaraco]

So essentially going from a 35 to 37 with my gearing wouldn’t be detrimental? I’d probably in all reality lose 100 RPM anyways. I’ve also realized that it is like driving a tank but I haven’t done much to alter aerodynamics. Maybe I’ll go a bed cover but not a topper. I’ve got dirt bikes so it’s a bit of a pain to have to remove the topper every summer (had the same issue running a cap on my old f-150). Thanks for the input though!


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Not sure the bed cover will do anything for you. They have the trucks figured out so you can drive with the tailgate up to get the best fuel economy. It's always been that way. But if you do want one, they make a folding one that would be out of your way without removing it.

Sidebar and a rant
I just had the misfortune of replacing a spring mount on an F150. What a hunk of junk! The quality of assembly and engineering of the Dodge is sooooo much better.

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[TexNeck]

My main concern would be clearance. You leveled or lifted? Going 2” taller and 1” wider is asking to rub depending on offset and added clearance

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[Amd241997]

Not sure the bed cover will do anything for you. They have the trucks figured out so you can drive with the tailgate up to get the best fuel economy. It's always been that way. But if you do want one, they make a folding one that would be out of your way without removing it.

Sidebar and a rant
I just had the misfortune of replacing a spring mount on an F150. What a hunk of junk! The quality of assembly and engineering of the Dodge is sooooo much better.

Sad to hear about that! Mine was the cam phasers and I didn’t wanna drop $1300 in parts to have them fail again. Truck had 150,000 and the Cummins has almost 300,000 but I have t had to do anything but an oil change.


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