Tire Size / Final Drive Ratio - Reality Check (More-fun-with-math!)

[leety]

 

So the truck came with a set of Rickson wheels 19.5" and a set of tires that were less than a year old.  I liked the old forged-in-America wheels so I decided ot keep them on the truck.

 

I have never had one of these trucks befoer this one so I have no comparative baseline on how it "should" run and drive.  In LA these trucks are like hen's teeth and I have not found anyone near me that knows much abou them at all.  My local dodge dealer is a waste of real estate and every time I've gone there they turn up their nose at my old truck and try to sell me a new truck :-)  I couldn't even get a radiotor cap from that palce.

 

After studdying this issue of tire size, on this website, more closely I decided to take a closer look at my set up as believe there is a fly in the ointment.

 

Now for the more-fun-with-math part.

 

So according to Mopar1973Man the goal is to have a Final Drive Ratio that is between 3.55 & 3.73

 

My truck started its life with a final drive ration of 3.55 (Dana 80 in the rear and Data 60 in the front)

 

Stock tire size was 235/85-R16

Current tire size is  203 245/70-R19.5

 

So I have a lift of about .85"

https://tiresize.com/comparison/

 

 

 

Upon running the calculation I now have a paultry effective final drive ratio of 3.41, so far so bad.

https://tiresize.com/gear-ratio-calculator/

 

 

Now the question is to should I lower the water or raise the bridge?

 

I could keep the wheels and change the gearing in the Axels or I can get a set of wheels and tires.

 

Either solution will cost about the same in the greater LA Area.

 

So let's say I decided to go the regearing route.

 

I believe Dana offers 3.73 and 4.1 gearing.

 

The question is this, if I went the re-gearing route which would be preferable (either 3.73 or 4.1) and why?

 

Also I have the NV5600 at (0.73:1) and I'm not exactly sure how to add that into my math here or that's even relevant.

 

Thanks @Mopar1973Man for this article this is super helpful in trying to sort this all out.

 

Edited October 13, 2020 by leety

[Mopar1973Man]

Yes there is a option for 3.73 gears. 

 

The last one in McCall ID that did an gear change from 3.55 to 4.10 gear for 37" wheels it was roughly $4,400 buck for both front and rear axles. 

 

17 minutes ago, leety said:

I can get a set of wheels and tires.

What I tell people is to barrow a set of stock wheels and tires and go for a drive. Most people afterwards would just about sell there bigger tires and wheels.

 

Yeah your sitting at 3.37:1 final to the ground so you will benefit from changing out to 3.73 gears to fit your current wheels. The 3.73 would bring you right back to the 3.55 final ratio for the most part. Like I opted for lower yet aiming for 3.73 finals and you find that the truck is even quicker. Does cap out the top end a bit but still can reach 100 MPH easy. So if you did the 4.10 gears it would put you at 3.89 which is a bit too low in ratio so you most likely right at 3.73 gears. 

[leety]

8 minutes ago, Mopar1973Man said:

Yes there is a option for 3.73 gears. 

 

The last one in McCall ID that did an gear change from 3.55 to 4.10 gear for 37" wheels it was roughly $4,400 buck for both front and rear axles. 

 

What I tell people is to barrow a set of stock wheels and tires and go for a drive. Most people afterwards would just about sell there bigger tires and wheels.

 

Yeah your sitting at 3.37:1 final to the ground so you will benefit from changing out to 3.73 gears to fit your current wheels. The 3.73 would bring you right back to the 3.55 final ratio for the most part. Like I opted for lower yet aiming for 3.73 finals and you find that the truck is even quicker. Does cap out the top end a bit but still can reach 100 MPH easy. So if you did the 4.10 gears it would put you at 3.89 which is a bit too low in ratio so you most likely right at 3.73 gears. 

Thanks that was kind of what I was thinking would make the most sense.

[Doubletrouble]

 No trying to hi Jack but @leety I see you have a dually. What was the oem tire size? What size you running now? Just curious. 

[leety]

1 minute ago, Doubletrouble said:

 No trying to hi Jack but @leety I see you have a dually. What was the oem tire size? What size you running now? Just curious. 

Stock tire size was 235/85-R16

Current tire size is  203 245/70-R19.5

 

So I have a lift of about .85"

https://tiresize.com/comparison/

 

 

 

Upon running the calculation I now have a paultry effective final drive ratio of 3.37, so far so bad.

https://tiresize.com/gear-ratio-calculator/

[LorenS]

You're actually at 3.41, right? You ran the calculator with 19.9" wheels.

And @Mopar1973Man's magic numbers for ratio is based on 245/70/16 tires, I believe. That yields a need for 3.97 gears; the jump to 4.10 makes sense at that point.

EDIT: it's 245/75/16. So, it'll be 3.85; I don't know what makes sense from there!

 

Edited October 13, 2020 by LorenS
Wrong tire size used.

[Doubletrouble]

 I was just making sure I was using that calculator correctly. I punched in my stock size in both fields and came up with 3:55 which is the ratio of both axles. So my final drive ratio is at 3:55-1 ?

[leety]

45 minutes ago, LorenS said:

You're actually at 3.41, right? You ran the calculator with 19.9" wheels.

And @Mopar1973Man's magic numbers for ratio is based on 245/70/16 tires, I believe. That yields a need for 3.97 gears; the jump to 4.10 makes sense at that point.

 

Doh!  Thank you and corrected.

 

1 hour ago, Mopar1973Man said:

Yes there is a option for 3.73 gears. 

 

The last one in McCall ID that did an gear change from 3.55 to 4.10 gear for 37" wheels it was roughly $4,400 buck for both front and rear axles. 

 

What I tell people is to barrow a set of stock wheels and tires and go for a drive. Most people afterwards would just about sell there bigger tires and wheels.

 

Yeah your sitting at 3.37:1 final to the ground so you will benefit from changing out to 3.73 gears to fit your current wheels. The 3.73 would bring you right back to the 3.55 final ratio for the most part. Like I opted for lower yet aiming for 3.73 finals and you find that the truck is even quicker. Does cap out the top end a bit but still can reach 100 MPH easy. So if you did the 4.10 gears it would put you at 3.89 which is a bit too low in ratio so you most likely right at 3.73 gears. 

Sorry made a typo earlier in the calculator at 19.9 wheel size (non existant) so it actually landed at effective ratio of 3.41. So it seems like the 3.73 gears put just a bit higher than than would have been the 3.55 ratio right?

 

45 minutes ago, LorenS said:

You're actually at 3.41, right? You ran the calculator with 19.9" wheels.

And @Mopar1973Man's magic numbers for ratio is based on 245/70/16 tires, I believe. That yields a need for 3.97 gears; the jump to 4.10 makes sense at that point.

EDIT: it's 245/75/16. So, it'll be 3.85; I don't know what makes sense from there!

 

According to my build sheet the my truck started it's life with 

235/85-R16 so I think I should be okay with the 3.73 solution.

https://www.jeep.com/webselfservice/BuildSheetServlet?vin=1B7MF33791J565630

Edited October 13, 2020 by leety

[Royal Squire]

You should probably make sure gears haven’t been changed previously 

[leety]

4 minutes ago, Royal Squire said:

You should probably make sure gears haven’t been changed previously 

For sure.  I'm reasonably certain they were not but I have to be 100% sure and that's a great point thank you for bringing that to my attention.

[Mopar1973Man]

Count the numbers of driveshaft turns to the one turn of the tire. 

 

Either going to be 3.55 or 4.10 turns of the driveshaft.

12 hours ago, leety said:

And @Mopar1973Man's magic numbers for ratio is based on 245/70/16 tires, I believe. That yields a need for 3.97 gears; the jump to 4.10 makes sense at that point.

 

My magic number of 3.69 is the 265/75 R16 to 245/75 R16 with 3.55 axles. Still a stock tire size. The early years was 245's and the later years to favor the big tire crazy were the 265's.

 

I was also wondering about the 19.9 rims. 19.5 that makes more sense. But that still puts you a lower ratio than 3.55. You could run it but always have that extra loading on the driveline. I was opting to head towards 3.73 because of the power and towing reasons. So for all the long hauls, I've done to Arizona and up north. Been a super good final ratio. Super fast to build power and pass other vehicles. Like a few days ago I took Jacob @Wet Vette, son, to town doing little oil leak hunt on my truck. When I came home through the slide I pass the construction zone and got open ground and laid on the throttle. She snapped the boost gauge to 49 PSI and the rear tires started letting go RPM took off and this was at 45 MPH (4th gear) and damp asphalt.

 

3.69 ratio is a super quick ratio and puts me at a perfect 66 MPH at 2K on the tach. I'm staying in 5th gear longer now in the mountains and not having to shift down as much. Being that 55 MPH isn't at 1,400 anymore. I can leave it high gear and pull easy up to 100 MPH in about 5 second from 65 MPH. Make it really strange that on CANBus fuel alone no wiretap from the Quadzilla. 

 

Truck below is the 245/75 R16 tires on Stock 16x7 wheels from early 2nd Gen. Both picture and sig below)

 

Edited October 13, 2020 by Mopar1973Man

[leety]

12 hours ago, Mopar1973Man said:

Count the numbers of driveshaft turns to the one turn of the tire. 

 

Either going to be 3.55 or 4.10 turns of the driveshaft.

 

My magic number of 3.69 is the 265/75 R16 to 245/75 R16 with 3.55 axles. Still a stock tire size. The early years was 245's and the later years to favor the big tire crazy were the 265's.

 

I was also wondering about the 19.9 rims. 19.5 that makes more sense. But that still puts you a lower ratio than 3.55. You could run it but always have that extra loading on the driveline. I was opting to head towards 3.73 because of the power and towing reasons. So for all the long hauls, I've done to Arizona and up north. Been a super good final ratio. Super fast to build power and pass other vehicles. Like a few days ago I took Jacob @Wet Vette, son, to town doing little oil leak hunt on my truck. When I came home through the slide I pass the construction zone and got open ground and laid on the throttle. She snapped the boost gauge to 49 PSI and the rear tires started letting go RPM took off and this was at 45 MPH (4th gear) and damp asphalt.

 

3.69 ratio is a super quick ratio and puts me at a perfect 66 MPH at 2K on the tach. I'm staying in 5th gear longer now in the mountains and not having to shift down as much. Being that 55 MPH isn't at 1,400 anymore. I can leave it high gear and pull easy up to 100 MPH in about 5 second from 65 MPH. Make it really strange that on CANBus fuel alone no wiretap from the Quadzilla. 

 

Truck below is the 245/75 R16 tires on Stock 16x7 wheels from early 2nd Gen. Both picture and sig below)

 

 

With the 19.5" wheels the tires are commercial, same as you see on gas powred motorhomes.  These tiers prety much don't get flats, I had two nails in one of my tires last week, all the way through, and not even a peep of a leak, also I like the side walls being so stiff with the camper always being on the truck.  These tires help with side to side movement with the camper on top that helps.  I may add a rear sway bar.  I live on a very steep and windy road.  Stunt I think you know that road.

 

I think I can get in and out of Hooper for about $1500 to regeare the front and back bumping up to a 3.73. It's a bit more to go to 4.10 becuase the carrier would need to be changed. That will cost me less than a complete set of wheels and tires x7 of them all together.

 

I do realize that the mass of my wheels is greater than stock and that increases my loand and will diminish my speed, gas mileage etc.  I'm not worried about speed so that's not a factor for me.  But the commercial tires give me a marked advantage for the application.

 

Edited October 14, 2020 by leety

[Mopar1973Man]

Yup a lot of members have switch over to 19.5 tires for long haul use. Not unheard of. Not look at speed as a factor but how much does it take to get to speed... That's all.

[leety]

1 hour ago, Mopar1973Man said:

Yup a lot of members have switch over to 19.5 tires for long haul use. Not unheard of. Not look at speed as a factor but how much does it take to get to speed... That's all.

Oh yea my zero-to-sixty rate won't make any records that's for sure.  It'll be good enough to get me where I want to be.  Right now getting up to speed is rather slow, like Mother's molassas, and I'm guessing if I get the difs regeared to 3.73 I'll see a marked improvement.

[Mopar1973Man]

I've had suggest gear swaps for several vehicles now and all of them seen marked improvements. Even a Dodge Cummins here locally he got 37" tires swapped from 3.55 to 4.10. Afterwards it was much better performance and MPG. 

[NIsaacs]

6 hours ago, leety said:

Oh yea my zero-to-sixty rate won't make any records that's for sure.  It'll be good enough to get me where I want to be.  Right now getting up to speed is rather slow, like Mother's molassas, and I'm guessing if I get the difs regeared to 3.73 I'll see a marked improvement.

 

 

Yes, don't even consider doing away with the 19.5 commercial tires with your overhead camper.

 

To re-gear for $1500 from 3.55 to 3.73 will only give you a slight bump in performance.

 

For $472 you could install an Edge EZ and walk all over a gear change as far as cost, simplicity and performance. Changing out your oem (low milage) set of gears, (they last forever) and taking a chance on a fubar gear changer is a big risk. Dana axles are the worst axle for re-gearing, unless you are an expert at the trade. They don't grow on trees. I have rebuilt a bunch of them (Dana's) never lost one but I still hate working on them 

[leety]

1 hour ago, NIsaacs said:

 

 

Yes, don't even consider doing away with the 19.5 commercial tires with your overhead camper.

 

To re-gear for $1500 from 3.55 to 3.73 will only give you a slight bump in performance.

 

For $472 you could install an Edge EZ and walk all over a gear change as far as cost, simplicity and performance. Changing out your oem (low milage) set of gears, (they last forever) and taking a chance on a fubar gear changer is a big risk. Dana axles are the worst axle for re-gearing, unless you are an expert at the trade. They don't grow on trees. I have rebuilt a bunch of them (Dana's) never lost one but I still hate working on them 

If I do the regearing it's gong to the best shop in LA who's been doing this stuff since before color, because I totally agree with your philosphy.

 

Thank you for the other option to consder!!!

[LorenS]

Tire Size         Diameter

245/75/16       30.5"

265/70/16       30.6"
235/85/16       31.7"

265/70/17        31.6"

265/65/17        30.6"

240/70/19.5     33"

 

The 19.5 option is about 8.2% larger than the stock size (30.5-30.6") for us 2500 owners, which is what Mike's Magic Ratio (MMR) references.  Not measles...
3.55 * 1.082 ~ 3.84, which would be a very custom ratio, indeed.  From this, it's probably the best choice - if you regear - to go to 3.73.  This is 3% difference from the MMR.  In the real world, this means you'd be going 68 MPH at 2,000 RPM versus Mike's 66 MPH (assuming that you have same transmission final ratio).  I drive an Auto, so my overdrive is even more ridiculous and puts me at 74 MPH with 3.55s and 30.5" tires, according to the math.  I have 31.6" tires right now, the 265/65/17's should go on before snow flies.  Just what I need, even more torque to the ground in snow and ice!

 

[leety]

7 minutes ago, LorenS said:

Tire Size         Diameter

245/75/16       30.5"

265/70/16       30.6"
235/85/16       31.7"

265/70/17        31.6"

265/65/17        30.6"

240/70/19.5     33"

 

The 19.5 option is about 8.2% larger than the stock size (30.5-30.6") for us 2500 owners, which is what Mike's Magic Ratio (MMR) references.  Not measles...
3.55 * 1.082 ~ 3.84, which would be a very custom ratio, indeed.  From this, it's probably the best choice - if you regear - to go to 3.73.  This is 3% difference from the MMR.  In the real world, this means you'd be going 68 MPH at 2,000 RPM versus Mike's 66 MPH (assuming that you have same transmission final ratio).  I drive an Auto, so my overdrive is even more ridiculous and puts me at 74 MPH with 3.55s and 30.5" tires, according to the math.  I have 31.6" tires right now, the 265/65/17's should go on before snow flies.  Just what I need, even more torque to the ground in snow and ice!

 

Thanks for the fun with math!!  Very cool!!!

 

I have  a 6 speed manual NV5600.  I don't purchase automatic vehicles.  Gears were meant to be rowed through in my view.

 

Mike has an NV4500 at (0.75:1)

My NV5600 is at (0.73:1)

 

Let me know if you know how to add that to the algorytm cause I don't.

Edited October 14, 2020 by leety

[LorenS]

14 minutes ago, leety said:

Let me know if you know how to add that

Okay, so we take (0.75/0.73) = 1.0274  Your final gear is 2.74% higher than Mike's, so instead of 68 MPH it's about 69.9 MPH.  That's with the 3.73 gears and your 19.5s. discussed above.

That 3.84 'custom gear set' would need to be 3.95 gears (1.0274*3.84) to match the MMR resulting in 66 MPH at 2k RPM.  So now you're creeping closer to 4.10s, which would put you at about 63.6 MPH at 2,000 RPM.  Be nice if you had a 7 speed, with a "five and a half" gear of about 0.85.  Then you could keep your 3.55 gears, and that gear would be like having 4.18 rear gears for uphill, and your 0.73 final gear for flat and downhill.  Or drafting semis...

 

Are you a lead-foot, 80 MPH driver with that permanent sail attached?  When one factors in drag, you may want the 4.10s for the higher speeds - which seems quite counter-intuitive.  Pushing that camper through the air will take some power!  Other folks on here would have far more experience with this type of operation than I do, hopefully they will chime in.  This might be a good question in the RV/travel trailer forum.