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I have 3.42 gears I'm looking to get 37's to run with my 4" bds lift, how do 37's do with 3.42 or would I be better off with 35's. I do very little towing just a 17' bass boat in the summer locally.
 

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Cummins motor has plenty of torque to push the 37's. If you pulled a lot then maybe regear. You'd notice more of a difference regearing then running 35's in my opinion.
 
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Discussion Starter · #4 ·
Thank you for the fast responses y'all.
 

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35" tires probably won't make to much of a difference without regearing. 37"s put you really low in the rpm range to where 6th gear is useless until freeway speeds. I'm going to regear with 4.10s to get my shifting close to normal. A 3.83 would make 37"s the same as stock 33 and 3.42. It is doable but bugs me how it has to drop to 4th then hit 5th for every little hill. I drive 60mph and 70mph on the two main roads to work everyday.
 
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Slight de-rail here. Can you put 35's on factory 17" rims without lifting or leveling and not have rubbing issues? Or do you have to at least do a level? In my searches the largest I've seen somebody go without a level is a 33" on factory rims.
 

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You can also go to ringpinion.com and use their calculators. One will tell you what rpm you will be running at what speed. It has you input your ring and pinion ratio plus tire size transmission gear ratio (it has a list of all Trans ratios) and speed then tells you the rpm's. It was very helpful for me to decide on gears. Cruising rpm's for this motor should be between 1400 and 2200.
 

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Slight de-rail here. Can you put 35's on factory 17" rims without lifting or leveling and not have rubbing issues? Or do you have to at least do a level? In my searches the largest I've seen somebody go without a level is a 33" on factory rims.
Trowel, I would think you would have to at least go with a level kit with 35's...:thumbsup:
 
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Here's the chart I made. It'll save you a little time. I have the 68rfe and was comparing gears for 37" tires. Pretty self explanatory. Another thing to consider is fuel mileage. My evic mpg thing went from 20 mpg now I might see 14.4 at the highest. I know it's not completely accurate but it's a good estimate. Also my speedometer isn't corrected so it's off by 12.8%. Keeps some miles off the truck til I get new gears. Bigger tires are heavy so consider that too.
 

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I was hoping to only lose 1-2 mpg with my 35" Toyo AT2s on stock rims and 2" level but it looks like it is closer to 3-4 mpg. My 2001 24v manual took less of a mpg hit with a similar setup but it also didn't get as good of mpg to start with. I make the same 450 mile round trip each week.

I have not had the speedometer reset yet. I have wondered if the bigger tires and already tall 3.42s knock the turbo out of its efficiency range. Are light throttle shift points based on mph or rpm?
 

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Here's the chart I made. It'll save you a little time. I have the 68rfe and was comparing gears for 37" tires. Pretty self explanatory. Another thing to consider is fuel mileage. My evic mpg thing went from 20 mpg now I might see 14.4 at the highest. I know it's not completely accurate but it's a good estimate. Also my speedometer isn't corrected so it's off by 12.8%. Keeps some miles off the truck til I get new gears. Bigger tires are heavy so consider that too.
What size lift do you have for them 37's.

I do 100% highway driving to/from work and I avg 22 sometimes 23mpg (when I'm most careful on throttle).
 

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5" lift. Most of my driving is highway with speedlimits of 55 and 65 and some hills. 60mph is to low and I usually end up staying in 5th unless it's flat road or down hill.
 

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5" lift. Most of my driving is highway with speedlimits of 55 and 65 and some hills. 60mph is to low and I usually end up staying in 5th unless it's flat road or down hill.
Mine is flat road with cruise control at 60 the whole way.
 

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Just to get a feel for it try it out at 54mph that's about the rpm you'll be running. If it's flat though you should be fine. Doesn't feel like I lost much acceleration off the line. Once the turbo is making boost you can't really tell you have bigger tires. Another reason why I'm regearing is I was gonna do electronic lockers but decided on a set of Eaton Detroit lockers. I don't want this truck to get stuck my old tacoma had an electronic locker and it was nice and never got stuck in mud snow or sand.
 

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On a very powerful diesel truck, you won't notice the effects of larger tires as much as you would say with a mid size gasser. But those larger tires have higher rolling resistance, and by lifting the truck you also decreased its aerodynamics.By changing the Ring & Pinions in your vehicles differentials, you can match the ratio to the tire size to bring the RPM’s back to where the manufacturer intended. Typically it is recommended to select a ratio slightly lower (higher numerically) than what the math will tell you.

In an effort to improve fuel economy, late model overdrive transmissions are geared very high which means RPM’s are very low on the highway. When even a slightly taller tire is installed the RPM’s are reduced below acceptable levels that the engine was designed to run. This causes constant shifting out of overdrive, sluggish acceleration, poor economy and rapid wear on the transmission and other components.
 

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Just to get a feel for it try it out at 54mph that's about the rpm you'll be running. If it's flat though you should be fine. Doesn't feel like I lost much acceleration off the line. Once the turbo is making boost you can't really tell you have bigger tires. Another reason why I'm regearing is I was gonna do electronic lockers but decided on a set of Eaton Detroit lockers. I don't want this truck to get stuck my old tacoma had an electronic locker and it was nice and never got stuck in mud snow or sand.
Can you explain lockers for me as it relates to differentials. I've always heard people doing this upgrade but don't know exactly what this does over stock differential and what the pros/cons are.

Thanks!
 

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Can you explain lockers for me as it relates to differentials. I've always heard people doing this upgrade but don't know exactly what this does over stock differential and what the pros/cons are.

Thanks!
Basicly makes a true 4x4. All 4 wheels pull. Electric/air gives ability to disengage them. Full time lockers can be dangerous on wet slippery roads
 

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5th gear is .816:1
68RFE Transmission Gearing

Tires = 37" OD
Speed=60
RPM=1522

5th gear is .816:1
Tires = 37" OD
Speed=70
RPM=1774

6th Gear = .625
Speed = 60
RPM=1166

6th gear = .625
Speed = 70
RPM=1360
 

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Just like stated above. I had an electronic locker in my tacoma and it was a 4x4 on the fly and I always had problems unlocking the rear end and getting out of 4 low. This truck I have the manual 4x4 shifter and don't want to deal with the locker not unlocking. A Detroit locker has clutches and only locks up when needed. It's like limited slip but better. If you're not gonna be off road alot in the woods or fields don't worry about lockers. You can't run a locked up axle on the street without the inside tire jumping and chirping.
 
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