2nd Gen Non-Engine/Transmission the right gears for an auto with 35" tires

[LVJUNIOR]

I am looking to change gears to 3. 73 or 4. 10 with my 35" tires which would you go with. I run 75mph most of the time on the highway and usually only gross #14,000 when towing

 

[Road Dog]

You want at least 4. 10 and I'd go 4. 56 but I never drive 75.

 

[LVJUNIOR]

I have 4. 56 gears now and it is tacking at 2250rpm at 75mph and a hair over 2400 at 80mph (yes the speed limit is 80mph some places I drive) I have been told that the trucks that came with the 3. 54 gears got a lot better mileage than the ones with 4. 10 gears. With 35's a 3. 75 would be 100rpm lower than 3. 54's with stock tires and the 4. 10's would be 69rpm higher at 75mph. With 3. 73's I would be turning 1853rpm and 4. 10's I would be at 2037rpm at 75mph

 

[Road Dog]

I'd be happy with 2,250rpm. The sweet spot for fuel economy (I hear) is around 1,900rpm. At lower rpm you start to lose boost and the torque that goes with it. I do a lot of mountain driving and I like to stay over 2,000 up hills just to keep the boost up and the exhaust gas temperatures down.

 

[jimdavidson413]

Check out this website for tire diameter vs gear ratio. , This graph shows gear ratios needed for various tire sizes. "4wheelparts.com/aux_incl/gear_ratio.html"


Jim Davidson

 

[AH64ID]

For the tire weight you are probably fine with 4. 56's. A few rpms lower may improve your mileage, but would take many thousands of miles to cover the cost of the change, and your towing performance would drop.

What kind of mileage do you see at 75 now?

 

[RHestand]

LVJUNIOR,
I just installed a new set of 8Rx19. 5 tires on 6 forged alum. wheels on my 98 4x4 12V dually with 4. 10 gears.
I'm out of state until next Wed. so I don't have access to my notes. But I did do some speed/RPM comps on the stock set of 16" tires and wheels and then the new ones, using a GPS, stock speedo. and tach.
I don't remember the tire heights or roll-out dimensions but I'm pretty sure the 8Rs calculated at about 14. 65% more roll-out than stock. Unless my calcs. are wrong, that should bring the 4. 10s down to about 3. 55s, plus or minus.
As soon as I get home I've got to make a 400 mile road trip so I'll be doing a MPG check/comp. to see what the difference might be. If you want I can get back to you with all the info. I've come up with.
I do know this, with what little driving I've done with the new setup, at 65MPH its much closer to the RPM sweet spot, and I don't notice any "seat of the pants" 0 to 60 MPH acceleration difference.
As a side note, each new tire and wheel only weighs 25lbs more than the old ones, which was much less than I had expected.
Ray

 

[AH64ID]

LVJUNIOR,
I just installed a new set of 8Rx19. 5 tires on 6 forged alum. wheels on my 98 4x4 12V dually with 4. 10 gears.
I'm out of state until next Wed. so I don't have access to my notes. But I did do some speed/RPM comps on the stock set of 16" tires and wheels and then the new ones, using a GPS, stock speedo. and tach.
I don't remember the tire heights or roll-out dimensions but I'm pretty sure the 8Rs calculated at about 14. 65% more roll-out than stock. Unless my calcs. are wrong, that should bring the 4. 10s down to about 3. 55s, plus or minus.
As soon as I get home I've got to make a 400 mile road trip so I'll be doing a MPG check/comp. to see what the difference might be. If you want I can get back to you with all the info. I've come up with.
I do know this, with what little driving I've done with the new setup, at 65MPH its much closer to the RPM sweet spot, and I don't notice any "seat of the pants" 0 to 60 MPH acceleration difference.
As a side note, each new tire and wheel only weighs 25lbs more than the old ones, which was much less than I had expected.
Ray

What size tires did you have stock? 235/85/16 to 8R19. 5 is about a 5. 6% increase, and makes 4. 10's like a 3. 87:1 ratio. Thou I am not sure what you had stock.




To the OP(I am assuming 245/75/16 stock). 4. 56 and 35's is like 4. 02:1. 4. 10's and 35's would be like a 3. 61:1, and 3. 73's would be like a 3. 29:1.

4. 10's are probably the optimal gearing for 35's and empty cruising, but really for a mix of towing and cruising 4. 56's aren't bad, and not worth the money to swap to 4. 10's.

I know my buddy had an 02 with 35's and 3. 55's and hated towing with it, even light trailers.

 

[LVJUNIOR]

This truck has had 35's since I have owned it. I would like to see what your mileage is I would appreciate it if you send me a copy that is exactly what I'm looking for

 

[CBlack]

What does it cost to put different gears in?

 

[LVJUNIOR]

The qoutes I've been getting are for gears and lockers and I've been running between $3000 $4000

 

[AH64ID]

The qoutes I've been getting are for gears and lockers and I've been running between $3000 $4000

If you wheel you can't count the locker cost, but gears alone will be about 2K, which even at 3mpg gain would take about 50K miles to get a 2K return (assuming 14 mpg to 17 mpg freeway)... and if you wheel you would want the 4. 56 anyhow.

 

[LVJUNIOR]

The gears are only around $300 per axle. Eveything else has to be purchased to do the lockers anyway. I am only getting. 12. 5mpg on the highway if I got 17mpg that would be a 40% improvement and only take 8570 miles to pay back. However, I doubt I will improve that much. The more important thing to me is dropping down closer to 2000rpm on the highway and right now I find myself at 2300-2400 very often and for hundreds of miles at a time. At these rpms the motor is out of its efficiency rating, is much louder and builds more heat. I was slightly concerned with the off-road drivability but in 4-low I am not even close to needing all of the torque I have now. Do you think the drop in gears will hurt me that bad off-road? I spend much more time going down the highway than driving off-road. My last hunt was 500 miles round trip and my next one is over 1000.

 

[AH64ID]

I am doing 2275 at 80 with my gears/tires and get 16. 5. I would assume that you would get about 15 at most with 35's, but I could be wrong.

Are you correcting the speedo to get 12. 5? If you are the difference in 12. 5 and 15 would be 1K in savings every 23K miles. 3. 73's would put you just under 2K at 75.

Your axles are D60's right? Doesn't that take a new carrier to go to 4. 10 or 3. 73? Have you factored that into the cost?

It depends on how hard you wheel, but if its just hunting type wheeling you will be fine with anything and 4Lo.

 

[LVJUNIOR]

My speedo is accurate. I used a superlift calibrator for the speedo and odometer and verified it with gps. I have a dana 60 front and a dana 70u rear and yes it requires a case split on the front but if I put a locker in it makes that not a factor in cost

 

[AH64ID]

Yeah, that is one nice thing about doing gears and a locker at the same time, decent money savings on labor.

 

[RHestand]

AH64ID,
I can't remember all the #s but I think you are correct on my stock tire size being 235/85/16. I'm just hoping for a taller gear ratio than the one you calculated, but I'll take what I've got.
In measuring the roll-out (Center of tire tread, with a cloth tape measure. ) it seems to me that the worn out 235s were about 97-98" and the new 8Rs were about 110".
The 12-14" more roll-out is how I came with the 14-15% higher gear ratio.
I'll just have to wait till I get back home to check this out.
Thanks for your input,
Ray

 

[AH64ID]

I don't know for sure what tire came on it, nor what one you are using now, but I used rev/mile of 2 common tires in those sizes. 235/85/16 roll about 656 and 8R19. 5 roll about 621.

 

[LVJUNIOR]

I just measured my tires and found that my 35" tires are actually only 33. 5" which made all of my calculations wrong. It looks like I may have 4. 10 gears in it now

 

[AH64ID]

What make/model are your tires? Most tire mfgr's post rev/mile data on their websites, and yes most 35's roll in the 33" range.

2250 at 75 could either be a tall 35" tire, or 4. 56's on a short 35" tire.

With rev/mile I can give you more exact numbers.