Thoughts on gear ratios for highway use?

[Minuit]

I'll be acquiring an MJ D44 rear axle soon. It will be going in my '91 4.0 HO with 235-75/15 tires (721 rev/mile). I do not plan on using tires any larger than that size, and I plan on rebuilding this axle anyway, so regearing cost is not a factor I'm worried about.

 

My truck originally came with tiny 195-75/15 tires. I can't even find a solid revolutions/mile number for them so I'll have to use the diameter for my math. With these tires and 3.55 gears, at 70 mph the engine would be spinning at right around 2400 rpm. With my tires, the engine is spinning at 2250 or so rpm at the same speed. Subjectively, the engine feels very lazy under about 65 mph, and there appears to be more power available at higher speeds even after considering drag. According to the dyno charts I've seen, the 4.0 HO starts picking up more torque right after 2250 rpm, and my butt dyno results seem to agree.

 

What's my reason for worrying about this? I have observed that my truck appears to get slightly better mileage at 80 mph than at 70. In fact, there seems to be noticeably more power available at that speed and it just seems happier in general at 80. That's a strange characteristic for an almost 30 year old pickup truck, but hey. My truck is in excellent mechanical shape and I have no problems driving it that fast, but I'd like to bring the optimal mileage point down a little bit. How do I know this? That's the speed I have to go to not get murdered on I-65 heading towards Nashville in the morning.

 

What are my goals? I'd like the best highway mileage possible. Any butt dyno improvements are a bonus. My truck is capable of 19 mpg at 80 mph, verified over several tanks. About 1-1.5 mpg less at 70. The 4.0 HO is not the tractor engine people think it is, and likes to rev a little bit. That's why I'm thinking I might get a little bit better mileage with slightly deeper gearing. I also think playing with axles might be fun.

 

So, that leaves me with a few options as far as gears.

 

- 3.55, the "correct" ratio for my truck. BORING! Would make my life the easiest of all, but it would leave me slightly undergeared at 70, according to my butt dyno. I could just slap in any Dana 30 from a 4.0/automatic vehicle later on and be good to go gear-wise.

 

- 3.73, which is a closer but not exact match to the original gearing. It's also the numerically highest ratio that does not require either a new carrier or thick-cut gears. Would make my life harder in the long run, since there would be absolutely no bolt-in D30 options for the inevitable 4wd conversion. This would result in a 5% increase in rear-wheel torque and engine RPM. Enough to be worth the trouble of regearing a D30 later on? Was there any factory offerings of 3.73s at any point in the XJ or MJ?

 

- 3.92 would return me almost exactly to the factory gearing with the 195-75 tires. This is not an actual option, since I have to be able to build a matching D30 and calibrate the speedometer.

 

- 4.10, which might be a little too much for my tire size. With a 15% increase in rear-wheel torque and engine rpm, the truck is sure to feel a bit quicker. However, that might be enough to reduce highway mileage. A D30 from most 4-cylinder vehicles would be a direct bolt-in later on.

 

Any thoughts?

[JMO413]

The XJ had an option of 3.73. I noticed them on carpart while debating this myself. Not sure what all years but if you look up a 95 it's shows as an option with some out there. The 4.10 is hard to find in the non CAD.

[Limeyjeeper]

This is a great calculator that will give you all the answers to your question

 

 

http://www.grimmjeeper.com/gears.html

[gogmorgo]

I want to say you could get 3.73's in tow package XJs. Early years especially you could get all kinds of weird combos... I know of an '86 2.5 MJ with the "fuel miser" spec 3.31s. But that's the wrong direction. I have seen a few XJ d30's on [choose your classifieds page] that have already been regeared to 3.73 and sometimes mildly (or extremely) built, generally not too far off what junkyard asking price would be. 

ZJ axles also came in 3.73's with the v8 and tow-pack 4.0s, but that's a low-pinion d30.

[Pete M]

4.10s all day, every day.  

[Jeep Driver]

12 minutes ago, Pete M said:

4.10s all day, every day.  

[Eagle]

2 hours ago, Minuit said:

I'll be acquiring an MJ D44 rear axle soon. It will be going in my '91 4.0 HO with 235-75/15 tires (721 rev/mile). I do not plan on using tires any larger than that size, and I plan on rebuilding this axle anyway, so regearing cost is not a factor I'm worried about.

 

 

I found revolutions-per-mile for 195/75-15s. https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/pub?hl=en&hl=en&output=html

 

Quote

I'll be acquiring an MJ D44 rear axle soon. It will be going in my '91 4.0 HO with 235-75/15 tires (721 rev/mile). I do not plan on using tires any larger than that size, and I plan on rebuilding this axle anyway, so regearing cost is not a factor I'm worried about. With these tires and 3.55 gears, at 70 mph the engine would be spinning at right around 2400 rpm.

 

Actually, much closer to 2200 RPM. 2448 would be for your factory 195/75-15 tires and 3.55 gears. To match that with 235/75-15 tires you are mid-way between 3.73s (2360 RPM @ 70) and 4.10s (2594 RPM @ 70).

[johnj92131]

Think Don's truck had 4.10 to replace his stock gears when he changed to a Dana 44.  Think he posted a 2 mpg improvement.  Remember his engine was a 4.6 stroker.

Go with the 4.10

 

My old 91 with the 3.07 factory Dana 44 and Auto - I always ran 225 or 235/75 x 15 tires.  Sometimes in the mountain passes, I would have to downshift to 3rd to hold interstate speeds.

[Eagle]

When I bought the 3.73 gears that ended up in the '88 MJ, they were intended to be used in the '88 Cherokee, and they were intended to be used with 30x9.50-15 tires. 30x9.50s are very close to the diameter of 235/75-15. I think that's a good ratio for street driving and highway driving with tires of that size. I wouldn't go to 4.10s unless I planned to run at least 31" tires.

 

Your criterion seems to be 80 MPH, so let's look at that. 3.73 gears with 235s would have the engine spinning at 2,697 RPM at 80 MPH. 4.10s will have you turning 2,964 RPM at 80 MPH. The latter will give you better performance, but I don't know if it will give you better fuel economy. I couldn't find any horsepower/torque graphs for the 4.0L HO engine. My guess is that it won't be much different regardless of which you choose, but it's probably easier to find axles with 4.10s than 3.73s.