Hey everybody, new member here.

A question for you: I'm going to be starting a project XJ soon, and selling my old XJ ('99 Limited). Here's the question I have:

I was driving a WK2 the other day with the 8-speed transmission in it. I noticed that, even at 80MPH, my RPMs in 7th/8th gear with around 2000, which is a 20%-25% reduction in engine speed, at roughly 10MPH above the speed limit!

My '99 runs at about 2,400 RPMs at 70 MPH (20MPG) - and even at about 2,000 RPMs at 55MPH (27MPG); so, Logic dictates that, if I'm able to swap the 8-speed 845RE transmission into my XJ (assuming my new project XJ is around the same age), that I could quite possibly crack the 30MPG threshold!!

Has anyone here just done a transmission swap with the 4.0, that wasn't an AX-15 or AW4, or wasn't just the popular Automatic-to-manual swap?

Thanks in advance for any help/advice you might have, and make sure to get out and explore this weekend!!

 

https://www.novak-adapt.com/knowledg.../manual/nsg370

 

Yes sir, this one I know about. However, I was trying to get into something with more gears, and I'm not 100% certain that the Novak conversion kit would work with either the 6L80 or 8L80 transmissions.

I should reach out to them and ask, though.

 

Logic seems to be forgetting about aerodynamics and engine power curves. It's not all about gearing. Run the engine too slowly and you will get worse mileage, not better. There's something called the Law of Diminishing Returns. There's no way to know what mileage you will get without some sophisticated computer modeling or just try it and see. But I very much doubt you'll get anywhere close to 30MPG.

But even if you do crack the 30 MPG barrier, how much will you save? That depends on several factors, like how much you drive and how expensive gas is. The more you drive, and the more expensive gas is, the sooner you see the payback. If it's a daily driver, you'll pay it back before a weekend warrior. If you live in L.A. you''ll pay it back sooner than if you live in Texas.

Let's do some math:

Assumptions:

1. You drive 50,000 miles per year.
2. You get 20 MPG.
3. The price of gas is $4.00

That means that you use 2,500 gallons per year, at a total cost of $10,000 per year. (Scary, ain't it! )

Now change that 20 MPG to 30 MPG and we get 1,667 gallons for a total cost of $6,667, for a savings of $3,333 per year. That is not too shabby! I'd bet your trans swap will run in that neighborhood, maybe a bit more. A one year, or year and a half payback is probably worth doing if you plan to keep the beast.

Now lets look at some more numbers:

30,000 per year @ $3 per gallon saves you $1,500 per year.
10,000 per year @ $3 per gallon saves you $500 per year. (If your project cost 4 grand, that's an 8 year payback.)

Now, at a more realistic 25 MPG (if that), you are looking at:

50,000 per year @ $3 per gallon saves you $1,500 per year.
30,000 per year @ $3 per gallon saves you $900 per year.
10,000 per year @ $3 per gallon saves you $300 per year. (If your project cost 4 grand, that's a 13 year payback.)

Now, if that transmission only buys you 23 MPG, you are looking at:

50,000 per year @ $3 per gallon saves you $978 per year.
30,000 per year @ $3 per gallon saves you $587 per year.
10,000 per year @ $3 per gallon saves you $196 per year. (If your project cost 4 grand, that's a 20 year payback.)

Looks to me like a "do it for fun" project, not a "do it to save money" project.

Always do the math!

 

If a simple gear ratio change would have given near double the mpg, jeep would have offered that back on the first XJ.

Ask some of the folks that installed big tires on their xj without doing the gear ratio change. they too can cruise at 70 or 80 mph at 2000 rpm or so. But they are not getting 30 mpg.

 

Your transmission swap may get you 1 MPG better at best, you can't cruise at that low of an RPM it would be downshifting all the time and you may not be able to use 8th gear at all unless you put lower gears in the axles. If you hand calcuclate the milage I highly doubt you are even getting 20 MPG at 70 MPH.

 

I understand what you want to do and why. However you need to take into account your engine torque curve. That engine speed is where the sweet spot is for efficiency. As many who have put on larger tires, effectively reducing engine speed like a gear change would, do not see an increase in mpg. Instead, it is the opposite. The other thing is that the 845 is computer controlled, so normal shifting would not work. You could possibly rig up something, but it would be a real pain to use.

 

If you want to cut your fuel bills, run on Propane