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Unread 09-13-2013, 02:25 PM   #1
Necromancer_tat
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What gas mileage does your V8 swap get?

I have a YJ with a 302 V8 and I'm only getting about 6 mpg... I have a CJ7 with an AMC 360 getting about the same. Both are fuel injected, the YJ is on 36" swampers and the CJ is on 35" all terrains. Do I need to try getting them tuned in better, or is that about what the rest of you guys are also getting? If you're getting a lot better mileage than I am then please let me know what you did to improve yours.

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Unread 09-13-2013, 03:05 PM   #2
jp360cj
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I'm getting 10-11 mpg around town on my '7 since going to 35s. Mostly stock motor except for the cam. What's the rest of your setup?
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Unread 09-13-2013, 06:23 PM   #3
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Quote:
Originally Posted by jp360cj View Post
I'm getting 10-11 mpg around town on my '7 since going to 35s. Mostly stock motor except for the cam. What's the rest of your setup?
CJ7 is AMC 360 with a 4 speed granny first, Chevy TBI injection. I think the injection is where I'm having problems with both Jeeps. I think it's running to rich or something. Both Jeeps run pretty good, but if I run my finger inside the exhaust tip it comes out nice and black. I'd be a lot happier if I was getting 11 mpg in town.
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Unread 09-13-2013, 07:34 PM   #4
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We have an xj in our club with a 318 efi. 20 on highway 14ish in town.
Have you regeared?
Running rich sucks. I know. Check your o2 sensors. Did you use the o2's for the specific engines?
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Unread 09-13-2013, 08:11 PM   #5
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Quote:
Originally Posted by uberxj92 View Post
We have an xj in our club with a 318 efi. 20 on highway 14ish in town.
Have you regeared?
Running rich sucks. I know. Check your o2 sensors. Did you use the o2's for the specific engines?
That's good mileage for an XJ, even better with a V8, but XJ's get better mileage than CJ's YJ's to start with being as they're unibodys and have better aerodynamics. I think they're lighter also. As far as gears go I think I'm stock gears on the CJ, and I think I'm 373 in the YJ, but I'd be lying if I said I was 100% on those numbers. Both Jeeps have plenty of get up and go, and turn about 2k on the odometer at 60 mph in 4th gear.
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Unread 09-20-2013, 09:13 PM   #6
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I've got a 95 YJ with a 2003 GM 5.3, 4.11 gears and 33" BFG AT's. I average around 14mpg and have seen over 16. Running 80+ on the interstate will drop that to around 12.5 mpg.
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Unread 09-21-2013, 10:49 AM   #7
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I get 13-14 mpg with mine.
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Unread 09-21-2013, 03:57 PM   #8
89YJBuilder
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I'm gettin ready to do a Ford 5.0l NP435 35s and 4.88s in my YJ and have been told that I should be looking at around 20 hwy and 14 city.
I know a cousin of mine has an LSI in his with a 700R4 and is getting 20 and 16.
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Unread 09-23-2013, 09:39 AM   #9
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Quote:
I think I'm stock gears on the CJ, and I think I'm 373 in the YJ, but I'd be lying if I said I was 100% on those numbers. Both Jeeps have plenty of get up and go, and turn about 2k on the odometer at 60 mph in 4th gear.
If your running stock gears with 35/36" tires, you've found some of your problem. Regardless of mileage, those tires are going to require something in the 4 series to bring the cruise rpm's down and close the shift points. Happy speeds are generally considered to be in the2200 to 2500 range for V8's at 60 MPH. Insure your speedometer is adjusted for the tires). Most folks are happy in the 4.56 range, with 4.88 taking the 5 speed crowd. Do some research, and think about a swap.
Not saying you don't have other issues, just that you see an improvement in pulling, shifting, overall power and mileage.
Insure the vac advance on the dist is operating properly. That little canister is worth 1 to 2 MPH at cruise.
Insure your timing is correct. You can play with it a bit, advancing it.
You may check to insure your FI module is correct for the application. This is especially true for the 360.
You can also build a relatively inexpensive O2 meter. Do a google search and study up a bit on it. This would allow you to see what your mixtures are at various speeds and loads.
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Unread 09-24-2013, 09:01 PM   #10
anthonyc12
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What gas mileage does your V8 swap get?

Finished my swap about 4 month's ago on my dd/wheeler . 1992 4runner with 1994 gm 4.3 cpi , l460e auto, sas , 35" tires 4:88 gears. I thought I'd be getting a little better mileage. If i really keep my foot out of it i get about 13 mpg. ? Everything has been rebuilt and runs great . New injector , o2 sensor...

Would going v8 get any better mpg ? The thought being it wouldn't have to work as hard to get going. I don't know a lot about the different v8 models but i would like to keep it " obd 1 " so it's easier to keep it smog legal in ca.


I know this isn't a hard core question but i thought the orange paint might make up for it .
1.jpg

2.jpg

3.jpg

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Unread 09-24-2013, 09:41 PM   #11
89YJBuilder
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According to reason a more powerful engine would get better mileage as it doesn't have to work as hard to do the same amount of work. With 2.5l, AX5, NP231, 4.10 gears and 31s I get 13 and I live in the Mountains and on flat highway I can't get over 65 with a tail wind. Gears are the second most important part of planning a swap. I mean all the gears, trans, t-case, and diff. Just because you have a 5 speed don't mean better mileage than a four speed if the gears don't work together well. As well if the drive train gives bad gearing for the amount of engine than you still won't do well.
What I am saying is research the crap out of it and then plan, plan, plan, and research some more. You really can't over study this one, though you can over think it.
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Unread 09-24-2013, 11:01 PM   #12
jp360cj
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Quote:
Originally Posted by anthonyc12 View Post
Finished my swap about 4 month's ago on my dd/wheeler . 1992 4runner with 1994 gm 4.3 cpi , l460e auto, sas , 35" tires 4:88 gears. I thought I'd be getting a little better mileage. If i really keep my foot out of it i get about 13 mpg. ? Everything has been rebuilt and runs great . New injector , o2 sensor...

Would going v8 get any better mpg ? The thought being it wouldn't have to work as hard to get going. I don't know a lot about the different v8 models but i would like to keep it " obd 1 " so it's easier to keep it smog legal in ca.

I know this isn't a hard core question but i thought the orange paint might make up for it .
Is that mileage in town or hwy? If its in town, its not to bad. If its highway, then a V8 may do better but a lot will depend on the gearing. You'll need to match the cruising rpm with the engines power band.

here's a great calculator for it:
http://www.grimmjeeper.com/gears.html
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Unread 09-25-2013, 02:46 PM   #13
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Quote:
Originally Posted by 89YJBuilder View Post
I'm gettin ready to do a Ford 5.0l NP435 35s and 4.88s in my YJ and have been told that I should be looking at around 20 hwy and 14 city.
I know a cousin of mine has an LSI in his with a 700R4 and is getting 20 and 16.
I have a 1990 YJ with a 5.0 Mustang engine and an NP435 with 36's and 373 gears and if I baby the crap out of it I can get 10.5MPG on the highway keeping it under 60mph. 488 gears and 35's with no overdrive would have to guzzle gas in a YJ... where did you see that you should get 20mpg?
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Unread 09-25-2013, 07:46 PM   #14
89YJBuilder
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http://www.grimmjeeper.com/gears.html

Run the numbers. The 435 wide ratio has a nice wide gear ratio over the span. Check them against your engines power band and the rpm that is achieved at desired hwy speed.
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Unread 09-25-2013, 08:14 PM   #15
89YJBuilder
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OOPS! You are right I screwed up the axle ratio. Either way if you use the given calculator you can figure your desired numbers. I do not use my jeep as a DD, it is a toy and only see's enough pavement to get to the trail. For example my current 31" MTs have a 3/8" of tread but are dry rot to the point they are leaking. I bought them in the spring of '04. and have only 37k on them. They are missing some chunks of tread and I know that don't help.

It would be more like 3.75 or 3.80.

I will more than likely still go 4.56ish.
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