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  #1  
Old 03-06-2009, 03:49 PM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
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RPM, tire size, and axle ratio questions

I am in the process of an engine build and that got me thinking about how to best build my rig for my driving (mostly daily driving). Though I am not that concerned about mileage (afterall it is not a Prius), I would like it to do it's best (I guess the Cub Scout in me wants this). From a couple other threads, I have a general understanding that overall weight of the GW (1988) is approximately comparable to my wife's ;04 Grand Cherokee (with the skid plates etc). So, I started to compare the two (I know somewhat apples and oranges)...

**Currently the GW has the 2.72 axle ratio, TF 727 tranny, and NP 229 t-case (pretty much stock). I will be increasing tire size to 30X9.5X15 (free tires). If I follow things correctly, this will give me a final RPM of around 2200-2300 at 70mph (highway speed). (when I drove it last it averaged 16-18 mpg on the highway with the smaller tires)

**Wife's GC has the 4.0L, 3.73 axle ratio, NV 42RE tranny, NV 242 tcase, and tires that are 30" in diameter (perfectly stock). Her final drive ratio puts her rig at around 2000 RPM at 70mph. (averages 17-19 mpg on the highway)

So, on with my questions:

The high axle ratio will make this feel like a dog...(expecially compared to the GC) right?

Will the high axle ratio kill gas mileage in city???

Would a lower axle ratio kill gas mileage on highway???

Would a lower axle ratio kill my new engine???

I guess ideally I would put in lower axles AND and overdrive transmission...but that is not an option... I am just trying to understand all sides of this and make a choice that will keep me happy with the rig for a long time to come.
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  #2  
Old 03-06-2009, 04:10 PM
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Tornado230 Tornado230 is offline
350 Buick
 
Join Date: Sep 14, 2008
Location: Fleetwood, PA
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You can get used to anything over time.
I have the J-10 with the 258/auto/2:72 gears.
It will cruise at speeds up to running out of numbers on the speedometer, but hit the hill and the thing dies. Around here on our hills, I do a lot of running in second. Virtually no power in drive under 45 MPH.
I can get 17-18 MPG on the highway at speed. Around town the mileage drops to around 13-14 or so.
The 360 has a little more power, but you will not be running within the power band at moderate highway speeds.
The thing to do is install a vacuum gauge and watch what it does when you are driving under varying conditions. You will probably see the needle down in the red area quite a bit. That's the result of having your foot into it to maintain speed.
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Old 03-06-2009, 04:47 PM
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bushwood44 bushwood44 is offline
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I think the 2.73 will suck (gas)in town,but ok on the highway.The towing package gears(3.31) would be better.Personally, if I was going to bother changing them I would go with 3.73, but I don't mind the engine turning a few rpms.
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Old 03-06-2009, 05:07 PM
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jdaniel83 jdaniel83 is offline
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I had a similar setup on my GW (2.72, 727, np229, etc.) and it was quite a dog especially on hills. Of course I have a 4" lift and 32" Swampers so I wasn't getting nearly what you guys are getting. I averaged about 10 city and 12 at best highway. However, after I swapped in some 3.73's my city mileage stayed about the same at around 10 but my highway dropped to around 9 to 10 MPGs.

The performance difference was day and night though (I can actually drive up hills in D and maintain speed instead of having to drop to 2). Maybe if I drove around 60-65 instead of 70 the mileage would be better but then again I don't want to be that slow guy on the freeway.
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'89 GW; 4" Susp Lift on 32" Wild Country MTX; 360, HEI, Edelbrock Intake, Melling MTA-1 Cam, Summit 600 carb, 3.73 Gears

'70 Wagoneer; stock Buick Dauntless 350, TH400; 3.73 stock gears

'83 Wagoneer Limited; stock 360, 727, 3.31 gears.
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  #5  
Old 03-06-2009, 05:44 PM
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Eugene 1 Eugene 1 is offline
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Try this :http://www.ringpinion.com/Calc_RPM.aspx
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  #6  
Old 03-06-2009, 06:23 PM
incommando
 
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http://www.grimmjeeper.com/gears.html

This caluclator is amazing. It has most every gear ratio for trannies, transfers, etc...pre-loaded!
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  #7  
Old 03-06-2009, 06:38 PM
GWChris GWChris is offline
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My '89 had the 2.73's - they were nice at 70 -75! 30X9.5X15 tires are just barely oversize - not enough to be significant.
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Old 03-06-2009, 07:04 PM
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Serious Johnson Serious Johnson is offline
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Location: Pumpkintown, South Carolina
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Tall gears and a 727 (or T-400 for that matter)don't necessarily translate to decent highway fuel efficiency. That old slushbox is extremely inefficient at low revs. My Wag, when running the stock setup you describe, got a little bit better fuel mileage at 75 MPH than it did at 50. 3.31s and 30-inch tires might be pretty good for a primarily highway-driven rig. 3.73s are a wee bit much for that, but sure would feel great everywhere else.

For my Wag, as currently equipped with a T-18a, 33s, and a 6,500 RPM-capable 360, 3.73s are a touch too tall for heavy towing on the highway.

S.J.
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  #9  
Old 03-06-2009, 10:36 PM
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jdaniel83 jdaniel83 is offline
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To be honest if I had to do it over again I might have gone with the 3.31 or 3.54's. I do a little more highway than I used to nowadays and the 3.73's are a little too high RPM's for my comfort going down the highway. With the 32" tires I'm doing about 2800 at 65, but I bought some stock rims with 235/75 tires to see if they would work better on the highway and it was tipping the 3100-3200 range at 65 which in my opinion is too high for highway driving. If you are going to be primarily on highway then I'd look into the 3.31 or 3.54 ratio.
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'89 GW; 4" Susp Lift on 32" Wild Country MTX; 360, HEI, Edelbrock Intake, Melling MTA-1 Cam, Summit 600 carb, 3.73 Gears

'70 Wagoneer; stock Buick Dauntless 350, TH400; 3.73 stock gears

'83 Wagoneer Limited; stock 360, 727, 3.31 gears.
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  #10  
Old 03-07-2009, 12:43 AM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
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I guess my real question is where do I want the RPMs to be for mostly on-road driving (probably 60% interstate 40% city)? From what I have read, for mileage on a small block V-8 you want it between 1800 and 2800 RPM...that is a pretty large range. I used to have a CJ with a 4-cylinder and ran that thing at 2400+ RPM all day (it loved higher RPM)... So, where do I want to run this 360?
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Old 03-07-2009, 12:09 PM
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badaboom badaboom is offline
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simply check here

http://www.superioraxle.com/feature.html
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  #12  
Old 03-07-2009, 01:00 PM
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TPICherokee TPICherokee is offline
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Quote:
Originally Posted by incommando
http://www.grimmjeeper.com/gears.html

This caluclator is amazing. It has most every gear ratio for trannies, transfers, etc...pre-loaded!

Best gear calc out there most all of the charts don't compensate for things like lockup converter and overdrive transmissions...
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