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View Full Version : Freeway RPM's VS gas mileage.



AgitatedPancake
09-21-2009, 11:46 PM
Alright so ever since I snapped the rear axleshaft I've been lookin at a new rear axle. I've got a local guy with a built up35 spline 9" that I'm really interested in. The only catch is its running 5.43 gears right now compared to my 4.56's. My question is would it be bearable to have 5.43's for freeway driving DD status, on the freeway about 20 miles a day? It's nice that my tranny has a double overdrive (final ratio 0.67). I crunched numbers, right now at around 70MPH I'm doing 2000RPMs. With the 5.43's I'd be pulling about 2400RPMs. How much would that absolutely destroy my gas mileage?

Here are my numbers I ran

70mph = 6160 feet per minute
35" tire circumfrence = 9.163'
35" tire 672 RPM's for 70MPH



engine @ 2000 RPM's
Final gear(0.67), shaft spins 2985 RPMs.
4.56 gears tires spin 654 RPM's
35" tires = 68mph

engine @ 2000 RPM's
Final gear(0.67), shaft spins 2985 RPMS.
5.43 gears tires spin 550 RPM's
35" tires = 57mph.
35" tires @ 70MPH = 2444 engine RPM's.

ArloGuthroJeep
09-22-2009, 09:16 AM
Looking outside of gas mileage, your driveshafts will be spinning that much faster and more susceptible to vibrations at higher speeds if they aren't balanced perfectly.

rstrucks
09-22-2009, 09:55 AM
It sounds like you'll be changing a ring and pinion set either way right? So you either change out you front to match the new rear ratio or you swap out the rear to match you current 4.56's. I'd guess that the 400 rpm difference or so will not hurt your gas mileage too awful bad but it will make it worse. How much, I don't know.

AgitatedPancake
09-22-2009, 12:51 PM
The driveshafts would be more susceptible to vibes, but I'm most likely having my rear rebuilt when I do this, it's trashed and dented in a few spots from planting it on things haha. Also yeah I'm going to be swapping one set of gears either way, and a bonus is I've heard the 9" is easy as pie compared to the danas to set up so it looks like I may stick with the 4.56's. I'll probably keep the 5.43's around for the next step up in tire size though.

Thanks guys.

LSRGreg
09-22-2009, 05:06 PM
I played around with my mileage a little last weekend going to southern california.

i am at 6" of lift, 35" tires, stock 3.73 gears with the same 5 speed auto tranny (.67 final gear)

i kept the speedo at 75 mph during this test... in overdrive my RPM's were about 1600 to 1700 if i remember correctly.. and the digital mileage was saying about 18 to 17 mpg.. on a dead flat road.. (any slight grade, and it went down).. then i took it out of overdrive (pushed button on shifter) then RPMs went up to 2800 to 3000 area, and my mpgs registered at 11 on the gauge up top. my rpms may not be exact, because i wasn't paying that much attention, but the mph was the same, and there was a huge difference in mpg while running in 4th gear, instead of 5th, but that is a 1000 rpm difference.

but since i have stock gearing, it would constantly be shifting out of overdrive whenever i had any uphill grades (even very minor ones) at that speed, so overall, my mileage was around 13 to 14 throughout the trip most spent in overdrive doing 75 to 80 mph. Those numbers above were just instant type numbers that it was getting for a couple minutes on flat road. I was just curious about the difference.

maybe that will help you out a little bit..

G-