Re: How much does rear gear ratio effect HWY gas mileage?
[Re: ragtop]
#2504377
06/04/18 03:06 PM
06/04/18 03:06 PM
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Joined: Jul 2011
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PurpleBeeper
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super stock
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Chicago
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This is over-simplified, but at a steady speed on the highway, just do a little math problem.
For example, if you got 10mpg with 4.10's and you swapped out to 3.23's, the you'd get about (4.10/3.23) x 10mpg = 12.7mpg traveling at the same speed. This should be really darn close.
70 Roadrunner convt. street car
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: PurpleBeeper]
#2504393
06/04/18 03:28 PM
06/04/18 03:28 PM
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Joined: Jan 2005
Posts: 12,419 Kalispell Mt.
HotRodDave
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This is over-simplified, but at a steady speed on the highway, just do a little math problem.
For example, if you got 10mpg with 4.10's and you swapped out to 3.23's, the you'd get about (4.10/3.23) x 10mpg = 12.7mpg traveling at the same speed. This should be really darn close. I have actually found this to be close to a point, you can not keep adding higher ratio and just always keep getting better MPG as your engine will only make enough power to keep you moving at a certain RPM and if its always downshifting to get up the slightest incline you just reach a point of diminishing returns but if you have very deep gears already than this formula is kinda close. I have contemplated throwin a 2.20 rear diff in my daokta with a .69 OD ratio in the trans and really put it to the test but have not had the time or ambition.
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: ragtop]
#2504465
06/04/18 06:27 PM
06/04/18 06:27 PM
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Joined: Jan 2004
Posts: 8,290 fredericksburg,va
cudaman1969
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fredericksburg,va
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I had thought of going from 4.10 to 3.54 in my van but the cost, time and changing the pcm to change mph would not make it worthwhile I'm my opinion. Maybe only change 3-400 rpm and not sure where the optimum efficiency rpm for a 5.9 Magnum is at. Majority of my driving is around 55 mph. 440s can lug, 318s can't.
Last edited by cudaman1969; 06/04/18 06:28 PM.
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: ragtop]
#2504488
06/04/18 07:52 PM
06/04/18 07:52 PM
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Joined: Dec 2005
Posts: 1,159 Cruising!
QuickDodge
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super stock
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Cruising!
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Gas mileage is difficult to predict at times. Many years ago, I swapped a 2.94 or a 2.76 for a 2.45??? It made virtually no difference. In fact, the highway RPM's did not change much. The fairly loose factory torque converter seemed to be slipping more. Later I swapped in a transmission with a lock up converter. That dropped the RPM's a little bit. It did not make much difference in the mileage though. I suspect it was because there was not much difference in the gear ratios. (I primarily did the swap because the new axle was a sure grip.)
In some cases, swapping in an overdrive transmission is a better option than changing the gears in the axle.
Last edited by QuickDodge; 06/04/18 07:53 PM.
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: a12rag]
#2504528
06/04/18 09:47 PM
06/04/18 09:47 PM
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Joined: Nov 2010
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Spaceman Spiff
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On my 74 Duster360, I took out the 3.55 rear end and swapped in a 2.45 rear end (complete, drum to drum) before going to Carlisle, back in 1999. The RPM at 70mph went down to 2500-2600. Was able to get 23mpg all day long at 70mph. With the 3.55, mileage was 18-19mpg . . . big difference on long trips. The 360 had more than enough torque to spin the right wheel (open rear) with the 2.45 gears. 2500-2600 rpm with 2.45 @ 70? that's more like a 2.93 gear.
526 cubes of angry wedge, pushbutton shifted, 9 passenger killer!
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: topside]
#2504677
06/05/18 07:41 AM
06/05/18 07:41 AM
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Joined: Nov 2010
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Spaceman Spiff
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It's not hard to understand on carbureted cars; remember vacuum gauges? You want the thing running at max vacuum (minimum throttle opening), which is generally at torque peak. That peak on stockish engines is usually in the low-2000-rpm range. Like Barry experienced, my last 225/6 Dart got no better hwy MPG than my 318 Dart; it had to be flogged a bit where the 318 didn't. We have no idea what engine, or car, the OP is talking about.
526 cubes of angry wedge, pushbutton shifted, 9 passenger killer!
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: PurpleBeeper]
#2504679
06/05/18 07:54 AM
06/05/18 07:54 AM
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Joined: May 2005
Posts: 4,734 Florida
BDW
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This is over-simplified, but at a steady speed on the highway, just do a little math problem.
For example, if you got 10mpg with 4.10's and you swapped out to 3.23's, the you'd get about (4.10/3.23) x 10mpg = 12.7mpg traveling at the same speed. This should be really darn close. Best answer, held true in the 2 gear changes I've done.
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: ragtop]
#2504689
06/05/18 09:13 AM
06/05/18 09:13 AM
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Joined: Jan 2003
Posts: 8,028 Tulsa OK
Bad340fish
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It just depends on the combo. My 11:1 360 with a big roller cam got better MPG at a 3800 rpm cruise than it did at 3000. I did 4 drag weeks with that 360, the first year we had to run down I-40 and I had to run fast to not die in traffic, 3800-4000RPM for 100 miles netted just under 15 MPG, it usually got 13@3000-3500 cruise. Getting close to peak torque and getting to the tighter side of the converter is the reasoning I am sure.
68 Barracuda Formula S 340
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: Spaceman Spiff]
#2504776
06/05/18 01:06 PM
06/05/18 01:06 PM
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Joined: Apr 2007
Posts: 2,920 Calgary, Alberta Canada
a12rag
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On my 74 Duster360, I took out the 3.55 rear end and swapped in a 2.45 rear end (complete, drum to drum) before going to Carlisle, back in 1999. The RPM at 70mph went down to 2500-2600. Was able to get 23mpg all day long at 70mph. With the 3.55, mileage was 18-19mpg . . . big difference on long trips. The 360 had more than enough torque to spin the right wheel (open rear) with the 2.45 gears. 2500-2600 rpm with 2.45 @ 70? that's more like a 2.93 gear. 235/60/14 tire . . . with that 2.45, that was what the rpm was - 2500-2600. Gotta think that all the other variables (intake, carb, exhaust, cam, convertor, tranny, etc) factor into all this. Of course each engine needs to be in "it's sweet spot" where it is happy . . .
Last edited by a12rag; 06/05/18 01:08 PM.
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: ragtop]
#2504808
06/05/18 01:51 PM
06/05/18 01:51 PM
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Joined: Nov 2006
Posts: 2,255 IL
furious70
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IL
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95 5.9 ram 3.55 to 4.10 actually picked up 2-3mpg in most driving, the OD + lockup was too lazy
My Fury 383 commando spec with 727/3.23 and then a500/3.91 the mileage is actually a little worse with the OD because I don't drive fast enough. The 3.23's were no fun @ 80mph commuting but I don't spend enough time at that speed. The 727/3.23's were better at 60-65mph. With a bigger cam I think it'd make even more pronounced difference as you lug the slobbery cam.
70 Sport Fury 68 Charger 69 Coronet 72 RR
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Re: How much does rear gear ratio effect HWY gas mileage?
[Re: a12rag]
#2504814
06/05/18 02:06 PM
06/05/18 02:06 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
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It depends entirely on the engine load and the engine's efficiency map. So there's no real answer, but in general as rpm increases, internal friction goes up and economy goes down. So, IN GENERAL, lowering the numerical gear ratio should increase mileage.
But here's a true story of my '64Dog. It's a 318 Poly stocker D100 and it came with 3.91 gears. Rear tires were 32-11.50 15, 636 revolutions per mile. I used to drive long distances and it would get a maximum of 15mpg. Much of that time was spent on Interstates at 80. Calculating that out that's 3320rpm at 80.
Then, I swapped rears as I had a donor truck and the '64 axle was leaking onto the brakes. The donor truck had a 318 and 727, 3.23 gears. New rpm,2740 at 80.It's like overdrive, right? To my dismay the next trip fuel mileage was same as before, 15mpg.
Changing the gear ratio that much really made for a better road truck but around town it lacked the snap it had with the 3.91s.
R.
Last edited by dogdays; 06/05/18 02:13 PM.
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