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How much does rear gear ratio effect HWY gas mileage? #2504303
06/04/18 12:03 PM
06/04/18 12:03 PM
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ragtop Offline OP
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Anyone measured there highway gas mileage before and after a gear swap?
If so, can you post some numbers. Even if it was somewhat steady state rural road driving. I know that city or around town driving won't be hardly effected by gear ratio.
Thank you.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504311
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Only vehicle I've measured it on is my GMC Dually. Going from 4.10 to 3.20 picked up 4 mpg years ago, from 7 to 11 highway (65-70 mph). 2500-2800 RPM is where it makes max torque with least throttle opening.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504316
06/04/18 12:45 PM
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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.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504324
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Great info. Let's here more replies. Keep them coming.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504377
06/04/18 03:06 PM
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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.


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Re: How much does rear gear ratio effect HWY gas mileage? [Re: PurpleBeeper] #2504393
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Originally Posted By PurpleBeeper
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
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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.
Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504472
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When I was messing around with the slant in the duster it seemed like everything I did that made it more fun killed the gas mileage. I put super six 2 brl, 2 1/2" free flowing exhaust opened up the manifold, recurved distributor, and then put 3.73. With the 3.73 the mileage tanked. I decided ok I'm getting v8 mpg, time for the slant to go.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504477
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LOL! yep I have change gears enough to tell which is which by the mpg

my 85 truck 318/727/2.76 gets about 18 mpgs 3.55 down 15 mpgs

my lumpy whiplash cam 440 with a street dominator intake 750 edelbrock headers/3" pipes/muffs/29x15x15 M/Ts gets a steady

10 mpg with 3.91 or less depending on how long at WOT

10-11 mpg with 3.23

12 mpg/hwy with a 2.76

when I had the 85 truck 318 & NV4500 granny gear 5 speed turning a 3.55 gear/235/75/15 I rolled a steady 20 mpg/hwy @ 60 mph

my 82 iron head sportster 4 speed got 56 mpg with a 2.2 final gear ratio but dropped to a 43 mpg with the 2.5 final gear ratio

BUT! going from a 3.5 fuel tank down to a 1.9 fuel tank = walking sooner the first ride.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504478
06/04/18 06:53 PM
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I put thousands of miles on two different 1970 A-bodies when I moved here to PA & I was surprised at the mileage difference.

1970 Dart, 225 auto with 2.76 gears 14.5 MPG

1970 Valiant, 318 3-speed manual & 3.23 gears - 21 MPG

Both engines bone stock with under 38K miles.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504488
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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.
Re: How much does rear gear ratio effect HWY gas mileage? [Re: a12rag] #2504528
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Originally Posted By a12rag
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.


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Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504627
06/05/18 01:18 AM
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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.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: topside] #2504677
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Originally Posted By topside
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.


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Re: How much does rear gear ratio effect HWY gas mileage? [Re: PurpleBeeper] #2504679
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Originally Posted By PurpleBeeper
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.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504689
06/05/18 09:13 AM
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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.


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Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504701
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Because tire diameter and 3rd and higher gear ratios
are involved too,
it is better to discuss this in terms of
Miles per Hour per thousand rpm of engine rotation.

For most heavy V8 vehicles maximum fuel economy on LEVEL highways
comes between 40 and 50 MPH per 1000 rpm.
If your tach is showing 2000 rpm at 80 mph you are in the ballpark.

Manifold vacuum should be
under 12 inches Hg but not less than 4
( you do not want to be in fuel enrichment mode of the Fuel Injection or carburetor)

Camshaft specs matter too. Less lift and less duration help.

One MPG was gained at a steady 70 mph on mostly level blacktop pavement
On a 5400 lb 2wd 360 Ram truck with 39.4 square feet of frontal area and a Aero Cd of 0.42
When the diff ratio was reduced from 3.55 to 3.21.

The $139 computer software modelling program from the Performance Trends company called Fuel Economy Calculator correctly predicted this real world result.

If the diff ratio was reduced from 3.21 to 2.96
Or if the engine displacement was reduced from 360 to 318
the program Fuel Economy Calculator predicts a further improvement of 0.7 MPG at 70 mph.

Another interesting prediction of FEC that I confirmed in the real world was that
if this same pickup is climbing a 4% grade hill,
it does not matter whether you are in 0.69 ratio overdrive gear or 1:1 ratio third gear,
in either gear MPG drops down to 13 because of the added power needed to climb the hill.

Re: How much does rear gear ratio effect HWY gas mileage? [Re: Spaceman Spiff] #2504776
06/05/18 01:06 PM
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Originally Posted By Spaceman Spiff
Originally Posted By a12rag
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.
Re: How much does rear gear ratio effect HWY gas mileage? [Re: ragtop] #2504808
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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.


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Re: How much does rear gear ratio effect HWY gas mileage? [Re: a12rag] #2504814
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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.

efficiency map.jpg
Last edited by dogdays; 06/05/18 02:13 PM.
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