Ring & Pinion Swap. How much MPG improvement to expect?

"Luc - The Acadian Oppo" (luc5)
06/10/2015 at 11:35 • Filed to: PROJECT BIG STEEL

Hey guys I’m been contemplating doing a ring and pinion swap in my ‘86 Chevy C20. This truck guzzles gas like nothing I’ve ever seen before. I’d be literally surprised if it got double digit hwy fuel economy.

The truck has a 350 V8 mated to a TH400 (no overdrive) with 4.10:1 gearing. My calculations say that at 100km/h (62mph) the truck would rev 2810 RPM. If I swapped out the gears to 3.42:1 (lowest numerical availble for 14 bolt rear end) it would drop my RPM to 2343 RPM. This is a 17% in RPM How much do you think this would improve fuel economy? 2-3 mpg or more?

I don’t use the truck for any heavy towing or anything so I’m not at all concerned with the lesser pulling power. I just don’t want to spend $30 bucks in gas everytime I drive the truck.

I not doing a tranny swap or putting in a gear vendors overdrive(I would really like the latter though) so don’t even mention it.

DISCUSSION (41)

Racescort666 > Luc - The Acadian Oppo
06/10/2015 at 11:37

The amount of money you’d save in gas would likely be offset by doing the swap. Honestly, your best bet would be to try and find an axle that has a final drive similar to what you want if you’re gung ho about it.

Luc - The Acadian Oppo > Racescort666
06/10/2015 at 11:39

I plan on keeping this truck for a very long time so a ~$400 initial investment doesn’t phase me one bit.

Jcarr > Luc - The Acadian Oppo
06/10/2015 at 11:40

Not sure if this is relevant or not, but my ‘04 Envoy (~4500lbs) has a 3.42 rear end connected to a 4L60 and the Atlas 4.2 I-6. I get about 15 around town and 18-20 on the highway depending on how much stuff I’m carrying.

crowmolly > Luc - The Acadian Oppo
06/10/2015 at 11:41

I know you just bought the truck. Did you do a complete tune-up yet? Carb and timing settings can make a big difference.

To answer your question, 2-3 mpg probably isn’t that far out of line.

CammedCTSV > Luc - The Acadian Oppo
06/10/2015 at 11:41

You are losing mechanical advantage in this swap. I would ditch the Turbo 400 for a 700R4 (.67 Overdrive) and keep your nice gears

Berang > Luc - The Acadian Oppo
06/10/2015 at 11:41

Do you do a lot of highway driving? Because around town you probably won’t notice much if any difference.

Captain of the Enterprise > Luc - The Acadian Oppo
06/10/2015 at 11:42

I don’t know for sure but I have a crown vic with 2.73s and get 19 on average but I know a guy with a p71 and 3.55s and he gets 16 so It could be noticeable especially with something like a truck that will already be getting poor gas mileage where every mpg really matters.

MontegoMan562 is a Capri RS Owner > CammedCTSV
06/10/2015 at 11:42

Having overdrive will change his world.

I recommend this route. It’s a long term truck, it’s transmission swap time.

Luc - The Acadian Oppo > crowmolly
06/10/2015 at 11:44

Not yet I have a carb kit on order. But being in Canada you have to wait DAYZ for anything to show up.

I changed the plugs and I buying a timing light to check the timing.

Luc - The Acadian Oppo > Berang
06/10/2015 at 11:46

I sort of live off the beaten path so the get anywhere includes at least 40 miles of highway driving each way. My ratio is probably 80%hwy/20% city

RamblinRover Luxury-Yacht > Luc - The Acadian Oppo
06/10/2015 at 11:46

350 V8 mated to a TH400 (no overdrive) with 4.10:1 gearing

Kinja'd!!!

That will do it. That will do it for sure. Also, contrary to Racescort666, there’s a certain advantage not only in gas price, but in convenience, noise, and wear and tear on the engine and trans. It doesn’t *have* to pay for itself in gas directly to be worth it.

Oh yeah, and Gear Vendors setups *are* cool. Putting one in the family motorhome bumped MPG (6.2 diesel) from something like 11 at best to over 13.

DrScientist > Luc - The Acadian Oppo
06/10/2015 at 11:51

not a specific answer for your set up, but my jaguar XJS has the same th400 3 speed. talk about cross-platform use.

in order to get reasonable mpgs out of the v12, they were produced with a 2.88 rear end. talk about slow off the line. but i do manage 15-19 highway.

the more costly options, which i’m sure you’ve explored, are an overdrive kit, or switch to a 4L80. but youre probably looking at 4 digits for either.

just be aware your off the line acceleration will start being measured in minutes. :)

Luc - The Acadian Oppo > RamblinRover Luxury-Yacht
06/10/2015 at 11:53

I was thinking the same. Not working the motor as hard definitely sounds like a plus. This is just a hobby truck for the occasional trips to the hardware store or to haul a piece of furniture ect. Since I don’t need the extra gearing I would be better off extending the longevity of the truck and saving a few bucks at the pump in the process.

now imagine the lower gearing WITH the gearvendors. If they weren’t so much money that’s what I’d do.

CammedCTSV > MontegoMan562 is a Capri RS Owner
06/10/2015 at 11:54

I was told by old men that the 700 wouldnt live long behind a big block in my K5 Blazer, but once I realized I could drive on the highway with 35” tires, I was happy

blacktruck18 > Luc - The Acadian Oppo
06/10/2015 at 11:57

Have you considered junkyarding for a 12 bolt diff with 2.73’s? You could probably buy the whole axle at a junkyard and swap it in for maybe $200 more than the new gearset.

Also I know you don’t want to do a tranny swap but, 700-R4. Just saying, 2.73 gears and a 700-R4 could get you very close to 20mpg highway. If you are planning on feeping the truck forever it might be a worthwhile investment.

HammerheadFistpunch > Luc - The Acadian Oppo
06/10/2015 at 11:57

When I went from 4.10 to 4.56 i saw a drop of about 1 mpg, but I also got a lift at the same time. You will see a gain for sure but how much I don’t know...just get bigger tires, its cheaper.

Luc - The Acadian Oppo > DrScientist
06/10/2015 at 11:57

Yes a 4L80 would be ideal but it would cost more than I paid for the truck.

My Lincoln has a 3spd Auto with 2.47:1 gears. You can almost count the rpms the thing is so slow. 0-60? 20 seconds or so.

Luc - The Acadian Oppo > HammerheadFistpunch
06/10/2015 at 11:59

The tires on the truck are literally brand new and they look plenty big on my stock height 2wd truck.

BigBlock440 > Luc - The Acadian Oppo
06/10/2015 at 11:59

There is a point of diminishing returns though, if your truck has to struggle too much to lug the weight around. Some personal experiences below.

Had a friend that got around 5mpg with his chevy 350. Dropped a 350 crate motor in, got a power bump and fuel mileage went to 8. His truck was on 44s though.

My brother got 12-13 in his car with a 383 and 3.21 rear, swapped to a 3.73 for performance gains, now he’s at 8-9 mpg.

My truck has a 3.55 rear. I used to get 18-19 highway, went a tire size up effectively lowing the numerical ratio. Now I’m about 15 or so, partly because the engine has to work harder due to the lower ratio, partly because the tires are heavier. When I drive in hilly areas, it drops to 12, where it used to only go down to 16. I expect if I regeared to a higher numerical ratio, I’d gain a mpg or two back.

It’ll help with mostly flat highways, but hilly and city mileage it may not help at all, or may make worse. Just something to consider.

Luc - The Acadian Oppo > blacktruck18
06/10/2015 at 12:02

I like that my truck is heavy duty and don’t want to downgrade it all for the sake of fuel economy.

In terms of strength

14 bolt > 12 bolt Also I’d have to convert the 12 bolt to 8 lug

TH400 > 700-R4 The TH400 is a superior unit the only drawback is no overdrive.

MontegoMan562 is a Capri RS Owner > CammedCTSV
06/10/2015 at 12:07

Which means it should do just fine behind a 350.

He said he’s not towing so could even pull one from an S10 Blazer that has a 4.3L.

CammedCTSV > Luc - The Acadian Oppo
06/10/2015 at 12:13

Build a 700R with 4L65 internals and “The Beast” sun shell, it can be done for under a grand and will last forever. Ten years in my 2002 SS Camaro with a ton of 12 second passes

Jeff-God-of-Biscuits > Luc - The Acadian Oppo
06/10/2015 at 12:17

Could be a silly question, but what about aero? Bed cap or net tailgate?

Luc - The Acadian Oppo > BigBlock440
06/10/2015 at 12:22

Good info. I live in the prairies where it is as flat as can be around here. I don’t think it will be struggling to much as they came with that ratio from the factory and this thing has no much torque it should have no problem with it.

Luc - The Acadian Oppo > Jeff-God-of-Biscuits
06/10/2015 at 12:25

I actually just removed the bed cap the truck came with.

Went from this:

Kinja'd!!!

To this:

Kinja'd!!!

No way am I ever putting a cap back on.

crowmolly > Luc - The Acadian Oppo
06/10/2015 at 12:25

Biggest thing is to establish the best baseline tune you can before spending money on anything else. Check a service manual for tune-up specs if it’s stock. You may want to buy a vacuum gauge for tuning too.

Luc - The Acadian Oppo > crowmolly
06/10/2015 at 12:27

Good plan. Is a vacuum guage very expensive?

Jeff-God-of-Biscuits > Luc - The Acadian Oppo
06/10/2015 at 12:28

Been awhile, but if I recall correctly, there was pretty significant aero advantage to either a flat “lid” on the bed, or removing the tailgate. Might be worth a look sometime.

crowmolly > Luc - The Acadian Oppo
06/10/2015 at 12:28

Actron one at most parts stores is $15 USD. You don’t need a permanently mounted one in your dash.

Oh, and FYI, when you buy the timing light a dial-back one is cool for stock stuff, but if you plan on eventually going MSD you can’t use it. A basic one is fine in that case.

Luc - The Acadian Oppo > crowmolly
06/10/2015 at 12:37

Yeah I aware with the issues with the MSD confusing the light.

My truck is stock whats the advantage of a dial back one. Do I need that? or can I just get the most basic one?

BigBlock440 > Luc - The Acadian Oppo
06/10/2015 at 12:54

There’s a sweet spot with gearing and hp somewhere, you’ve just got to find it. From what your situation sounds like, you’re almost guaranteed to gain a few mpg.

Also, don’t forget to re-gear your speedometer.

Luc - The Acadian Oppo > BigBlock440
06/10/2015 at 13:27

Is it a difficult job to re gear the speedo?

BigBlock440 > Luc - The Acadian Oppo
06/10/2015 at 13:30

On my car, 727 Torqueflite, no. It was a $10 gear and two bolts to take off a cover.

I have no idea what it would take for your trans though.

crowmolly > Luc - The Acadian Oppo
06/10/2015 at 14:10

Nah, just stick with a basic one. Dial back allows you to check the advance curve.

Rykilla303 > crowmolly
06/10/2015 at 14:57

This. Plus if you don’t have it, I recommend using the original quadrajet. I know they get a lot of hate, but spread bore always helps the milage. (mine is 77 k15/350/rvcam/nv4500trans swap with 35” tires and i can get double digits easily with my qjet)

Luc - The Acadian Oppo > Rykilla303
06/10/2015 at 15:08

Yeah I still have the factory quadrajet. I need to check the idle mixture screws. when I looked at then the other day they were 3 1/2 turns out but everything I’m reading states that it should be in the neighborhood of 1 1/2 turns out. Once I get a vacuum gauge I’ll fine tune it again.

Rykilla303 > Luc - The Acadian Oppo
06/10/2015 at 18:31

Yup, vacuums the only way to do the idle mixture accurately. Close as much as possible, open each equal turns until max vacuum. I’ve had two identical quadrajets that needed a different number of turns out. If you want information overload, check out “How to Rebuild & Modify Rochester Quadrajet Carburetors” by Cliff Ruggles. The best tuning goes way beyond just the idle mix.

Luc - The Acadian Oppo > Rykilla303
06/11/2015 at 21:46

I just bought myself a timing light and a vacuum gauge. The timing was spot on at 6deg BTDC as per the truck’s specs but the carb was WAY out. The idle mixture screws were at 3 1/2 turns out and I achieved the absolute best vacuum at 1 3/4 turns out. It slowed the idle right down and the truck seems to idle a lot smoother now. I took it out for a quick jaunt and the truck ran perfect so I’m hoping this might help quite a bit. I’m hoping this leans out the truck a bit since it was was running way too rich before.

Luc - The Acadian Oppo > crowmolly
06/11/2015 at 21:47

crowmolly > Luc - The Acadian Oppo
06/11/2015 at 21:51

NICE!

Report back with what happens if you can! It should definitely clean up the idle circuit.

Luc - The Acadian Oppo > crowmolly
06/11/2015 at 22:24

Will do!