Swapping to numerically lower gears

[jrw429]

I have a 70 Bronco that is fairly stock. I very rarely take it anywhere off road. I often take it on the highway. It has the stock 4.11 gears. With this, I max out at about 60MPH on the interstate. Gas mileage suffers as the engine revs so high. My goal is to get to about 70 MPH at a much lower RPM, and still maintain city driveability. I'm not worried about off-road gearing - using the low gear in the transfer case would provide more than enough for my needs.

I've been planning on changing the transmission to a NV3550, but cost and amount of work has kept that from happening. I just got the brilliant idea of simply putting in different gears. I found these: http://www.precisiongear.com/dana30.htm and http://www.precisiongear.com/ford9.htm. Looks like I could put a 3.07 in front and 3.08 in back.

Any reason not to do this?
Would those ring and pinion sets work in my Bronco?
What other parts or special tools are needed?
How easy would it be to do this for someone who has never touched a differential?
Any guesses on cost for a shop to do this? And advice on choosing a shop?
Has anyone seen even lower gears?

Thanks!

 

[Lmfp]

Do you have an auto or manual? If i was you i wouldnt go any less than 3.50. Especially if you have an auto, the torque converter will be slipping way to much.

 

[broncnaz]

What tire size are you running? Also what engine? I'm guessing 302 since you mentioned a 3550 swap. 302's have no problem spinning 3000 RPM all day long if yours cant get you there then you have other issues. Also broncos are not mileage masters you should be able to get 12-14 if you cant then maybe its out of tune or just old and tired or something else is out of wack
While yes 3.08 gears can be swapped in I dont think you'll like them in the least. You've already got a poor first gear so now taking off will be even worse and harder on the clutch. Also depending on tire size you may end up with your RPM's being way to low and will still lose mileage as the engine struggles to maintain speed.
For the price vs results I wouldnt change axle gearing unless you do it all yourself. In your case your going to need a new front carrier as your 4.11 carrier will not work with 3.08 gears. If you have someone else do the gears well it may cost you almost as much as a NV3550 swap.
better idea is find a used 28 spline center section for the rear axle. you can usually find 3.50 easily out of other broncos or older vans. also common in old vans is a 3.25 ratio could be some 3.08 ratios out there as well. Anyhow with the different center section you just swap it in and try out the gearing see if you like how it changes things. Just dont use 4wd. then you can either decide to change the front to match or go a different direction.
other option is to run the tallest tires you can. That effective changes your gear ratio without having to change anything else.
One last thing is there are other OD transmission options that are cheaper than the NV3550.

 

[Itsa67]

I like broncnaz's idea about trying out the third member first and seeing if you like it (I'd go 3.50's). If you still have the dana 30 in front you will probably be able to get a complete one with 3.50 gearing for under $100 bucks if not free...then you can avoid setting up gears altogether.

 

[jrw429]

Thanks a bunch for the ideas. I think the first issue to address is that I don't have solid information on RPMs - no tachometer! I've ordered one today and might have it by the weekend. It will be interesting to see how well I've guessed the situation. I'm guessing it turns about 4300 RPM at 60 MPH. I'll report back once I have solid information, but it sounds like a transmission upgrade might in fact be a good choice.

Yes, 302. I believe original engine but previous owner put an Edelbrock performer manifold on and an Edelbrock 4-barrel. Stock three speed manual transmission, but the column shift has been removed and a floor shifter inelegantly grafted in. With 13 months of records, I get 11.6 MPG.

 

[Bronco Junkie]

Do you have an auto or manual? If i was you i wouldnt go any less than 3.50. Especially if you have an auto, the torque converter will be slipping way to much.

X2 on not going less than 3.50's.

 

[Apogee]

Thanks a bunch for the ideas. I think the first issue to address is that I don't have solid information on RPMs - no tachometer! I've ordered one today and might have it by the weekend. It will be interesting to see how well I've guessed the situation. I'm guessing it turns about 4300 RPM at 60 MPH. I'll report back once I have solid information, but it sounds like a transmission upgrade might in fact be a good choice.

Yes, 302. I believe original engine but previous owner put an Edelbrock performer manifold on and an Edelbrock 4-barrel. Stock three speed manual transmission, but the column shift has been removed and a floor shifter inelegantly grafted in. With 13 months of records, I get 11.6 MPG.

I could be wrong, but 4300 RPM at 60 MPH (assuming 3rd gear = 1:1) would mean that you would be running 20 inch diameter tires give or take...something doesn't sound right.

If you want to calculate your engine RPM, you can backsolve for it relatively easily if you know your tire diameter since you know your speed, transmission, t-case and differential ratios.

 

[Broncobowsher]

Borrow someone's 3.50 3rd member (or buy one used for cheaper then buying a new set of gears) and put just that in and go for a test drive. You can play with junkyard 3rd members and different gear ratios until you dial in what you like. Just don't use 4WD until you get the front to match.

 

[broncnaz]

Your really guessing on the RPM's I doubt your over 3000 RPM. 11.6 is not to bad a few guys would love to have that. I'd say your fairly close to pulling about all the mileage you can out of it. but you may pull a little more with some engine tuning. on a slightly built 302 I had to lean out a edelbrock about 4 steps. so its definetly worth checking. Also a ignition upgrade would be a good idea get a good advance curve and stable timing.

 

[wildbill]

Don't go higher than 3:50's or you wont be happy good luck.;D;D;D Bill ;D%)

 

[blubuckaroo]

You get your best engine efficiency when it's running at its RPM of maximum engine torque. For example, say your car's max torque comes in at 2500 RPM and you're running only 2200 RPM at 70 MPH. You would be better off with lower gears.
If you are still wanting to swap ratios, I'd be willing to bet you could find someone who would swap their 3.50 axles for your 4.11s.

 

[Explorer]

Play with this a little bit: http://www.grimmjeeper.com/gears.html Post your engine, tranny and tire size so you get informed answers.

 

[Larry B]

Don't go higher than 3:50's or you wont be happy good luck.;D;D;D Bill ;D%)

2x on this.
My son as a teen ager 20 years ago told me our stock 74 with 350 gears and factory rubber would run 100 mph with long straight away. It does 80 for me easy and quick now and has more. Will get 15 mpg at 55 or 60. I don't think Broncos will do much better on the HYThan this with what ever gears you choose. JMO

 

[crankman]

Get the tach and verify cause i cant see you at 4000 rpm. I just went to 4.11 and run hwy all day long under 3000 rpm. Im stock 302 auto trans with 33 tires. I would sure rethink going back IMO but you do what you want. The 4.11 is one of the best all around gear out there.

 

[jrw429]

So it took quite a while. Wild Horses was back-ordered on the tach, and a spinal injury knocked me out for a while. But I've finally got it in place. The wisdom of ClassicBroncos forum shines through - I was WAY outta line on what I thought was happening.

Original 302, original 3-speed, original transfer case. 30x9.50R15 tires. At 70 MPH I'm doing just about 3200 RPM.

So the idea of a gear change is out the window. I'll go back to the idea of a transmission swap to get both more gears for city driving and overdrive for highway. I'll look again at what people have done, but the NV3550 sounds pretty nice. No rush on this, since things are far better than I thought.

I've been wondering why I thought it was spinning so much faster than it really is. I easily take my MGB to 5000 RPM and my JCW Mini to 6500 RPM without worry. But the truck really feels like after 3000 there's not much left. It seems to me a 302 ought to rev a bit more than that.

 

[broncnaz]

Tuning and or other issues may be causing you to feel a lack of power at the higher RPM's. You have a aftermarket intake but no way for the extra airflow to leave the engine. Small fords need more help on the exhuast side then they do on the intake. A good flowing exhuast system will help and a camshaft matched to your needs will really help. Also going to taller tires would help lower the hiway RPMs as well. get you some 33x9.50's and you'll probably be happy its almost like a free OD.

 

[Ranchtruck]

Original 302, original 3-speed, original transfer case. 30x9.50R15 tires. At 70 MPH I'm doing just about 3200 RPM.

But the truck really feels like after 3000 there's not much left. It seems to me a 302 ought to rev a bit more than that.

My uncle used to say his Bronco could only do 55mph. I didn't believe him until I took it out for a drive. It was gutless and at 55 it really felt like it was working hard. I put in a rebuilt motor and it's done 110 with me driving and everything else stock. That's with 4.11's and stock 29" size tires too.

I put a tach on it because the new motor will keep pulling past 5k but I don't want it too.

The reason it sounds like it's turning so many rpms to your ear is the fan. Put a clutch fan on it and it sounds much less like it's screaming.

 

[blubuckaroo]

If you were to offer a trade on the "For Sale - 66-77 Bronco Parts", I'll bet you could find someone who would be willing to trade.

 

[DirtDonk]

...But the truck really feels like after 3000 there's not much left. It seems to me a 302 ought to rev a bit more than that.

It would if it was newer. It would feel peppier too if it was in a lighter, more aerodynamic car chassis. But it's in a 4000+ lb EB, maybe with old worn out wheel bearings and other things.
Original engine from '70 you say? How many miles?

Conditions you might have:
1. choked off exhaust.
2. worn out valve seats causing a loss of combustion pressure.
3. worn out cam lobes, causing loss of power especially at higher rpms
4. you're definitely pushing wind at highway speeds with a Bronco! Any loss of power is going to be magnified by that.
5. Even in it's new condition, these engine/cam combinations would fall pretty flat after 5000 rpm anyway. Pretty noticeable usually.
6. Improperly jetted carb.
7. Improperly functioning advance curve in the distributor.
8. Other potentially tired ignition components making for lower HP than when new.
9. Truck tires with high rolling resistance. Old worn tires being that much worse.

So while it might be running decently and have another 100,000 miles left in it yet, it's very likely tired enough for you to feel a deep lack of power available to you at speeds above 55 mph. Simply from trying to push so much air ahead of the square-bodied truck, as opposed to your more aerodynamic MGB (not to mention less than half the weight!) and your rev-happy and much newer JCW.

So, while it's not right, and won't make you happy, it's probably "normal" for your vehicle's condition.
All the usual tweaks that count for the other two cars, also make a difference with an old 302 as well. Better exhaust, valve-job or better heads, fresh cam (not even a big one, just fresh), better tires and a lower stance. Things like that.

Paul