Gearing and MPG

[JWestfall]

I am just finishing up getting my 71 Wagoneer, 360, TH400 all fixed up. I have an Edelbrock Performer intake,1406 carb, HEI distributor and 29" tires. I have the car all tuned up and jetted pretty lean. With my 3.73 gears, I can squeeze 10 mpg. I would like to re-gear it to 3.08's for highway drivability and gas mileage. When I go 65 I'm at about 3000 rpm, so I'm the old guy going 55 in a 65 on my way to work every day. For those who have done it, how much MPG gain can I expect? It would just help me decide if it is a worthwhile investment or something to do later after my ship comes in. The tranny seams real strong, so I really don't want to swap it out for a 700R4 unless I have to replace the tranny anyway. Thanks

[akguy09]

Might be cheaper to just go up a size or two in the tires.

3000 rpms with your current combo should put you closer to 70 MPH,

[tgreese]

What do you mean by drivability? The engine will be fine at 3000 RPM. You can add more insulation to the interior surfaces if the noise bothers you. Look to the tires, fan and and exhaust to reduce noise further.

Yes, 3000 by 29 by 3.73 is 69 mph. That's about how fast I would drive on a long trip.

Perhaps your carburetor does not promote the most economical driving. Logically adding secondaries should reduce the amount of fuel you need, but I expect it's no longer as lean as the original carburetor. Most drivers, when given the performance capability, don't seem to be able to resist using it.

You could get a vacuum gauge and an air/fuel monitor and work to improve your use of fuel.

Most of the problem with higher speed comes from wind resistance, which goes like the square of velocity. Slowing the engine down will improve economy some, but you will be hitting second gear more when you need acceleration.

Kinda think the most you could expect is a few more mpg. How much will you spend to regear? It's a simple calculation to figure out how long it will take to make that back in fuel costs, if you have an idea of how many miles per year you'll drive.

[JWestfall]

I have an AFR gauge and when cruising I'm at about 15.5. I drive it pretty easy and almost never get into the secondaries. No noise issue, I have new carpet with sound deadener underneath, so its pretty quiet. I have always been a believer that keeping the rpm's down will limit the wear on an engine and it will last longer, not to mention better mpg. So far shops around here are quoting $2500-$3000 for a gear swap. I was just looking to see if anyone has done it to get an idea if "a few more mpg" is 1-2 or 4-5. Thanks for the help.

[tgreese]

Likely the gear change will not be recoverable when you go to sell, even if you make back the cost in improved economy. I would think your TH400 is more ripe for replacement than the axle gears. Strong, reliable, durable but inefficient. I recall reading something like 20% of the power going through these old automatics is lost in slippage/heat? If true, that would give you 12 mpg right there and provide the opportunity for an overdrive gear, with some small additional gain?

You could trade up to a newer vehicle. My KL does a reported 26 on the highway at 70, and I can push it to 27 if my foot is really light. I would attribute much of that to aerodynamics and advanced transmission design. Probably makes as much HP as a mildly-souped vintage 360.

[Stuka]

By far and away the cheapest solution is to move up in tire size. Regearing to 3.07 would really hurt acceleration.

Going to 3.07 gears would drop to you about 2500rpm at 65mph. When calculating RPM at speed, its important to use the rolling diameter, not the total diameter. As the weight of the vehicle will cause the tire to budge a bit. So a 29" tire is more like a 28" tire for a rolling diameter measurement.

Its important to note that RPM at speed isn't the only thing to consider. Changing the gearing will also cause the ramp speed to change. Which is the speed at which the engine will move through its RPM range. Going with higher gears (numerically lower) can actually hurt fuel economy in many cases. It may help you when traveling at a static speed on the HWY, but will hurt around town economy.

There is two things I would look into before regearing.
1: Tuning the carb and timing. In stock form, the 1406 is jetted very rich, which will hurt economy. Get that tuned first. The distributer can also hurt economy. Not just base and total timing, but also the advance curve.
2: The single biggest impact is the driver. Every time you push down on the accelerator, a big bunch of fuel gets shot into the carb. The farther you push it, the more fuel gets shot in. Also, every time you go into the secondary it will suck down fuel.

And really, you need to consider how much money in fuel you would save vs the cost of regearing. Going to 3.07's (assuming you can even find 3.07 gears) would probably run you $1800 or so if you have a shop do it. Thats a LOT of fuel. Even if you daily drive it, it will most likely take years to offset that. And thats assuming your economy actually improves.

Oh, and here is a handy tool to test out gearing and tire sizes: http://www.grimmjeeper.com/gears.html

[tgreese]

The circumference of the tire matters to the effective gear ratio. The diameter only matters when figuring the circumference. If the tire flattens on the ground and appears shorter, that does not affect the circumference, and does not affect the gearing. More common is that the stated diameter of your tires is more than the actual diameter.

[JWestfall]

I'm just trying to decide if re-gearing will pay for itself in about 2 years or so. If I can increase mpg by 4 or so, it will based on current gas prices and my yearly miles. Less then that and it will take longer to pay for itself so it's more of an expense and its more risky with an old vehicle and old, but healthy, transmission. I don't want a lift kit or larger tires and my current tires are pretty new, so no point in replacing them yet anyhow. Thanks again for the help and insight.

[will e]

You can lean out the mixture a bit. I would shoot for something closer to 13.5 ish.

[Stuka]

The circumference of the tire matters to the effective gear ratio. The diameter only matters when figuring the circumference. If the tire flattens on the ground and appears shorter, that does not affect the circumference, and does not affect the gearing. More common is that the stated diameter of your tires is more than the actual diameter.

The revolutions per mile does change depending on the air pressure in the tire. If the tire is at 10psi, it will have fewer revolutions per mile than it would at 35psi.

That's why on say, a newer jeep, when you program the computer for the tire size, you measure from the ground to the center of the hub, and multiply that by two to get the diameter that you feed into the computer. If you measure the overall height of the tire, your speedometer will be off. I have been through this on both of my Wranglers to get the speedo to be correct.

I'm just trying to decide if re-gearing will pay for itself in about 2 years or so. If I can increase mpg by 4 or so, it will based on current gas prices and my yearly miles. Less then that and it will take longer to pay for itself so it's more of an expense and its more risky with an old vehicle and old, but healthy, transmission. I don't want a lift kit or larger tires and my current tires are pretty new, so no point in replacing them yet anyhow. Thanks again for the help and insight.

Like I said in my post above, before considering regearing, I would suggest tuning the carb, and paying attention to your driving. Grand Wagoneers with 2.73/3.31 gears could get 14 on the HWY. But they also have a 2v carb, you have a 4v. And they are tuned to run on the lean side. And they also have things like air dams under the bumper. You could also put a stiffer spring on the secondaries in the carb so its more obvious when you are using them. Or, remove the 1406 and put on a MC2150. You use the power of the 4v, but you get better economy.

[tgreese]

The circumference of the tire matters to the effective gear ratio. The diameter only matters when figuring the circumference. If the tire flattens on the ground and appears shorter, that does not affect the circumference, and does not affect the gearing. More common is that the stated diameter of your tires is more than the actual diameter.

The revolutions per mile does change depending on the air pressure in the tire. If the tire is at 10psi, it will have fewer revolutions per mile than it would at 35psi. ...

Easy to test. Measure circumference of the tire inflated. Deflate and repeat. The difference ratio is the change in miles traveled exactly.

[Srdayflyer]

there are a multitude of factors involved in drivability gear ratio , tire size, my cherokee came with 2:73 gears with stock tires , but some where along the way someone installed 31x10:50 x r15s , that tire change alone changed the effective ratio to 1:92 and the drivability was great for highway driving but the trans was shifting all over the place, and was awful off road driving. i searched online did my research and determined that with my new tires 32x11:50x15s the effective ratio to have the original ratio and drivability i needed to change to 3:73 gears and i installed a posi in the rear end and am quite happy with my drivability and milage, 14-15 mpg but i have a holly sniper f.i. and hyper spark ignition, which made a huge , huge difference in idle ,drivability. obtw mpg. just my experiance

[tgreese]

Just a footnote - it seems that tire companies are all optimistic about the actual diameter of the tires they offer. Look on TireRack.com at various sized tires. In their listings they will show the diameter. You will find the diameter of say a "32 inch" tire is actually 31.6" or some other slightly smaller value. Get a tailor's tape and measure around the equator of the tire. say it's a 30x9.5R15 and it's 92.5 inches in circumference. Divide by pi (3.14159) and you get 29.44 inches diameter. It seems like the companies always round up when they state the diameter.

Metric sizes are little more complicated, since the diameter is hiden in the treadwidth to height ratio; say 285/75R18 is a 285 mm tread width and the tread height is 75% of the width. You have to find the tread height and add twice that to the wheel diameter to get the overall diameter. There are calculators online that will do this for you.

Then use a pocket calculator or one of the online calculators to find your speed at a given engine speed.

[letank]

I'll trade you my 3.08 for your 3.73... really, 3.08 suck big time... if you calculate the 10 to 12mpg.... it is not a lot of dow... only less stops for refuelling

[JWestfall]

Thanks for all the insight and experience of what has worked or not worked for you all. Maybe I will live with it until my tires wear out and see if I can squeeze some 32's on there. Seems like the simplest and most cost effective adjustment.

[Stuka]

Thanks for all the insight and experience of what has worked or not worked for you all. Maybe I will live with it until my tires wear out and see if I can squeeze some 32's on there. Seems like the simplest and most cost effective adjustment.

Its still worth checking how rich your carb is. If you have never tuned it, its rich.

[cone]

I not sure if it would ever pay for itself in your situation but I have an 87 Wagoneer, I swapped the auto transmission for an NV4500 5 speed. The 360 now has Howell throttle body FI, an Melling MTA1 cam and removed the pollution crap. I also installed hubs on the front axle. It has the standard 3:31 gearing and wheels. I drove it cross country a couple of years ago and the on the highway my mileage was 16-20 depending on conditions.

[devildog80]

Cone, that is some nice mileage.

Do you feel it was worth the investment?

[cone]

Cone, that is some nice mileage.

Do you feel it was worth the investment?

Well, it will take a while to recoup my investment but with the price of fuel now it might not take as long as I first thought. I installed the NV 4500 not so much for the improved fuel economy but because I prefer a manual transmission, I enjoy shifting. I also serves another purpose as a Gen Z anti thief devise.

[devildog80]

In todays world, the manual sure does confuse most thieves