Boat Tailing a Pickup

[omgwtfbyobbq]

Originally Posted by theclencher

Yeah but mostly no. Would it help? Sure, but let's not go bananas here.

If it increases the mpgs enough and Snax has the truck for long enough, why not? It really depends on how much it can increase mileage, the cost, and gas prices. Lets say, $2,000 for the unit, and mileage goes from 14mpg to 17mpg. We have two mileage values to compare to figure out if it's worth it or not with gas at, say $3 a gallon. Xm is the number of miles we need to travel to pay off.

Xm($3/14mpg)-Xm($3/17mpg)=$2000
And the result is a payoff time of 50-60k miles. After that, the device saves money, at the rate of $2000 every 50-60k miles. If it's a 1.5mpg increase, the payback mileage is doubled, and we only save $1,000 every 50k miles. Now, some people may not have their vehicles for that long, but if I had the itch to get a big/used gasoline 4x4, a gear splitter would be one of the first things I'd do after tracking down the factory tranny/rear end with the lowest possible OD ratio. This is also good insulation against gas price spikes. If it goes up to $5/gallon for whatever reason, the payback drops to 30k miles. YMMV

I used this expression to figure out whether it's worthwhile to drop in a five speed, and based on a 3mpg increase at the same speed, a used $150 transmission will pay itself off in ~10k miles iirc. The gear splitting unit would be harder to justify, and payback would probably be in the ~100k mile range, but given that my pickup is a Toyota, I think the truck would make it that far and it'd be worthwhile. Or, maybe I can get a custom .5:1 fifth gear cut for $100-1000, that'd definitely pay itself off in under 50k. And it won't look as different, so a wife will probably be o.k with it.

[Snax]

I think a gear splitter would take too long to payback, as the gearing on the truck seems reasonable with being able to cruise at 60 mph and 1500 rpm locked up. Any further overdrive isn't likely to return much economy there IMO. Even at 42 MPH with the converter locked up, the truck won't do much better than 24 mpg vs. the 22+ mpg I recorded recently at 60 mph. So I'd say it's running right where it should be at 60. Then again, maybe I'd like to go 70 . .

[omgwtfbyobbq]

That's not a problem with your overdrive, it's a problem with your kickdown cable imo. Like I said before, an auto makes good mileage via lower gearing a PITA, but if you had a M/T, the lower the ratio, the better the mileage. Besides, if basjoos can pull ~24mpg in a M/T V6 4x4 F150, I think a V8 with really low gearing and a boat tail, lrunning ~1000rpm@55mph, could see 30+mpg cruising. Automatics make this a PITA, but there's probably some way to adjust the kickdown cable so we don't drop into another gear the second we give it some foot. And if the Global Warmer is a M/T, any changes in gearing shouldn't be a problem.

[Snax]

Agreed clencher. There is definately a sweet spot in load factor, valve overlap, intake, exhaust, and fuel metering that just yields the greatest efficiency. Changing any one of those things just hurts the system. IMO, this is likely the very thing that Ford spent allot of time to get right on the new F150 since it's drag coefficient is worse than it's predecessor.

[omgwtfbyobbq]

What's your buddy's gearing, and how fast does he drive? As for mileage, people who have enough to buy new big trucks have enough to pay out the nose for gas and usually don't care about mileage, or care enough to do anything about it. Anyone who cares and has a big truck mods, drives slower, and/or gets a GV unit. It's kinda like the situation with fuel efficient small cars. People here are pulling out all the stops to go from 30mpg to 40mpg, but that won't save as much as going from 10mpg to 12mpg. Then again, the ones getting such crappy mileage don't care anyhoo... Even though that's where there's the most improvement. Anyway, messing with the kick down probably isn't worthwhile, since you'll still have a top gear that's too high. But, lower is almost always better with any engine made today. It's just that people don't give two ishts about it.

[The Toecutter]

OK so here's what you need to do: form a consortium between one of the pickup manufacturers, Leer, GearVendors, and yourself. Get Ford, Dodge, Chev, Toyota, or Nissan to donate a full-size pickup for the "demonstration unit", get GearVendors to donate an overdrive unit, and get Leer (manufacturer of everything necessary to pimp your truck out) to fab up a fastback topper. Mix it all together and voila!- get 30+ mpg and market it as the "HyperTruckMaxUltraFuelSipper" package. When the millions start to roll in (right after gas hits $3 again) all I ask is that you remember my free gift of this business plan and I want a helicopter ride out to your yacht sometime when you're partying out there!

This supposes that the auto industry would actually want it to come to market, under the condition that this advancement is possible.

[omgwtfbyobbq]

Originally Posted by theclencher

FWIW EPA says V8 5.0 4sp a/t F150 14/18/15 while V6 4.3 5sp m/t 1500 16/19/17 so 1mpg more on the open road and 2 more in town- pretty underwhelming difference I think with his 2 fewer cylinders and a nice efficient stick tranny with one more gear; individual vehicle differences could cancel that small an FE difference out.

Yeah... The EPA for the Camry is the same way. The problem is, even though the E153 M/T Camry has an extra gear, the FD/OD ratio is way higher than it is on the automatic in it right now. Which is why I can set the cruise control and get ~10mpg better than the EPA at the same average speed with city/stops. If I swap in M/T, my city mileage would go up, but my cruising would drop since my engine was running ~700rpms higher all the time. Not worth it in a highway cruiser imo. Regarding making trucks that pulled 30+mpg, the people that really want them can make them with little hassle, and those that don't care wouldn't be interested anyway. Most people with large trucks wouldn't go for a sloping bed cap or tall OD because they think it'd look funny, and be too slow on the freeway. Course explaining that they'd get better acceleration by downshifting anyway, or that taller gearing means better mileage would probably be as fun, or useful, as headbutting a rock for a half hour. Otoh, some hot rodders understand this...

Prior to the TKO install, gas mileage was firmly mired in the 11 to 12 mpg range. It's not so much that our mild 383 was a gas hog, but that the 3.73 gears and loose converter kept engine rpm in the stratosphere. Rolling along at a glacial 55 mph had the engine cranking 3,000 rpm. We can now cruise at 70 mph with the tach at a more reasonable 2,000 rpm. This huge difference is the result of the engine turning 36 percent fewer revs, and that's assuming the converter was locked to start with (it wasn't). Account for 5 to 10 percent more rpm from converter slippage with the old TH350, and the difference is even higher. We guess you could say we weren't surprised that we got a 33 percent improvement in fuel economy, from 12 mpg to 16 mpg--and that was with a heavy right foot.

Course, like you said, if only driving ~5,000 miles a year, it'd be close as to whether the gear splitter paid itself off before the truck was sold or broke down, but if you think the vehicle's going to be with you for long enough, why not go for it? In my case with the Toyota, if I can have a custom 5th gear cut that's low enough, I'll probably see 40mpg+, w/o having to get an expensive small diesel pickup with less power.

[omgwtfbyobbq]

Originally Posted by theclencher

I'd like my truck to get 30+ and I wouldn't mind a fastback cap or a tall OD but I know this for sure: those things won't get that truck anywhere near 30; I'd be shocked if it broke 24.

A tall enough OD (~1000rpm@55mph) should add ~7-9mpg to the truck provided you could keep it in that tall gear, but... can you make one yourself, and rig a way for the truck to stay in gear with lots of pedal? Are there any feasible manual swaps with lower gearing to help out? It's possible, but it's all up to how much you want to put into it.

Originally Posted by theclencher

Come on man, if 30 is so easy to get with a full-size 4x4 how come the pint-sized Toyota flatbed isn't doing it?

Because it's stock, I'm poor, and it's the only vehicle used for short city trips. Going by the mileage I've gotten cruising on the highway, I'm pretty close with a 25 year old engine, ~220,000 miles, 3.42 rear end, and a 1:1 top gear (on tires I haven't inflated in months). A conversion to fuel injection or 5speed with a .8:1 ratio could push me over 30mpg cruising on the highway individually. Hell... leaning the engine or advancing timing may push me past 30mpg highway. A .5:1 gear ratio may bring me up to ~40mpg cruising at 55mph. Maybe even a combo of leaner engine and .8:1 gearing, who knows?

The rear end ratio alone doesn't not engine speed make, what's your total OD ratio? How much power does it take you to go 55mph stock? With aero mods? The gear vendor comment was probably aimed at those who would put an overdrive on a small block and complain about not being able to go 70-80mph uphill w/o the tranny hunting, because that's what most people do. I bet an overdrive would work just fine on a small block truck going 55-65mph with attention paid to the kickdown mechanism, which is probably what someone who wants to save gas will do, unless they're not all here. Not to mention the potential reduction in drag from a sloping back end, grill block, and underbody tray, which will drop the energy needed to move the truck even more. The point I'm making is that it's possible... If you want to convince yourself otherwise, that's your choice.

Otoh, I wouldn't want something like this living in Colorado, but if it's flat enough, my vehicle doesn't need to be a 10-20% throttle when it can sit a 50-60% and get better mileage imo. I don't doubt your reasoning for not bothering with it, a Ford pickup that's 10 years old and rusting probably won't last long enough (12 years) to recoup the costs. Diesel and gasoline peak efficiency are pretty much the same, so if Chebby can build a diesel in the 80s that could get near 25mpg at 55mph (a guess compared to ~23@65mph), a gasoline pickup with all the aero trim could probably get ~30mpg at 55mph with the right gearing.

P.s. What's with the small truck potshots? You're scaring me man... It usually starts with the macho remarks, and before you know it, you've moved to Alabama and grown a mullet.

[omgwtfbyobbq]

Originally Posted by theclencher

Ford OD .71:1 x 3.31 axle = 2.35:1; 245/75R15 tires (already 2.04% oversized) = 706 revs/mile x 2.35 = 1,659 engine revs/mile; 55 mph = 1,521 engine rpm. Stock engine rpm @ 55 = 1,552 rpm; when it had 265/75R15s it was 6.128% over stock or 1,462 rpm @ 55 and lemme tell ya that thing was a dog AND the FE didn't increase. How do you figure a 500 rpm drop 1: works and 2: leads to 8% FE increase?

So I'm here to tell ya 1,000 rpm in a truck that actually gets used as a truck if only even on occasion will severely compromise the utility of the truck which is the reason for it's being in the first place.

First off, did you test the Crr of both tires? The green seal report shows that some tires of the same size have twice the rolling resistance of others. Second, well... There is no second. It really depends on if you know the Crr of the tires. If they have the same Crr, then I guess dropping engine speed won't help. Otoh, you may just have bad luck with tires... A 500rpm drop should increase fuel economy quite a bit, at least according too all the gas engine BSFC maps I've seen. From ~1500rpm to 1000rpm it's about a jump of 1/3rd in throttle to make the same power, which can really move up the point on the BSFC map. It wouldn't be as much as the 280g/kwh compared to 450g/kwh comparison I made in the recent PM, but I could see a 50-100g/kwh drop in fuel consumption all things being equal. And, a gear vendors unit should be able to get you to ~1100rpm. Not that I'm suggesting the purchase of a new one, but if you ever see a used one at a junk yard for a few hundred, why not go for it? There's one on car-part dot com for ~$1100, but that's still pretty expensive imo.

As for utility, well, imle, when towing more than the truck's weight, I keep it in 3rd, and go under 40mph on side streets. Hauling 1000lbs is np in 4th at 55mph, so the trucks already pretty useful. Adding another super tall gear wouldn't impact my FE or ability to haul/tow, but if I was moving furniture, or picking up something bulky but light, I could drop it in my really tall fifth and hopefully get great mileage. YMMV.

edit- One last question I gotta ask, about how much throttle are you using at 55mph on flat ground in good weather? If it's already at half, then you're right and there's not much you can do, maybe see a 10% increase. Otoh, if it's at 1/4 or less, I really think you could see a nice increase from a gear splitter.

[omgwtfbyobbq]

Originally Posted by theclencher

I have no idea what the Crr of the different tires was although they were both passenger car tires with smooth highway tread designs so it shouldn't have been a huge difference. It was only a 90 rpm drop yet it really killed performance.

Well, that's the nice thing about a gear splitter, you only engage it when you want to. So you'll have the same acceleration and gearing until you get up to speed and engage the splitter. Then the engine speed goes down and efficiency goes up, unlike using tires to gear the entire drivetrain down, which hurts acceleration in all gears. And unless you did coast down tests on the tires or know the Crr, who knows how well they roll?

I'm saying, how can one conclude 30+ is "easy" with a vehicle that is practically twice as big, frontal area-wise, weight wise, and displacement wise when 30+ doesn't even come easy for the little guy? Remember you are claiming almost a 100% FE increase!

If the vehicle weight for your 4x4 F-250 is right, then it's only ~1000lbs more, which is 33% more weight than my little pickup. Given the oh so aerodynamic bed on it, I bet it's CdA is ~ a third less than your trucks'. So, given a 4000lb truck with a ~.45 Cd and maybe 35 square feet, and a 3000lb truck with a ~.55 Cd and ~27 square feet, there's not gonna be a difference of ~2, probably more like ~1.3. And, since the ford geared way lower than the Toyota, that's probably not why it gets worse mileage. The only thing left is engine displacement and pumping losses since the 5.0L engine is twice as big as 2.4L. Short of dropping in a smaller engine, the easiest thing to do would be increase the amount of air the engine sucks in at a given rpm to simulate a smaller engine. And if it's already at ~1500rpm, then a gear splitter should drop it down to 1000rpm and probably increase efficiency.

Originally Posted by theclencher

Also adding a GV is adding even more bearings, seals, oil, shafts, and gears that the hp must flow through so while the GV helps in extreme gearing cases like all the pre-OD old junk I'd have to think it can't compare to a properly geared newer OD trans.

It will be another loss, but it will also allow the engine to operate way more efficiently. Unless you're already at half throttle at 55mph, a splitter will drop your engine speed and reduce pumping losses, which will reduce BSFC provided you can delay kickdown. Otoh, the downside is we'll have to upshift when we normally wouldn't, we'll top out at ~65-70mph on flat ground in that double OD, and we won't accelerate very quickly. So, this isn't something most manufacturers would use because it may only increase mileage by ~7mpg at 55mph, but will "Feel" too slow. And when we see truck ads, what do they go on and on about? Power, towing, etc... occasionally we'll see a blurb about fuel efficiency, but it's mostly power.

But, if we want to get better mileage, especially if we're thrifty and buy used, this is a great upgrade for a big truck imo, and a FE minded person would probably accept those disadvantages if they could get ~23-25mpg cruising at 50-55mph. I know I would, especially if it was a bolt up option.