Gearing for Gas Mileage?

FiveOEric · 05-23-2009, 12:35 AM

I'm curious what gears would provide the best mileage? 2.73's for low-RPM's on the highway, or 3.27/3.55's because cruise speed put the RPM's more in the powerband of the engine?... Or would more gear be better because the RPM's can bring the car to its powerband faster? Let's hear your thoughts...

Ralph Greene · 05-25-2009, 01:01 PM

It's a little complicated.....otherwise cars would just come from the factory turning 1000 RPM at cruising speed (some almost do).
I can tell you what engineers have told me....and what I have observed on my vehicles with many differnt gear ratios installed.

First....cruising where? Level terrain mostly? At what speeds? Slower is better with computer controlled engines almost all the time. Aero drag is a huge part of the equation...which is different on each model and MPH speed.

For each 5% you lower cruising RPM, you lower fuel consumption by around 1 MPG. (roughly) To a point. Example....3.55's are about 5% taller than 3.73's, so going from 3.73 to 3.55 should improve hiway fuel economy about 1 MPG under ideal cruising conditions. Not much differenve around town.

You don't need to be in your most efficient power band to get the best fuel economy. Minimal fuel usage is the goal. That is very RPM related. It only takes 10-15 HP to cruise around 70. If not for emissions testing, a good tuner can make your engine run leaner than 14.7 air fuel until point of surging for even better economy. But exhaust not legal sometimes.

Around town, where you can up shift at low RPM no matter rear gear, and average driving RPM about the same no matter the rear gear, gears not a big deal....then when you average that with cruising fuel economy, you may not get enough fuel saving to warrant a gear change.

But if cruising down the road in the 60-65 MPH range at very light throttle on level ground in an aero efficient vehicle....the 2.73's will give best fuel economy under those ideal conditions...which is probably not how you drive. MHO

FiveOEric · 05-30-2009, 01:54 AM

Great examples Ralph. 60-70 mph is my ideal cruise speed. I'm visualizing a flip chip with a "lean" burn mode, and a little 2.73 gear. Thanks for helping keep the ideas flowing.

Michael Yount · 06-06-2009, 11:33 AM

.....or.....you stick with the 3.73 gear, and go to a 6 speed where 5th is around .75:1, and 6th is .50:1 --- so you have the best of both worlds. Plenty of torque multiplication/gear to move it off the line (which helps in town mileage) and low revs to help highway mileage.

The key is having an engine that makes good torque in the rpm range in which you'll use the engine. I built my motor for gobs of low end and mid-range. I'm geared with a 3.73 in the rear end, and a .625:1 5th gear with 245/45/16 tire size (a bit shorter than the Stangs). It would be like a Stang with a 3.90 gear in terms of engine speed to road speed.

It weighs 2960 lbs. and consistently gets 17-19 around town and 25-27 on the highway. I think it's amazingly efficient given the performance that's available and given that it has all the aerodynamics of a brick. Just goes to show that if you're careful - you can have your cake and eat it too. I'm turning over around 2200 rpm at 65 mph. Car makes close to 300 ft-lb of torque at that rpm - so cruising is at a VERY light engine load.

I'm about to have a Ford 8.8" rear end fabricated to fit the car - and I'm going to drop down to a 3.55 gear when I do that. My speedo is about 3% off (says I go further than I do) - so when I make the switch I'll be 2% off the other way. And as it is, I can easily start off in 2nd gear --- so a little longer gear in the back won't hurt a think around town. And it'll knock off 100 rpm or so while cruising.

Seek · 06-07-2009, 05:28 PM

Quote:

Originally Posted by Ralph Greene

It only takes 10-15 HP to cruise around 70.

It takes my car ~25hp/80ft lbs to go 65mph with a drag coefficient of 0.35 - loadx=30, load=21%. This is up to 30hp at 70mph, both around 1650 rpm's. These values are by going through a datalog and trying to find an area where I wasn't on an incline or decline. Inclines appear to rise to 60hp/200 ft lbs at ~1600 rpm's at 60mph in overdrive. Your values are only valid for going downhill, closed throttle, where I see <12hp at 65mph.

Kevins89notch5.0 · 06-15-2009, 09:37 PM

3.73s in my 89, cruising at 72-73mph with the cruise set netted me just shy of 26mpg. Oh, and thats with 36lb injectors and a matching pro-m air meter.

900cc_bored2_5.0 · 06-16-2009, 01:57 AM

Excluding accessories, it takes approximatly 1.5HP to keep a 1 square foot area at 55 mph.

With that being said, SN95 mustangs have Height and Width (outside dimensions to mirrors and top of roof) of about 71.8" x 52.9". Thus the assumption that the curved areas (mirrors, section between tires under the bumper, inward sloped pillars, etc.) brings the area down by 10 percent (this is a conservative assumption - if any of you can find the actual frontal area of an SN95 mustang (or other car), insert that KNOWN value here).

That value with the 10 percent reduction is 23.4 sq feet of frontal area (accounting for the area not taken up around mirrors and bottom side of the car). Take that times Cd (0.36) and you get an adjusted frontal area of 8.4. Take that times the 1.5 hp = 12.6 net hp (not including tire rolling resistance and accessories and other losses.)

or:
Net HP = 1.5x23.4x.36 = 12.6 Hp

ref: 1.5 hp per sq. foot from http://www.trackaero.com/factors1.html
by interpalation third order fit.

Michael Yount · 06-19-2009, 01:24 PM

Great article recently in R&T about what has to be overcome to move the car, and the aero resistance at typical road speeds (55 and below) is less than most might think - around a third of the total. Which means you'd multiply your 12 X 3 to get the amount of power necessary (estimation of course) -- around 35HP to move it down the road at 55 mph. If you know your brake specific fuel consumption at certain rpm, you could pretty easily get from the HP requirement to predicted mileage....

the_ghost · 08-01-2009, 01:44 AM

I'm not exactly in a stang but something fairly similar-- 96 Ford Thunderbird with an NPI motor. I am running 2.73 gears from a Thunderbird SC, but my fuel econ has actually suffered since I put in the 2.73's, since rpm on the highway is too low for the engagement of the torque converter... the stupid thing doesn't "grab", I suppose I should say, until maybe 1700 RPM, and unfortunately unless I'm going upwards of 80, it's impossible to get up there... but if I'm given the chance to go FASTER than 80mph, the car is capable of 26+ MPG from the 4R70W transmission. I imagine with a 5MT it wouldn't be an issue, or if I was using a newer transmission.

Buck268 · 10-11-2009, 10:53 PM

More subjective than objective at this point as I don't remember any hard numbers, but in my '94 Crown Vic I picked up a huge boost im around-town MPG when I rebuilt the rear with 3.73's and an alum. d/s. The highway mileage suffered as badly as the city mileage prospered; I was doing A LOT of driving at the time and figured out the magic number in that car was 2000 RPMS. Highway cruising below 2k netted 23-26mpg and city driving maintaining acceleration RPM's below 2k netted 19-21mpg as I remember. Of course other things like defroster useage, seasons and closing in on 200k miles overall also have effects.

Buttt my around town mileage definately made a HUGE jump and highway over 75mph I could almost watch the fuel gauge drop.

Kato Engineering · 10-12-2009, 12:25 AM

what you gain on one operating mode,

you will loose at the other end of the spectrum.

.....do more coasting.

jerseymike68 · 11-01-2009, 10:15 PM

Love the post..

patman0322 · 11-02-2009, 12:05 PM

As stated above: Around town, shifting early is the key.

As is minimizing 6K RPM WOT blasts through the gears

Easier said than done though. Every time I set out to see how well I can do with my around-town mileage it always turns out like the Owl and the Tootsie Pop

I'll add this to this already interesting discussion:

Needless acceleration should be avoided but somewhat counter-intuitively, when you do need to accelerate, use as much throttle as you can with out sending the it into fuel enrichment (maybe 3/4 throttle) while still shifting as earlier as possible without lugging the engine. The theory being, that your engine is most efficient at converting fuel into power at large throttle opening where pumping losses are at a minimum. Barely using any throttle to accelerate means a larger percentage of fuel is being used to overcome pumping losses and less for acceleration. You are using fuel at a much higher rate at the larger throttle opening, but you are also burning it at that rate for a much shorter amount of time and more of it is being converted to power for acceleration and you're getting more quickly to your steady-state cruise mode where you can minimize fuel use.

I've seen mild, but somewhat inconclusive results in practice though. It's too difficult to stay disciplined for the nearly 500 miles and 2 tanks of gas it would take me to test 2 different driving modes out... not to mention a 3~4 week stretch of similar weather conditions and no highway trips, plus varying morning and afternoon work traffic that can't be predicted. Too many variables that can't be controlled for and a test subject that is uncooperative

2.73 are pretty sweet on the highway. I've gotten as high as 30 MPG on the interstate with the cruise on 75 MPH when I've been able to drive straight through a whole tank of gas. Usually it's more like 26 (+ or - a couple) if I have to make normal stops for food and restrooms.

In town it's right at 18.5... occasionally lower.

MFE · 11-02-2009, 02:06 PM

Part of the effect of tall gearing is in minimizing pumping losses caused by closed-throttle vacuum. In other words, if you have to throttle an engine back at 3000 RPM so it'll only make enough power to maintain speed and not accelerate, you're generating more pumping loss (engine having to fight against its own vacuum forces) than you would if you had an engine that would make the same power with more throttle opening at, say 1500 RPM. Therefore the lower-rpm engine would use less fuel, assuming it wasn't far outside its torque peak and assuming the throttle position itself didn't trigger an enrichment circuit.

This was the fundamental principle behind BMW's "eta" series engines. They were hated by performance enthusiasts because they didn't make a ton of power and what they did make was down low. That low-RPM torque peak was exactly what the BMW engineers were after...because is reduced the pumping losses generated by throttling a higher-rpm engine.

toddturbo · 11-04-2009, 11:22 PM

What can one do to reduce the wind drag on a mostly stock '93 LX? Seems like reducing CD would help. 34 now but looking for more MPG.

Fox4dr · 11-08-2009, 10:27 AM

Quote:

Originally Posted by toddturbo

What can one do to reduce the wind drag on a mostly stock '93 LX? Seems like reducing CD would help. 34 now but looking for more MPG.

How much money ya got?? You crafty??
Cheap mod's could be:
Get it lower (than stock) to the ground, add a front airdam that is not full o holes. Seal up the front bumper /headlights so all the airflow is directed in through the rad. Add some side spat's/skirts. Vent the hood, preferably just after the rad to get that air out. Pull the bottom of the front fender & extensions out to cover more of the leading edge of the tire. Make sure your little airdam defector is below the rad. You want to stop airflowing under the car, basically.

All out would be kinda a toughie but fiqure out a way to get the windshield flush
remove wipers, mirrors, door handles and flush fit side glass

From the underside of rear axle back, if there is some way to deflect the air or seal the rear bumper/parachute it up would also help.
I'll be doing most of the above to a '90 LX hatch, using the Cervini style airdam (mostly sealed) with skirts and modified rear valance to let air out, and no rear spoiler at all... might ad a small lip.
If you can take a video of your car cruising along the freeway in the rain, paying attention to where the tire spray hits and how much it swirls around the back and such. This is a good visual of what air does too..
All of these will add up, but won't do squat if all your driving is in town or where you'll tear everthing off on poor roads or speed bumps...
As for being practical, well, you have to stay safe and street legal right? Only you know what will last/work on the streets you drive on.
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