Explain Gear Ratios to me

"zeontestpilot" (zeontestpilot) 03/24/2015 at 08:10 • Filed to: Question

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After making this !!!error: Indecipherable SUB-paragraph formatting!!! yesterday, some people kept mentioning different ratios for the transmission and gave different mpg numbers. How do gear ratios affect mpg, and is there a way to find out what the gear ratio is?

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MasterMario - Keeper of the V8s > zeontestpilot 03/24/2015 at 08:20

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The gear ratio is the ratio between the RPM of the input shaft to the transmission (so the engine crank) and the output shaft of the transmission (the driveshaft). The higher the gear ratio the more pulling power you'll have at the wheels since the engine will turn more times per wheel revolution than a lower gear ratio. But a lower gear ratio lets you enjoy better MPG since your engine will spin at a lower RPM, thus using less fuel. As to finding what it is, I think some vehicles will have it on a sticker in the door jam, or you can look up where to find it on the transmission and get under the vehicle, or if you're lucky the owner will have the spec sheet from when it was new.

Arch Duke Maxyenko, Shit Talk Extraordinaire > zeontestpilot 03/24/2015 at 08:27

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MasterMario - Keeper of the V8s > MasterMario - Keeper of the V8s 03/24/2015 at 08:27

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looking back at the comments from your other post people are actually alking about the rear axle gearing, same concept except the driveshaft is the input shaft and the axles are the output. again, in the door jam there should be a sticker, otherwise there should be a tag or stamp on the rear diff cover

zeontestpilot > MasterMario - Keeper of the V8s 03/24/2015 at 08:28

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So with a lower gear ratio on a manual, would that mean the times between gears would be shorter? Aka, shift sooner?

MasterMario - Keeper of the V8s > zeontestpilot 03/24/2015 at 08:32

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other way around...with a higher gear ratio you'll have to shift sooner (based on vehicle speed)

RamblinRover Luxury-Yacht > zeontestpilot 03/24/2015 at 08:47

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With a manual, and somewhat less with an automatic, the gear ratio at the axle combined with the size of the wheel and the ratio in the transmission gives the number of times your engine will turn per mile. A specific engine speed will come out as a specific vehicle speed. This is not good for a gas engine's mileage, as anything but a very new gas engine requires a specific amount of fuel to turn a given number of times, regardless whether all available power is used. If two vehicles are going at a given speed with the same size tires in the same gear, but have a 3.53 ratio rear axle and a 3.73, the 3.53 will get *about* 5% better mileage just because the engine is going slower. This has tradeoffs - less ability to tow well, worse acceleration - but at constant speed mileage is better the "taller"(numerically smaller) the rear axle ratio is.

Gear ratios in the transmission are different. If a transmission has an overdrive, it has the same effect as a taller rear end - slower engine, better mpg - compared with a "normal" transmission with drive only. Most transmissions nowadays have an overdrive, but the classic model was to have a succession of lower gears ending with one in which the engine was going the same speed as the driveshaft. Each of those lower gears allow the engine to go faster (using more fuel) to provide a mechanical advantage and get up to speed faster. Some gearboxes are "close ratio", some are "wide ratio". This refers to how big the steps between gears are. If first is not very slow at all, then it takes the car more time to get up to speed, but makes the car use less fuel when traveling slowly - too slowly to use "drive". If first gear is very slow, it's great for towing things, but in normal driving you'll shift out of it very fast.

deekster_caddy > MasterMario - Keeper of the V8s 03/24/2015 at 08:48

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You are talking about the gear ratios of the individual transmission gears. I think the OP is really asking about the differential gear ratio, which affects all of the transmission gears and what's called the "final drive" ratio. That's what keeps RPM down at cruise for better MPG, but at the cost of slower acceleration - or a 'Tall' gear (lower number). 'Shorter' gears (or higher ratio numbers) are good for acceleration, bad for MPG. This is usually not referring to the gears you shift through, that's talking about a close ratio gearbox. Generally inside the transmission the more forward gears you can choose from, the closer the ratios.

There are a lot of different ways to get to the final drive ratio, and that's the number that really matters.

Snuze: Needs another Swede > zeontestpilot 03/24/2015 at 08:57

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Trans used in the Ranger is actually a Mazda M50DR1 (Manula, 5 speed, OverDrive, R1 = light duty). The truck you're looking at should have these gear ratios:

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MasterMario - Keeper of the V8s > deekster_caddy 03/24/2015 at 09:00

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yea, OP said transmission, but I replied to myself once I read the other posts that were talking about the diff. also, I didn't know if Ford had different transmissions with a different final gear ratio in the transmission that would affect MPG noticeably

Snuze: Needs another Swede > Snuze: Needs another Swede 03/24/2015 at 09:03

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Also, as mentioned, look in the door jamb, you'll find a build sticker that will tell you what your final drive is.

http://www.therangerstation.com/tech_library/a…

Looks like they offered them with everything from 3.08 to 4.56 ratios.

zeontestpilot > Snuze: Needs another Swede 03/24/2015 at 09:07

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So, I guessing I'm looking for the lowest ratio then...

450X_FTW > zeontestpilot 03/24/2015 at 09:17

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In terms of automatic trans calibration there is a term used called MESAU, Minimum Engine Speed After Upshift. Different for every engine/trans/axle ratio/etc. This is a calculated engine speed based on torque that is required for the engine to be at after the upshift happens (upshift too soon and you'll bog the engine or stall it). Let's say that value is 1200 rpm. If you have a tall (numerically low) rear axle ratio, then you will have to rev each gear higher before upshifting so you do not dip below the 1200 rpm threshold. If you have a short (numerically high) rear axle ratio, then you won't have to rev it nearly as high before shifting into the next gear and your MESAU will be lower. This is due to the torque multiplication happening. For sake of argument, let's say you have a 3.30 axle ratio with MESAU of 1400 rpm and 4.10 axle ratio with MESAU of 1200. This means that in the same gear for both configurations, you'll be putting to the rear wheels the same torque with a 4.10 axle at 1200 rpm as you would with 3.30 axle at 1400 rpm.

So it is possible that lower gearing would help you in the city driving since you don't have to rev it as high before shifting and you can run at lower engine speeds. On the highway though at cruising speeds your engine speed will always be higher with the lower gearing. But more than just ratio affects the mpg, parasitic losses have to be taken into account. If you have ever driven a manual, coast to a stop in 4th gear then repeat it in 3rd gear. You'll notice the vehicle will slow much faster in 3rd gear. This is due to the parasitic loss from the lower gear ratio that is in 3rd. Also referred to as "reverse torque" since the wheels are now putting torque into the drivetrain. So with lower axle ratio gearing you'll be slowing to a stop faster, which is now going to affect fuel economy in a negative way.

Snuze: Needs another Swede > zeontestpilot 03/24/2015 at 09:22

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Short answer: Yes (because I think you mean lowest number).

Pedantic dick answer: No, you want the highest ratio (lowest number). Gears are backwards, so low or short refers to big numbers, long or tall refers to smaller numbers.

Long answer: You want the tallest ratio that will work within reason. Gearing depends a lot on available engine power, vehicle weight, and aerodynamics. Cruising down the highway takes a certain amount of horsepower just to overcome the rolling resistance of the tires, the aerodynamics of the truck, etc. That 4 cylinder probably makes about 125HP, but thats peak HP which probably occurs at something like 5500 rpms. Curising down a flat, straight, level highway at 65 in overdrive, you're probably turning more like ~2000 rpms and the actual horsepower you're making is like 50HP, maybe less.

To go 65 mph you probably need 40HP, so you have 10HP on reserve. This way when you get to a hill or a headwind or move to pass someone, you have some power on tap to accelerate. So you could have a higher final drive, lower your cruising RPMs and in theory save fuel, but that only works for the the straight, flat, level, no headwind situation. Any time you encounter these things, you'd need to downshift to move up in the RPM band to access more power to overcome them. I had a roommate with a 4 cylinder Jeep like that. Jeep offered them for a while to meet fuel economy standards, but it had so little power and was geared so poorly you had to downshift to climb even a modest highway grade, so it never really got any better fuel economy than the 6 cylinder models.

Most likely the 4 cylinder truck has a 3.73 or 4.10 final drive ratio. The engine is so small it needs that extra bit of mechanical advantage, while the V6 trucks probably have a 3.08 to 3.55 because they will have extra power on tap, so turning the gears slower helps offset the extra fuel used by the bigger engine.

Tohru > zeontestpilot 03/24/2015 at 09:29

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Gear ratios work as follows: (Number of revolutions of the motor):(1 revolution of the axle).

A numerically-lower gear ratio hurts acceleration in trade for fuel economy at cruising speed. A low gear ratio is also not ideal for towing, as it gives you less mechanical advantage over the load. This is like riding around on a multi-speed bicycle in the top gear all the time.

A numerically-higher gear ratio hurts fuel economy in trade for acceleration and ease of towing. This is like riding around on a multi-speed bicycle in 1st gear all the time.

My 1976 Oldsmobile Delta 88 used a 2.73:1 rear axle ratio. It accelerated slowly, but cruised at under 2000rpm at Interstate speeds.

A dragster will use a something like a 4.11:1 rear axle ratio. It accelerates very quickly, but hits redline in top gear after 1/4 mile.

There's a few ways you can figure out the axle ratio. Fords usually have a tag on the axle that tells the ratio, otherwise it should say which axle it has in the door jam. You can also take the back cover off and count the teeth on the ring and pinion - divide the number of teeth on the ring by the number of teeth on the pinion and you'll get the ratio.

Gonemad > Snuze: Needs another Swede 03/24/2015 at 10:02

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TLDR version: you want the engine to rev as slowly as possible. Less friction losses, less explosions per minute, less heat, less fuel spent.

On the other hand you have less torque, and available power. And that's why you might get better economy with a larger engine revving slower.

On the other side of the scale you can get 1.0 liter engines screaming at 4.000 rpm, just to move people from A to B, no hooning. They drink more fuel per miles than 2.0 liter engines.

Snuze: Needs another Swede > Gonemad 03/24/2015 at 10:58

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Yeah, it's all about "right sizing." Thats why turbo engines are taking over, especially in economy cars. My little 1.4L Turbo Cruze has pretty tall gears in it, but it can chug along at 2,000rpms all day doing 65mph. If I hit a hill or something, tip the throttle in, build a little boost, get over the hill, then right back to chugging along.