How does final drive ratio affect gas mileage?

[fuzzybabybunny]

I'm a little confused on what final drive actually is after reading the wiki.

I totally understand this:

Gears: 1st 3.545, 2nd 1.888, 3rd 1.296, 4th 0.972,5th 0.780,

So in 5th for every 0.780 revolutions the engine turns the transmission turns once. So it saves on gas provided the load isn't too high.

final drive 4.44:1

This I totally don't get.

Does a final drive ratio of 3.70:1 get better gas mileage than a final drive of 4.44:1 if driven 100% on the highway? Or something?

 

[96Firebird]

Final drive is the ratio of transmission output (driveshaft) to wheel axle.

So in your example, in 5th gear, the transmission is moving the driveshaft one revolution per 0.78 revolutions of the crankshaft. The wheel axle is rotating one revolution every 4.44 revolutions of the driveshaft.

Final drive is not part of the transmission, unless you include the transaxle as part of the transmission. Its easier to think of it in a RWD car. The transmission has it's ratios for gears 1-5 or whatever. Then the rear axle has it's own ratio, known as the final drive. So, to answer your last question, 3.70 geared rear end will get better gas compared to a 4.44 geared rear end. At 60mph, the 4.44 rear end will need to be at a higher RPM compared to the 3.70 rear end (all other things equal).

Edit - Did you change the title? To answer that question, see Gillbot's response.

 

[Gillbot]

You want to be in the best efficiency range for the engine, so whatever drive ratio puts you there is the correct answer.

 

[Throckmorton]

It depends on the engine, amount of air resistance, etc.

Sometimes with a taller rear ratio (3.70 in this case), the engine is spinning TOO slow. This happens a lot with trucks when you increase tire size (effectively same as making final drive taller) and lose gas mileage.

 

[Rifter]

What they already said. Basically find out what rpm range your engine is the most fuel efficient in and then you want a final drive that puts it in that rpm range at highway speeds in 5th gear. This will give you maximum fuel economy.

 

[NetWareHead]

Going to a gear ratio (in the transmission or rear axle) that is too low puts too much load on the engine and results in worse fuel economy. Alot of people think that just because an engine is revving high, it automatically has to be using more fuel. Thats not to say the revs are not important: engines are designed to operate in their own power band where the right conditions are created for optimal power and fuel economy. It's all about the load the engine is handling that determines fuel economy. If your tranny is in neutral and you are revving your engine at 6000 RPM, it will probably still use less fuel than that same engine pulling a fully loaded trailer, yet only spinning at 2000 rpm.

 

[JCH13]

It depends on the engine, amount of air resistance, etc.

Sometimes with a taller rear ratio (3.70 in this case), the engine is spinning TOO slow. This happens a lot with trucks when you increase tire size (effectively same as making final drive taller) and lose gas mileage.

There are other factors in play when tire size is increased. Taller side-walls, more tire tread exposed to flowing air, and increased rotation mass all adversely affect mileage. You are over-simplifying things. Yes, increased tire size can reduce fuel efficiency, but not strictly because the final drive ratio is effectively changed.

You want to be in the best efficiency range for the engine, so whatever drive ratio puts you there is the correct answer.

Do you mean best BSFC efficiency?

Edit: OP, my Miata has a lower final drive ratio than stock, 4.1 (mine) vs 4.33 (stock). Even with a turbocharger and 'spirited driving' I get 27-30mpg combined. EPA rated mileage for the stock engine with 4.33 rear end is 24 combined, 28 highway. Clearly no answer is 100% absolute.

 

[Throckmorton]

There are other factors in play when tire size is increased. Taller side-walls, more tire tread exposed to flowing air, and increased rotation mass all adversely affect mileage. You are over-simplifying things. Yes, increased tire size can reduce fuel efficiency, but not strictly because the final drive ratio is effectively changed.



Do you mean best BSFC efficiency?

Edit: OP, my Miata has a lower final drive ratio than stock, 4.1 (mine) vs 4.33 (stock). Even with a turbocharger and 'spirited driving' I get 27-30mpg combined. EPA rated mileage for the stock engine with 4.33 rear end is 24 combined, 28 highway. Clearly no answer is 100% absolute.

The rotational mass makes no difference at highway speed, and hardly any difference even in city driving. Neither does the air resistance, when you look at how much total drag a truck has. My Cherokee went from 20mpg to 16mpg with 31" tires vs the stock 29" ones. That vehicle is a brick.

 

[Topweasel]

The rotational mass makes no difference at highway speed, and hardly any difference even in city driving. Neither does the air resistance, when you look at how much total drag a truck has. My Cherokee went from 20mpg to 16mpg with 31" tires vs the stock 29" ones. That vehicle is a brick.

Actual mileage or just what the odometer says? Because while taller tires could help out fuel efficiency. It won't look like it to the odometer because you are spinning the tires less and therefore to the odometer you haven't traveled as far. 91 inches of surface length versus 97 inches. The 31 inch tires would turn 653 times a mile. The 29 inch tires 696 times a mile. Those 40 less turns is a big chuck. This is just at Rim size depending on the tire choice and thickness you chose between the two rims sizes could increase or decrease this issue.

Edit:
Just reread what you were saying and now I am going to assume that we are talking about total wheel and tire height of 31 vs. 29. So that information should hold up as to what your Jeep is doing.

 

[JCH13]

The rotational mass makes no difference at highway speed, and hardly any difference even in city driving. Neither does the air resistance, when you look at how much total drag a truck has. My Cherokee went from 20mpg to 16mpg with 31" tires vs the stock 29" ones. That vehicle is a brick.

Rotating mass makes a noticeable difference, especially in city driving. I've done the math. Having the extra un-sprung weight forces the tire to deform more over bumps or surface imperfections, which consumes power. It doesn't seem like much, but it adds up.

So does air resistance from bigger tires. Tire surfaces are moving against air flow at 2x vehicle speed, making them contribute a disproportional amount of drag relative to the whole vehicle. Don't believe me? Here is at least one data point:
http://www.flyinmiata.com/projects/targa/TargaBuild.php?UID=301

About a 23% power savings at 100mph. That's about an 11% savings at 70mph.

The extra sidewall also impacted your fuel economy. Do you think it's an accident that tractor-trailers are switching to double-wide tires instead of dual-wheel setups?

 

[iamwiz82]

I'm a little confused on what final drive actually is after reading the wiki.

I totally understand this:

Gears: 1st 3.545, 2nd 1.888, 3rd 1.296, 4th 0.972,5th 0.780,

So in 5th for every 0.780 revolutions the engine turns the transmission turns once. So it saves on gas provided the load isn't too high.

final drive 4.44:1

This I totally don't get.

Does a final drive ratio of 3.70:1 get better gas mileage than a final drive of 4.44:1 if driven 100% on the highway? Or something?

The final drive multiplies the final output of the transmission. Lower will usually give you better gas mileage assuming all else is equal, unless it's so low that it lugs the engine.

 

[JCH13]

The final drive multiplies the final output of the transmission. Lower will usually give you better gas mileage assuming all else is equal, unless it's so low that it lugs the engine.

Hooray! A reasonable answer!

 

[herm0016]

So does air resistance from bigger tires. Tire surfaces are moving against air flow at 2x vehicle speed, making them contribute a disproportional amount of drag relative to the whole vehicle.

Not true.

There is one point on the surface of the tire moving twice vehicle speed, and one point moving at 0 speed relitive to the road. (the point contacting the road assuming a round tire).


The spliter works because it prevents air from the turbulence creating tread and rotating tire, and from going into the wheel well and under the car. not because it prevents the side of the tire from creating drag. it creates more downforce in the same manner, by preventing air from moving under the car. less air under the car creates a lower pressure area, and will actually compress the suspension and suck the car down to the road.

wider tires/wheels create more cross sectional area, and push more air under the car. It has nothing to do with the V or Speed of the tire and wheel.

 

[JCH13]

Not true.

There is one point on the surface of the tire moving twice vehicle speed, and one point moving at 0 speed relitive to the road. (the point contacting the road assuming a round tire).


The spliter works because it prevents air from the turbulence creating tread and rotating tire, and from going into the wheel well and under the car. not because it prevents the side of the tire from creating drag. it creates more downforce in the same manner, by preventing air from moving under the car. less air under the car creates a lower pressure area, and will actually compress the suspension and suck the car down to the road.

wider tires/wheels create more cross sectional area, and push more air under the car. It has nothing to do with the V or Speed of the tire and wheel.

Fine, I'll re-word, "part of the tire is moving forward at up to 2x the vehicle's speed."

The point of those aerodymic elements, diverters if you will, (not the splitter) is to keep air away from the tires. They do nothing to prevent air from going under the car, that's what the splitter is there for.

Wider wheels and tires (which is not at all what I originally posted about) won't magically push MORE air under the car. At worst there air going under will be under slightly pressure due to a slightly reduced flow area.

Having a taller tire (which is what I posted about) both raises the car off of the ground and creates more surface area moving faster than the car. You cannot possibly argue that this does not happen and that it does not contribute to reduced fuel economy.

Edit: even the fluid coupling between the tire surface and the wheel well gets worse with a taller tire.

 

[herm0016]

still has nothing to do with V.

 

[JCH13]

still has nothing to do with V.

I assume by V you mean speed. I never said that it did.

 

[herm0016]

So does air resistance from bigger tires. Tire surfaces are moving against air flow at 2x vehicle speed, making them contribute a disproportional amount of drag relative to the whole vehicle.

v=velocity. speed with direction. speed is more accurate in this instance.