Final Drive Ratio

[ToddG]

New vehicle has choice between "final drive ratio" of 3.517 (towing package) or 3.251.

Can someone please explain to me what this means in terms of practical performance (low end and highway speed acceleration, in particular)?

[Tamara]

New vehicle has choice between "final drive ratio" of 3.517 (towing package) or 3.251.

Can someone please explain to me what this means in terms of practical performance (low end and highway speed acceleration, in particular)?

The higher the number, the brisker the acceleration, but the higher revs you'll turn at freeway speed with concomitant lower mpg.

[Spr1]

I believe practical traction in sub-optimal conditions improves with a higher numerical final drive ratio. At least that has been my experience.
I read a write up from a GM engineer a while back that stated that final drive ratio has less impact on mileage than you would think.
YMMV......Hah!

[ToddG]

So then the question becomes, and maybe you just can't answer without knowing more, but is the difference here worth something tangible or will it just be noise?

I'm relatively ambivalent about MPG -- it's an SUV, not a Smart Car -- but the odds I'll ever actually tow anything are near zero. I'm just generally looking for that brisker acceleration and so I think you've answered my needs.

Tam. What would I do without you, chick? :cool:

[David S.]

I would prefer a lower number for highway cruising. I would prefer a higher number for stop and go or "spirited" driving.

Probably have little to no impact on real world gas mileage especially if you have a spirited driving style. Maybe good for 1 or 2 MPG....Maybe.

[TheTrevor]

The performance difference of a higher final-drive ratio is only apparent in the upper part of the towing performance envelope, i.e. with loads of 50% rated max or higher. If you're not buying the vehicle with the intent of towing a specific boat or trailer which pushes the upper limits of the tow rating, get the standard ratio and burn less gas.

[TGS]

The performance difference of a higher final-drive ratio is only apparent in the upper part of the towing performance envelope, i.e. with loads of 50% rated max or higher. If you're not buying the vehicle with the intent of towing a specific boat or trailer which pushes the upper limits of the tow rating, get the standard ratio and burn less gas.

For towing that might be true, but he's after the fun factor.

And if a higher final drive ratio didn't equal a bigger smile, there wouldn't be tons of BMW Z3s with 3.91 instead of 3.23 gears.

ETA:

Something to consider Todd is that you'll probably have faster 0-60 times with a lower final drive ratio, because you will likely have to shift an extra gear with a higher ratio. Some guys who took a liking to 1/4 mile drag racing with the WRX STi actually wanted to drop the final drive exactly for that reason....the gears were too short compared to typical 1/4 mile cars. Dropping the ratio, and an STi could easily do 0-60 in the low 4s range, whereas my Stage II with the original shorter gears still couldn't break 4.6s.

But, I was after the tangible fun factor, not the numbers. :D

[ToddG]

Sounds like I've got my answer, then. Thanks, everyone!

[Luke]

Basically a couple hundred RPM's at like 70

[John Ralston]

It's quite a difference even though the numbers seem comparable. I would go with the 3.25 if it were me...and I currently have 3.57 in my truck.

[jon volk]

The towing package also probably comes with a transmission cooler. If you are choosing a numerically higher gear, either for fun or towing, the cooler and any other ancillaries that go along with said package are the way to go.

[John Ralston]

The towing package also probably comes with a transmission cooler. If you are choosing a numerically higher gear, either for fun or towing, the cooler and any other ancillaries that go along with said package are the way to go.

Although the Trany Cooler is a nice addition, not sure it is necessary if you don't actually tow anything.

[Luke]

Don't have to have the cooler but they are nice. I build transmissions at a dealership (among other things) and heat is a huge issue. I've done 2 this week that went out due to over heating the unit. So a cooler is a nice option. Fluid is the heart of the engine and trans, best to keep it clean and cool.

[Haraise]

RPM at 74 MPH with towing package: 1922 RPM
RPM at 74 MPH without towing package: 1776 RPM

[Spr1]

If the object is to maximize mobility in dicey conditions, I would always opt for the higher numerical ratio.

[Erik]

If the object is to maximize mobility in dicey conditions, I would always opt for the higher numerical ratio.

Why?

[Spr1]

The tires "hook up" faster and you have less throttle sensitivity.

[Erik]

Thanks. I think I'd prefer to address grip with more/better tire rather than less effective torque. On throttle sensitivity, my complaint usually goes the other way. I don't think I've ever stepped on the gas and wished the vehicle was less responsive*, but I can think of plenty of times when what I wanted (in a few cases, really could have used) was more acceleration. On the other hand, I do appreciate traction control when conditions suck, so I see your point.

*Except for this one time when I decided to see how a RWD G35 coupe would do if I turned traction control off and floored it on the empty, snow covered street I was inching down. THAT was exciting.

[TommyG]

Thanks. I think I'd prefer to address grip with more/better tire rather than less effective torque. On throttle sensitivity, my complaint usually goes the other way. I don't think I've ever stepped on the gas and wished the vehicle was less responsive*, but I can think of plenty of times when what I wanted (in a few cases, really could have used) was more acceleration. On the other hand, I do appreciate traction control when conditions suck, so I see your point.

*Except for this one time when I decided to see how a RWD G35 coupe would do if I turned traction control off and floored it on the empty, snow covered street I was inching down. THAT was exciting.

Agreed. My truck takes it's time deciding if you were really serious when you mashed the go pedal. After a sufficiently judicious pause, it will finally deliver the throttle response that was requested. I don't run around with the pedal glued to the floormat but on those rare occasions where a little burst of throttle might be the difference between a collision and a normal day, it really stinks.

[Spr1]

Thanks. I think I'd prefer to address grip with more/better tire rather than less effective torque. On throttle sensitivity, my complaint usually goes the other way. I don't think I've ever stepped on the gas and wished the vehicle was less responsive*, but I can think of plenty of times when what I wanted (in a few cases, really could have used) was more acceleration. On the other hand, I do appreciate traction control when conditions suck, so I see your point.

*Except for this one time when I decided to see how a RWD G35 coupe would do if I turned traction control off and floored it on the empty, snow covered street I was inching down. THAT was exciting.

It is actually exactly opposite that. You have more torque but better ability to modulate the throttle because it takes a greater change in rpm to increase the tires rotational speed. The engine almost always has the ability to increase rpm faster than the car can accelerate to a an equivalent tire speed. The faster the surface speed of the tire (tire circumference times tire rpm) slows to match the velocity of the car/truck itself, the sooner the tires will stop slipping/spinning.
I have had 4wd trucks that were identical in all respects except final drive ratio, and the difference was noticeable.
And yes, good tires always make a difference.

[Erik]

It's entirely possible I'm getting my gear ratios backwards, conceptually. I'm a words guy. Numbers make me dizzy.

[saltydog452]

I have a high torque Diesel with 4.10:1 rears and limited slip. It can, and has, gotten stuck on wet grass but can drag a 20,000 lb Gross Vehicle Weight trailer border to border and water to water w/o breaking a sweat.

For use as a people/family hauler in the snow belt, the lower number rears, or transaxle, is more likely to get you home.

salty

[Corvus]

If you plan to use taller tires for that rugged 4x4 look the lower gearing would be offset some by the tires.

[BN]

3.517 = lower gear ratio = more lower end power and better acceleration = engine RPMs will be higher at a certain speed.

3.251 = higher gear ratio = higher top speed and better gas mileage = engine RPMs will be lower at a certain speed.

Todd, you probably want the 3.517 gears. ;)

Info from an old hot rodder. :)

[justintime]

This

Higher gear ratios are not that fun but they are more efficient! Unless your planning on adding tons of power - from a practical standpoint it does make a difference in gas mileage and high-way comfort. Cuts down on the droning noises you get from sitting at a high rpm on the highway. better better acceleration and drifty fun > everything else


3.517 = lower gear ratio = more lower end power and better acceleration = engine RPMs will be higher at a certain speed.

3.251 = higher gear ratio = higher top speed and better gas mileage = engine RPMs will be lower at a certain speed.

Todd, you probably want the 3.517 gears. ;)

Info from an old hot rodder. :)

[Erik]

For what it's worth, this is pretty much what I had in mind. I get tangled up in the higher numbers = lower ratio thing.


3.517 = lower gear ratio = more lower end power and better acceleration = engine RPMs will be higher at a certain speed.

3.251 = higher gear ratio = higher top speed and better gas mileage = engine RPMs will be lower at a certain speed.

Todd, you probably want the 3.517 gears. ;)

Info from an old hot rodder. :)

[Tamara]

I borrowed a guy's '65 Mustang for a weekend while he was busy getting my motorcycle impounded. (These are the sensible decisions one makes at 19.)

It had a mildly-breathed on 289 K-code motor which normally means sprightly, if not exactly eyeball-flattening, performance.

But this car also had a stripped-to-the sheet-metal interior and a four-speed swinging a 4.11:1 rear end, which made for a lot of sound and fury in daily driving. Very few street small blocks I've driven demanded that much delicacy in driving it out of the hole to avoid just sending the skins up in smoke. :o

[Spr1]

My '87 5.0 liter Mustang with a 3.08 rear was beyond helpless in snow, demanded your A game when driven enthusiastically in rain, and would absolutely incinerate its tires in the dry. The front /rear balance was horrible.

[John Ralston]

My '87 5.0 liter Mustang with a 3.08 rear was beyond helpless in snow, demanded your A game when driven enthusiastically in rain, and would absolutely incinerate its tires in the dry. The front /rear balance was horrible.

Surprising...my 70 Mustang has a 3.00 rear and can be a bitch to start out of the hole if you aren't paying attention (fall on its face). From a rolling start it is a beast though, cause you can run up to 40+ before you have to shift into second.

[ford.304]

Personally I don't see the point in the lower ratio in final drive unless you're towing. If I'm passing someone, I'm flooring it and downshifting anyway. Might as well get the better gas mileage and quieter ride the rest of the time.

[BN]

Surprising...my 70 Mustang has a 3.00 rear and can be a bitch to start out of the hole if you aren't paying attention (fall on its face). From a rolling start it is a beast though, cause you can run up to 40+ before you have to shift into second.

My 1967 390 Mustang had 3.23 gears in it and I put 3.90 gears in it. Whoo Boy. :) Fun. Hard on tires though. ;)

[John Ralston]

My 1967 390 Mustang had 3.23 gears in it and I put 3.90 gears in it. Whoo Boy. :) Fun. Hard on tires though. ;)


I have ruined my fare share...3.90's would make for a fun ride between stop lights!