Gear Ratio and engine wear

[tadaima]

Was wondering whether I should mess around with my car's gear ratio. I'm pretty sure the mfgs have already done the R&D to make the ratio where there's a balance between gas mileage and usable power.

Now my question is what ratio will make the car last longer? For example.

My civic hits 3000rpms at 65mph, if I change the tire diamater, so that it will lower my rpms at 65mph I will gain mileage, but how much stress is the engine taking from this gain?

I always wondered at what rpms is the optimal driving range. Seems like I feel the engine lug around 1800rpms (red line is at 7200rpms).

 

[moribundman]

Well, at 65 mph you still want some torque left for passing. But do you really want to mess up your speedometer's accuracy?

When I bought my 1989 VW Scirocco, I had the option of getting either a regular 5-speed or a 5-speed with a 5th "economy" gear. I opted for the regular 5-speed. With the other tranny, I could have reached top speed only in 4th gear, while 5th gear was simply for cruising efficiently. If I had wanted a car for cruising, I would not have bought a sporty car to begin with.

 

[tadaima]

True, however I am aiming for gas mileage. My other plan was to put a civic vx transmission in there. IN the civic vx, it cruised at around 1500rpms at 65mph but had no forward acceleration at ALL. I stepped on the gas and you would have to really wait for the needle to move to 70mph.

Like you said that last 5th gear was for cruising, you would get to the speed you want to on 4th and set it to 5th when you were done, I guess like the newew 6th gears

 

[MolaKule]

quote:

---

if I change the tire diamater, so that it will lower my rpms at 65mph I will gain mileage, but how much stress is the engine taking from this gain?

I always wondered at what rpms is the optimal driving range. Seems like I feel the engine lug around 1800rpms (red line is at 7200rpms).

---

Enlarging the tire diameter alone will increase engine rpms and torque to the tires. Decreasing tire diameter alone will lower rpms at the expense of torque.

The curves for your engine's torque and horsepower verses rpms determines gear ratios and fuel mileage.

Most generally, lowering the final numerical gear ratio increses mpg at the expense of performance. I define performance as acceleration, passing, and pulling abilities.

On the other side of the coin, increasing the final numerical gear ratio increases engine rpms and performance at the expense of mpg.

Take for example my (past tense) HotRoded '86 Surburban. It came with 3.07:1 final (diffy) gears for max mpg and lowest engine rpms at cruise. However, the performance was seriously lacking.

Enter the 3.43:1 gears WITH larger tires. With the larger tires, this increased the final gear ratio to about 3.71:1. Mpg went down (because of higher average engine RPMS), but overall performance increased, especially passing and pulling from cruise to acceleration.

For added performance, I installed a higher duration, higher lift cam, headers, dual exhaust, and a Holley induction system. MPG actually improved with these mods, since the engine was breathing better and had better fuel induction.

So you generally, you are always giving up one item for another.

For the newer Nissan vehicles, for example and depending on engine displacement, they seem to tack between 2700 rpm and 3200 rpm at a cruising speed of 75 mph with their stock gear ratios.

Sometimes, changing to a superchip or reflashing the computer gives gains in performance and mpg.

 

[White 03]

[/QUOTE]Enlarging the tire diameter alone will increase engine rpms and torque to the tires. Decreasing tire diameter alone will lower rpms at the expense of torque. [/QUOTE]


This sounds backwards to me or am I reading it wrong?

Increase the tire diamater,or circumference, and I should go farther with each revolution. Therefore less engine RPM for a given speed.

Larger tires should equal lower numerical diffy ratio.

 

[tadaima]

White: Yea yours sounded correct

 

[Gary Allan]

quote:

---

I'm pretty sure the mfgs have already done the R&D to make the ratio where there's a balance between gas mileage and usable power.

---

They cover a broader range of driving environments usually. If you're going to resign this car to a highway cruiser, you may be able to squeeze a few more mpgs out of it ..but you have to weigh the costs of the mod against the gains over the duration of you anticipated ownership.

You've got to find where your torgue peak is and target that for your typical cruising speed. That should give you enough rpm left for a downshift if higher power levels are needed for passing and afford you the most resilience for grades and such. You just gotta get used to winding it out in the lower gears in situations that aren't in the "warp speed" realm. That is, you'll pay for your highway economy with your sub highway speeds. This is the case with my wife's jeep ..but it aint about economy. She's got 3.73 gears, a 5 speed od trans ..and 33" tires. You can drive @ 65-70 in either 4th or 5th and the rpms are fine. Unless you hit a really long grade ..you're fine. The trade off is that she's got to be doing 45-50 to be in 5th gear ..or she's lugging it. The actual final drive in that situation isn't much different than having the stock 30" tires (28.5") while in 5th ..so it's a wash if you look at it that way.

That is, below your cruising speed ..it a game of drivetrain semantics when you play with either tires or the R&P ...except that tires can add rotating mass ..but in your case ..I can't see that being a factor of great magnitude.

[ November 04, 2004, 01:52 AM: Message edited by: Gary Allan ]

 

[mechtech]

Sometimes when you change gears or tire size to reduce RPMs at cruise, you have to give more throttle, with possible different mixture, timing, and cylinder filling, as the result. You can negate the gearing improvements under certain conditions.
I'm not sure what Molakule is talking about - his numbers are all wrong, and backwards.

 

[Hankrr]

quote:

---

Originally posted by White 03:

---

Enlarging the tire diameter alone will increase engine rpms and torque to the tires. Decreasing tire diameter alone will lower rpms at the expense of torque. [/QUOTE]


This sounds backwards to me or am I reading it wrong?

Increase the tire diamater,or circumference, and I should go farther with each revolution. Therefore less engine RPM for a given speed.

Larger tires should equal lower numerical diffy ratio. [/QUOTE]

Yep it's backwards. Your reading it right....... Putting on larger dia. tires will LOWER engine RPM for a given speed.

 

[eljefino]

My mazda 323 econobox did this too... 3500 rpm cruising at 70. Funny thing is, in 5th gear it would have gone from 65-70 more easily than 50-55 or 75-80. It was coming into its torque peak at legal speeds but needed the "short" 5th to push through the air at 80. Used to get on my nerves too.

I suspect the civic VX also had a different cam for better torque down low, to match its tranny. I wouldn't change one without the other. Your honda motor can surely go 500k if it were always warmed up and on the highway so I wouldn't worry about total rev count. Your cruising gas mileage is mostly shaped by the aero of the car body, weight, and tires... what are you getting for MPG anyway, close to 40?

 

[hungdynasty]

Please correct me if I'm wrong. I think in some cases lower rpm doesn't mean better gas mileage if the engine is running outside the appropriate rpm range. Just like if you move your car from stop in 5th gear(forget about dropping the clutch), I'm pretty sure you use more gas than moving the car in 1st gear.

 

[tadaima]

I wish I was getting 40mpg, well when I first got the car I was getting around 33 at best on all freeway, cleaned the Tbody/AutoRx/Castrol GC oil change and got 37 at best, but usually 35-36.

Changed the tires from 185 to 195/60/14 and got around 34-35mpg.

Finally I installed an injen intake just for kicks and for mileage, and got about 39mpg =), but that's until the ECU compensated for it. Now its back to the 34-35mpg range

The VX is amazing suppose to get 56mpg, but most people post around high 40's. I guess it has to do all cruising for 56mpg.

 

[cryptokid]

see if youre civic has a gear ratio swap available. civics are madly popular cars. i bet somthing exists for cheap.

my mazda use to cruise at 4000 rpm at 75mph. i swapped the 5th gears from a 929 into it and now it does 3500 rpm at 70mph. the swap was easy and i the only things i had to remove was 1 wheel and the transmission cover. it was a 30 minute job.

if you can find somthing similar to this, maybe a 5th gear from an accord or other honda which has lower cruising rpm's.

 

[ALS]

Been there, done that and got the tee shirt.
I did it all for better gas mileage. I went from a 15 inch tire to a 16 inch tire. I then swapped out the auto with a .69 OD for a manual with a .78 OD. Swapped out the 3.91 rear for a 3.31 rear. Gas mileage city didn't change much but highway jumped 13 percent. The increase depends on speed but at 65 mph (base line) the original factory set up I got 26 to 26.5 miles per gallon.
With my changes mileage has increased to 30 to 30.5 mpg. At 70 mph that increase in mileage is close to 16 percent over the old set up. At least with Volvo 200's 700's and 900's once the RPM's go north of 2500 gas mileage heads south quickly. The trick with my car is to keep the rpms in between 2000 and 2500 for optimum gas mileage. So with my set up that rpm range puts my speed range between 55 to 72 mph. You need to find out the torque curve for your car and set your gearing up to optimize on that information. Usually just dropping the rpms by 150 to 200 will get you a nice increase in mileage. In my case I dropped the RPM's by 167 at 65 mph and 181 rpm's at 70 mph. Here is a link to a quick little program to calculate gear, tire, and wheel changes. Be careful not to drop your RPM's so low your lugging the engine.
http://pw1.netcom.com/~sgalaba/m3e46/mph.htm#GearSpeed

 

[1sttruck]

I think that lugging an engine will do more damage than living with a few more revs. Higher revs on engine is similar to driving at 75 mph instead of 65 mph, something that engines are designed for. I don't use OD on the automatics in either sedan until we hit 65 mph, and maybe it's just the luck of the draw but the 93 Taurus has over 176k miles on it and it's still running very well.

 

[tadaima]

Maybe I should have shifted into 4th while going up a hill at 65. Can't really tell if its lugging a 3000rpms, but i just don't feel right downshifting to 4th having it run around 4200rpms for 65mph, at least not in a car with 130K+.

Or should I

 

[tadaima]

Maybe I should have shifted into 4th while going up a hill at 65. Can't really tell if its lugging a 3000rpms, but i just don't feel right downshifting to 4th having it run around 4200rpms for 65mph, at least not in a car with 130K+.

Or should I

 

[TheLoneRanger]

What about installing a performance exhaust system? That, combined with your aftermarket intake, should improve the engine efficiency and give more mpg.

I gained 1-2 more mpg in an Explorer with nothing but a muffler change. (Highway mileage should be even more.)

If your cruising speed is near the torque curve peak, that's about the optimum gearing. What about adding a front air dam and rear spoiler? Those things work very well in reducing drag and increasing mileage.

 

[MolaKule]

Good catch; it was late, what can I say; typo's typo's typo's.

Here is the corrected version:

quote:

---

Enlarging the tire diameter alone will decrease engine rpms and increase torque to the tires. Decreasing tire diameter alone will increase rpms at the expense of torque.

The curves for your engine's torque and horsepower verses rpms determines gear ratios and fuel mileage.

Most generally, lowering the final numerical gear ratio increses mpg at the expense of performance. I define performance as acceleration, passing, and pulling abilities.

On the other side of the coin, increasing the final numerical gear ratio increases engine rpms and performance at the expense of mpg.

Take for example my (past tense) HotRoded '86 Surburban. It came with 3.07:1 final (diffy) gears for max mpg and lowest engine rpms at cruise. However, the performance was seriously lacking.

Enter the 3.43:1 (Auburn LSD) gears WITH larger tires. With the larger tires, the final gear ratio went to about 3.23:1. Mpg went down (because of higher average engine RPMS), but overall performance increased, especially passing and pulling from cruise to acceleration.

---

 

[tadaima]

Thanks for the vast information. Well I was considering swapping the transmission as a whole, not into opening the case. But pretty much across civics d-series engine, the transmission are interchangeable. The general ratios changed between 92-95/96-00, and so did the market expectation of the speed limit, i remember early 90s had speedos that marked 55mph, but later on cars were geared towards 65mph.