What's highway RPM with short gears and 4.55 R&P?

[ryneen]

I'm thinking about getting the short gears and the 4.55 R&P for my 5 speed.

Does anyone know the RPM difference from stock? I drive my NSX nearly 80 miles per day and I cruise about 80mph, so I don't want to rev it to the moon. I'm hoping it's not more than a few hundred RPM's different.

My stock speed to RPM is:

70mph = 3000rpm
75mph = 3200rpm
80mph = 3400rpm

Thanks,
Ryan

 

[Yellow Rose]

Divide By 2, Multiple By 100

At 80 MPH with the 4.55 (short gears are not a factor, since they only affect 2nd, 3rd, 4th) you will be spinning at 4000 RPM.

60 MPH - 3000 RPM
70 MPH - 3500 RPM
90 MPH - 4500 RPM
100 MPH - 5000 RPM

etc etc etc etc

 

[nsxtasy]

You can convert the RPM's for any given speed from the stock R&P (which has a 4.062 ratio) to the 4.55 R&P by multiplying by 4.55 and dividing by 4.062

Short gears don't have any effect on highway cruising because they only replace second through fourth gears, and I assume you cruise in fifth.

60 MPH
70 MPH
90 MPH
100 MPH

Perhaps the estimates you posted were rounded off? For example, at 70 mph revs become 3000 RPM * 4.55 / 4.062 = 3360 RPM, not 3500... (Actual calculations using the numeric ratios of the gears show 70 mph stock equating to 3010 RPM, so these numbers should be pretty accurate.)

 

[Yellow Rose]

Tire Diameter

BTW....I run 285x35x18 on the back, so a variation of my posted RPM vs calculated RPM will vary if you are running a different rear tire.

 

[nsxtasy]

Re: Tire Diameter

BTW....I run 285x35x18 on the back, so a variation of my posted RPM vs calculated RPM will vary if you are running a different rear tire.

Have you had your speedometer corrected to adjust for your larger rear tire?

If not, then the speed indicated on your speedometer would not vary from the speeds in these calculations; however, there would be a difference between your indicated speed and your actual road speed, nominally corresponding to the 4.6 percent greater outer diameter of your rear tire size compared with stock.

(If you don't have your speedo corrected, then it sees how many revolutions it's turning per unit of time, and calculates the speed to display based on its assumption that the car has the stock size rear tire - in Andy's case, 245/40-17.)

 

[Yellow Rose]

Re: Re: What's highway RPM with short gears and 4.55 R&P?

Perhaps the estimates you posted were rounded off?

Yes and no, reading directly from the tach.....now granted if the tach looked to read 3540, I called it 3500. Point is for our thread originator, 3500 +/- 100 is "good enough" vs "revving it to the moon" that he is concerned with. Tire diameters will offset mathematical (in)accuracies.

 

[nsxtasy]

Re: Re: Re: What's highway RPM with short gears and 4.55 R&P?

+/- 100 is "good enough"

...or, he can do the calculations and find out what the exact change would be.

Tire diameters will offset mathematical (in)accuracies.

I don't understand what you mean by that.

 

[docjohn]

with 4.55 and 5-speed at 4000rpm my speedo says 80mph with 265/35/18 rears.

 

[ryneen]

I know Honda motors are made to rev, but I'm concerned about changing to the short gears and 4.55 R&P. Since I drive 80 miles round trip to work (and about 80 mph), I'm concerned about running the motor over 4000 rpms for so long each day.

My NSX is my daily driver, and I like to take it to the track as well. I would like the extra performance of the shorter gears and final drive, but I don't want to make a big sacrifice to engine life. Perhaps someone can shed some light on this and let me know if I'm being silly and shouldn't be concerned with the additional RPM's... or should I? I would assume the pistons, bearing, rings, valves etc have a lifespan and I don't want to shorten it too much on something as simple as my daily drive. Additionally, I'm not sure if there is a big difference in gas mileage at 4000rpm's.

I have 265/35/18 rears as well.

Thanks,
Ryan

 

[nsxtasy]

I don't think you need to worry about engine life due to the revs it turns. It's made to rev. The engine life isn't shorter if you drive it 85 mph all the time than 75 mph, and it isn't shorter due to the gearing either.

There are pluses and minuses to the various gearing mods.

Pluses: Slightly better actual acceleration. Better perception of acceleration (beyond actual) due to reaching redline sooner, albeit at a lower road speed.

Minuses: Cost to install. Gas mileage. May not provide as much value (acceleration gain per amount of money spent) as other mods.

Your decision.

 

[5inchfatlip]

Ryneen- you might want to go with the 4.23- you save a little top end with this setup, and still get the short gears...

 

[Lud]

You can run the engine all day at 5000 RPM and it's not going to have a failure simply as a result of running at higher RPM.

However, an engine cycle is an engine cycle. Certain things wear as a function of the number of cycles. If you increase the number of cycles to travel a given distance, you are increasing wear by some amount. The difference of a couple hundred RPM on a day to day commute is certainly pretty negligable, but it is more nonetheless.

The analogy of driving at 85 MPH vs 75 MPH doesn't hold because in that scenario your RPM is higher, but for a shorter period of time (since you reach your destination faster by driving faster). Changing your final drive ratio and then driving the same speed and distance raises the RPM while time of operation is unchanged.

The same is true for people who buzz around in a low gear all the time.

Will this marginal wear increase be the reason you have to rebuild the engine after 15 years of daily use instead of 16? Maybe... Or maybe something else will break first that is not directly related to engine cycles.

 

[Yellow Rose]

Forget The Revs.....

.....what about the noise?

Not meaning to get down to exact RPM at a particular speed, but can you tolerate the additional noise? When I was pondering the 4.55, I was asking myself if I could tolerate the noise at increased RPM. For about a week, at highway speed I would not shift into 5th gear. I soon became acclimated to the engine revs in 4th gear. Before the math pundits quickly point out that 4th gear revs with the 4:06 do not exactly equal 5th gear revs with the 4.55, I will counter by stating that my experiment above is "qualitative" not "quantitative".

you might want to go with the 4.23

The 4.23 will not fit all five speed cases. Beginning in 95, Honda changed something in the LSD carrier that will not accept the 4.23.....thus I have the 4.55 from Comptech.

 

[nsxtasy]

Keep in mind that the labor to open up the transmission and install the gears will be at least a full day, maybe part of a second day. Not cheap.

If you proceed with getting any of the shorter gear options installed, don't forget to check to see if your car's transmission is in the snap ring range. If it is, and has never been repaired/replaced for this problem, it would be a good idea to replace the snap ring and upper transmission case at this time, so you will never have to worry about the snap ring issue again.

 

[W]

Re: Forget The Revs.....

.....what about the noise?

When I was pondering the 4.55, I was asking myself if I could tolerate the noise at increased RPM. For about a week, at highway speed I would not shift into 5th gear. I soon became acclimated to the engine revs in 4th gear.

I did the same thing when pondering whether or not to get the 4.55 ring and pinion. I decided against it in the end due the stock setup being better suited for my situation. Funny thing is, I don't use 5th anymore except for on long trips.

 

[nsxtasy]

Re: Re: Forget The Revs.....

Originally posted by W
I don't use 5th anymore except for on long trips.

If you're in traffic or residential neighborhoods, or carving canyons, that may make sense. But for many other situations, like taking the expressway from one side of town to the other, you're going to pay a penalty at the gas pump for not using fifth, without getting much in return...