5th Gear Speed to RPM Ratio in a 240sx.

[rsmithdrift]

Maybe this should be stickied.

Much debate has been given about how fast the 240sx can actually go. Well, here is your answer:

In 5th gear you gain 23.22 mph for every 1k rpm using 195/60r15 tires. I used this formula to figure that out: RPM = (mph x gear ratio x 336) / tire diameter

So here is your 5th gear speed to rpm guide for using 195/60r15 and 205/60r15 tires: rpm = speed (195's) ; speed (205's)1k = 23.22; 23.681.5k = 34.83; 35.522k = 46.44; 47.362.5k = 58.05; 59.23k = 69.66; 71.043.5k = 81.27; 82.884k = 92.88; 94.724.5k = 104.49; 106.564.8k = 111.456; 113.6644.9k = 113.778; 116.0325k = 116.1; 118.45.2k = 120.744; 123.136 5.5k = 127.71; 130.24 5.8k = 134.676; 137.3446k = 139.32; 142.086.2k = 143.964; 146.8166.5k = 150.93; 153.92 6.8k = 157.896; 161.0247k = 162.54; 165.76

*Numbers in bold are commonly seen rpm and 7k is bold for being the redline.

**These ratio's were found for a stock 240sx with stock ka24de with a stock transmission and open diff with factory 7 spoke wheels on 195/60r15 and 205/60r15 tires.

***My car will only get to 5.5k rpm on its own on flat ground and 5.8k with a draft or slight downgrade. I have gotten to 6.2k on a steep downgrade. This is due to wind resistance and lack of power. Be realistic. 160hp will not get a 2,600lb large chunk of steel to 169 mph.

Disclosure: I know about tire deflection and warping (it's affect is so small it only affects by 2-3 mph at most) and areo resistance and power to weight ratio and so forth. I am making this tread to clear up the speed issue and pose as a guide to finding your speed in 5th gear past the range of the speedometer. Don't flame me for speeding, we all do it and I don't condone street racing, I am only putting this up as a guide. Also I see nothing wrong with going at outrageous speeds on an abandoned stretch of highway........the only person you are endangering is yourself and your license..........so go right ahead, I don't care if you hurt yourself going 140mph, just don't take anyone else out with you.

Edit: I modified this post using more accurate calculations than my speedo which is set to a 24.166 per 1k rpm ratio in 5th. That's 72.5 at 3k rpm, 140 at 5.8k rpm, 145 at 6k rpm and 169 at 7k rpm. So It clearly isn't accurate. Thanks to s13satch for the accurate equation.

Modified by rsmithdrift at 6:21 PM 2/22/2006
Modified by rsmithdrift at 8:41 PM 7/2/2006

[lrb_2000]

Wow...

I would be scared as sh*t to go that fast in a stock 240, with stock suspension, that's just crazy. But good info I guess.

[KDashy]

I like this thread.

I wish I had a taller 5th gear though.

[FliMSiCaL]

you do realize that the stock suspension on a s13 is better than most american made muscle cars? its the car being too light/no downforce at high speeds that makes it scary

[naed240sx]

If this is for s13, your math is flawed. s13 hits 112mph at 5.5k

[sensibleS13driver]

Yea... this isn't right, sorry, back to the drawing board buddy.

[ddgsxr504]

Don't forget that most factory tachs are slightly in accurate the higher you go up the rpm band.

[Alfador]

Seeing as how air temp affects a car, (any good technician records the ambient temp while doing a dyno run) it might be good to note the temperature this applies to. Also, yeah, I've cruised 85 at 3k on flat road at the Mass. turnpike, so there's something off with these numbers. At the very least I dont think they need to be taken to 3 decimal places seeing that the standard deviation is going to be in whole number mph more than likely.

[rsmithdrift]

You guys that say my math is off. Are you saying that my tach is wrong??

Also, do you have low profile tires, lsd, different diff, different wheels, etc. This will all affect speed.

My 300z has the same R200 diff, the same size tires and the same transmission and it has the exact same numbers for 5th gear. And I mean to the tenth. Sooooooo, this should be right for any stock north american 240sx (non-hicas, I have no clue about their final drive ratio)

[lrb_2000]

My car hits right over 115mph @ 5k rpms.. that was last week.. stock lsd, 205/60 r15 tires...

[s13satch]

Okay everyone, here is the correct math:

RPM = (mph x gear ratio x 336) / tire diameter

Lets assume you have the stock 195/60R15 tires on your s13 and you have the factory 4.083 rear end. Your formula for 100 mph in 5th gear would be:

RPM = (100 x (.76 x 4.083) x 336) / 24.21

RPM = 4306

rsmithdirft, your numbers are close but not quite right.

[BobWithBoobs]

the math s deff right, but i do agee ive hit in the 6k range in 5th geer on a down slope and was'nt pushing more then 120 (on the tack) eaither the tack is wrong or sumthing realy uked p about these cars haha

[BobWithBoobs]

nm i mean 5k , and it is just about right .

[s13satch]

My formula came directly out of a textbook, so I assure everyone that it is correct.

With the factory tires and differential, the s13 would hit the 6900 rpm redline at 160.218 mph. With the optional 205 tires, you would hit 163.395 mph.

[rsmithdrift]

My car is supposed to have 205/60r15 's on it. Are you talking about the steelies? Cause I have factory sevens. But that first number is just tire width, the tire hight is the second number and they are both the same. (60)

And with my equation........100mph is 4.15k rpm.

I have had my buddy following me when I got the car and he said that my tach/speedo was right, because we both set our cruise control right on 75mph on the way back to Chatt. with it and we were going exactly the same speed.

What is 5.8k rpm with that equation??
Modified by rsmithdrift at 5:36 PM 2/22/2006

[shockload]

dang, i didnt think ive hit such high speeds. no speedo

[s13satch]

My car is supposed to have 205/60r15 's on it. Are you talking about the steelies? Cause I have factory sevens. But that first number is just tire width, the tire hight is the second number and they are both the same. (60)

And with my equation........100mph is 4.15k rpm.

I have had my buddy following me when I got the car and he said that my tach/speedo was right, because we both set our cruise control right on 75mph on the way back to Chatt. with it and we were going exactly the same speed.

What is 5.8k rpm with that equation??

If you look in your owner’s manual you will see that 195/60 is listed as the standard tire size; 205/60 was optional. I personally have 195/60 on my seven spokes. As for your comment about the 2nd number being the same, you are correct. The aspect ratio is the same but the width is larger which means that the overall tire diameter increases from 24.21" to 24.69".

While you and your friend's gauges might have appeared to be identical, I'm sure that they were not a 100% match. Also, factory gauges are never 100% accurate, especially as rpm and speed increase.

5.8k with the 205 tires is equal to 137.346 mph.

[rsmithdrift]

Should I modify the OP to compensate for this new info??

Also one last question, what's the tire diameter on 215/50r15's??

[Red coupe]

tire diamiter depends on load... or equil to 2X the loaded radius...In other words this isnt gonna be standered car to car, and technicaly as speed increases the vehichle should lift increasing the loaded radius

For all you kids at home that wanna join in MPH= (RPM X tire diameter)/(gear ratio X 336)"Gear ratio" should be final drive * what ever gear your in

once I d/l the manual for the gear ratios I will calculate tire size based off the 112mph @ 5.5k rpm ...

I really dont see the point of giving your results to 5 sig. fig.'s though as the tach reads in 100rpm incriments and the speedo in 1 mph....Im sure you didn't take your actual mesurements that that strict a degree so your really only creating a false sence of accuracy...

K so with a .759:1 5th and 4.083 final drive I get a 21in loaded tire diamiter based off the 112mph @ 5.5k rpm....
Modified by Red coupe at 3:06 PM 2/22/2006

[eds13]

man i dont know what ya'll be talkin bout yo! i had my mine up to 200 and all i got is an intake, a can, and alltezza tailss. jdm tyytenesss yo!

[s13satch]

Yeah, these calculations aren't perfect since tire diameter changes with load and how worn the tread is. But, they are much more accurate than just reading off the gauges in the car.

Just to clarify (we don't want to confuse anyone any further Red coupe ) "gear ratio” is the total of the transmission and the differential. 5th gear is .76 and the differential is 4.083.

[Red coupe]

yeah the first time I did the tire diamiter I only used final drive, right after I finish saying not to do so....I got a 5" tire(BTW thats not the right answere)3.10 is the gear ratio to 3 decimal places.

[rsmithdrift]

Alright, I modified the OP using s13satch and red coups equations for both 195/60r15 and 205/60r15 tires. 195's in middle and 205's on right.

That should be more accurate than my speedo which is off by .4 mph per 1k rpm according to these new equations.

[naed240sx]

Did you all just overlook what I posted? Im not bothering with the math right now, but you have to realize that the s13's top speed from the factory is set at 112 to 115 depending on tire size, at 5.5k rpms. You cannot go higher than 5.5k in a stock s13. The computer will rev cut. If you are getting a higher tach reading than that, your tach is off.

[Red coupe]

he also mentioned that the car couldn't go that fast for lack of power...I think this post is more about the limits of the gear ratios of the car rather then how fast the car can travel...There are a few ways to disable to governer as well...

[naed240sx]

he also mentioned that the car couldn't go that fast for lack of power...I think this post is more about the limits of the gear ratios of the car rather then how fast the car can travel...There are a few ways to disable to governer as well...

Im not disagreeing with the intentions of the thread, only saying that its not possible for you to hit 130 mph at 5.5k, because 5.5k in 5th will only take you to 112-115.

[nismofly]

my speedo read ~104 or 105 at 4200 rpms, im on 225/50/16 tires

[naed240sx]

Practically everyones speedos or tachs are pretty inaccurate at around 100mph in s13s. You can't argue with the specs though.

[rsmithdrift]

I cut my fuel cut off and disconected the 4th and 5th gear sensors at my transmission. So I have gotten to 6.2k rpm (hwy 111 going down Signal Mtn). If my speedo went that far it would be 149mph. But my speedo is off by a factor of .4mph/1k rpm. So By using gear ratio and tire size and whatnot. that is actually 146.8 mph give or take due to tire warping.

On flat ground my car can go 115 in 4th at 6k or something. In fifth I am only at just under 5k. That is were my fuel cut off USED TO hit.

Naed240sx: You have the wrong sized tires on your car, or your speedo is WAY off. You are going 130 at 5.5k rpm if you have 205/60r15's on your car. End of story.

112 at 5.5k is total BS!!! If that were true you would be at 4k at 75mph. It is well noted that 240's hit 70-75 at 3k rpm. You are very wrong.

[masticatingcow]

I don't know if this thread has quite run its usefulness, but let's not turn it into a mathematical pissing contest, people. Estimating a vehicle's top speed is interesting, that's for sure, but only yields a set of GUESSES, albeit a set of well-educated guesses. Get a radar gun at the end of a 5 mile stretch and flippin' find out.

As has already been stated, the suspension geometry and chassis design will prevent the car from being particularly safe at such high speeds. Without significant aero work, one need not consider his car's top speed.

And rsmithdrift, while I appreciate your academic efforts, I have to say that this information is largely irrelevant. At ~163 MPH, the LAST thing you should be doing is looking at your speedometer.