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DMC
03-01-2003, 07:52 AM
ok this is a dumb question

if my car is at 3000 rpms and it is going 70 miles an hour the increase in rpms should be directly proportional to the increase in speed right(as long as you dont change gears)? so at 6000 rpms shouldnt i be going 140mph?!?!?!
Phlip
03-01-2003, 09:27 AM
There is that pesky ass top speed limiter I hit at about 120 or so... I'm usually in the 5*** range when I do it, so 140 at 6000 may be doable. Unfortunately, 120 is so fast I don't think I'd have the nuts for 140 personally.
thelinja
03-01-2003, 10:02 AM
Originally posted by DMC
ok this is a dumb question

if my car is at 3000 rpms and it is going 70 miles an hour the increase in rpms should be directly proportional to the increase in speed right(as long as you dont change gears)? so at 6000 rpms shouldnt i be going 140mph?!?!?!
Incorrect. Speed and RPM are not directly proportional. The other day I actually tried this. At 6000 RPM I was doing 120 and the limiter kicked in. It doesn't matter if you downshift or not, you're always going to end up at 120mph @ 6000 RPM in fifth. The speed vs. time graph is the left half of a concave down parabola, not a straight line, if it were a straight line, yes, you might hit 140. But, as RPM inceases, the amount of speed that increases gets infinitesmally smaller each RPM you move up to.
Phlip
03-01-2003, 01:37 PM
Originally posted by thelinja
Incorrect. Speed and RPM are not directly proportional. The other day I actually tried this. At 6000 RPM I was doing 120 and the limiter kicked in. It doesn't matter if you downshift or not, you're always going to end up at 120mph @ 6000 RPM in fifth. The speed vs. time graph is the left half of a concave down parabola, not a straight line, if it were a straight line, yes, you might hit 140. But, as RPM inceases, the amount of speed that increases gets infinitesmally smaller each RPM you move up to.
I don't doubt what you say here, but I have gone out to the top speed limiter on my car several times, and I have NEVER been to 6000 in 5th gear
mrdirty
03-01-2003, 01:41 PM
Originally posted by thelinja
Incorrect. Speed and RPM are not directly proportional. ...

Actually, it IS a linear relationship between engine RPM and velocity where the slope of the line is the gearing ratio and the origin is zero....
DMC
03-01-2003, 04:00 PM
if it isnt a linear relationship on your car than you have something messed up in your tach or speedometer becuz the engine is "directly" connected to the tires it isnt a cvt transmistion where it is variable.
mrdirty
03-01-2003, 04:03 PM
Originally posted by DMC
if it isnt a linear relationship on your car than you have something messed up in your tach or speedometer becuz the engine is "directly" connected to the tires it isnt a cvt transmistion where it is variable.

exactly.
thelinja
03-01-2003, 04:07 PM
Hmmm, I think I confused myself earlier. Sorry about that. They are directly related. Speed goes up while RPMS go up, duh. I'm retarded. I was reading your post DMC as if you were saying that 100RPM = 1mph, and 1000RPM = 10mph and so on. When I bounced it off the speed limiter I only caught a glimpse of the RPMs and it was somewhere between 5800-6000. I was just keeping my eyes on the road because I was going 120mph. I just don't understand how RPM's are directly proportional to speed here. If you increase speed by let's say, from 70-90, the RPMs don't go from 3000 to 5000. I'm probably wrong though, I'll just investigate the next time I'm on the freeway.
mrdirty
03-01-2003, 04:12 PM
Originally posted by thelinja
I was reading your post DMC as if you were saying that 100RPM = 1mph, and 1000RPM = 10mph and so on.
ummm....that is correct...

the formula is simple: gearing ratio = final transmission rpm / engine rpm. Since final drive rpm is directly and linearly related to velocity; velocity is linearly dependent on engine rpm....
DMC
03-01-2003, 04:23 PM
basically there is a ratio for each gear
like 5th gear should be 300:7 every 300 rpms taht go up u go up 7 mph
thelinja
03-01-2003, 06:02 PM
Originally posted by DMC
basically there is a ratio for each gear
like 5th gear should be 300:7 every 300 rpms taht go up u go up 7 mph
AHHHHHH, ok. Thanks for clearing that up.
misnomer
03-01-2003, 06:47 PM
Keep in mind not every 240sx will be going the same speed at the same RPMs, a larger tire radius will yield a higher speed (this will not be accounted for in the tach/speedo, as the speed is guaged at the tranny iirc). Was there any difference in the gearing between s13s and s14s?
AKADriver
03-01-2003, 07:56 PM
All 240SXs have the same gears for a given trans.

The factory tire heights are all vaguely the same, too.
240fluke
03-02-2003, 12:48 AM
Hmmm....I might be wrong about this, but I thought the car hits its limiter (is it speed/rev or both?) at 119 mph and 5400 RPM (at least in the S14). Maybe it was 5700 RPM, but I know it was under 6000 RPM.

Tim '95 SE
PabloHoney
03-02-2003, 01:12 PM
5000 RPM ~ 119mph
5400 RPM ~ 129mph

this is for an SE with 205/55/16s, it is very slightly different for base size tires/wheels.
Nismo241
03-03-2003, 02:52 AM
The only time when 1000rpms=10mph is in third gear. At least in my car. That is the gear that it is closest to 1:1. I know it's not exactly 1:1 but damn close.
mrdirty
03-03-2003, 06:14 AM
Originally posted by Nismo241
The only time when 1000rpms=10mph is in third gear. At least in my car. That is the gear that it is closest to 1:1. I know it's not exactly 1:1 but damn close.

I don't think he was referring to any gear ratio in particular; but was trying to say that a doubling of RPM does not double Velocity (which of course it does, but why harp on it.)