S2000 Under The Hood S2000 Technical and Mechanical discussions.

RPM's Vs. Speed

 
Old 03-18-2018, 11:47 AM
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Default RPM's Vs. Speed

Hi all, new to the forum. Forgive me if this has already been covered. I have tried many web driven engine RPM calculators and don't get anywhere near what I am turning in 6th at 70 mph. I am turning right at 3800 at 70. I just recently bought the car and I'm starting to wonder if a previous owner has put lower gears (higher numerically) in it. The car has not been modified otherwise, but soon will be. According to most of the calculators, with my tire height and and a 6th gear final ratio of .763 and rear gearing at 4.10 it shows closer to 3100 RPM's. Any one have some 4.10 speed Vs. RPM numbers I can look at?

Craig C.
2004 Suduka Blue w/ factory hard top.
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Old 03-18-2018, 06:39 PM
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In my stock '06, stock tire sizes, 4k rpm = 74 mph. So that works out to 18.5 mph per 1k rpm. That means 1.85 mph for every 100 rpm.

So, that wouod mean my car would go 70.3 mph at 3800 rpm.

Your car is normal. If you aren't ised to riding sport bikes, the hwy rpm can seem ridiculous. But the car can take it. Just get used to it. I know sometimes it feels like there should be two more gears after 6th.
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Old 03-19-2018, 02:23 AM
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There is a secondary reduction gear in the trans, OP.
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Old 03-20-2018, 01:01 PM
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Overall_ratio = gear_ratio * secondary * final

Speed (mph) = (tire_diam * rpm / overall_ratio) * (pi / 1056),
where tire_diam is in inches.

RPM = (speed * overall_ratio / tire_diam) * (1056 / pi)
where speed is in mph and tire_diam is in inches.

Values above model tire deformation under load by incorporating slight correction factors to reduce the effective tire diameter. The correction factor is bigger (smaller eff diameter) for taller sidewalls. The speedo does not make this correction, so at a given rpm the speedo will read ~3% higher than the numbers above. Also, of course, the speedo can't account for diameter reduction due to worn tread, so it's not uncommon for Honda speedos to read 5-8% high, even with standard-size tires.

Last edited by twohoos; 03-20-2018 at 01:53 PM.
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Old 03-21-2018, 02:58 AM
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I've been wanting to see a chart like that. Could you provide a link to the source for that chart?

The only thing I sometimes wish for, is the ability to cruise in 6th on the interstate at a lower rpm. But, I agree, the car is built to take long cruising at around 4K rpm, so I don't think it hurts it. It does take some getting used to. Especially for American drivers who come from the V8 world. Its has the effect of keeping my mind on oil levels, and that makes me check more frequently than normal, which is a good thing. I'm curious why the car is not designed with a lower cruising rpm for 6th gear. I don't think 6th has much use for most people except as a highway cruising gear. I'm sure there is a reason - I just don't know what it is.

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Old 03-21-2018, 08:43 AM
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The ap2 is geared such that you can drive in 6th at the US national speed limit of 55 mph, without rpm dropping to where you 'need' to downshift (so as not to lug engine). The chart that shows rpm you need to downshift at, IIRC says downshift to 5th if speed drops belpw 53 mph.

That seems to have been a design decision to allow 6th on hwy in USA. With that decision, you get 18.5 mph per 1k rpm. 4k rpm = 74 mph, etc.

If they had added a 7th gear, or made 6th taller, you couldn't use top gear on all US hwy at legal speed.
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Old 03-21-2018, 09:11 AM
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Great chart!
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Old 03-22-2018, 02:09 AM
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Originally Posted by Car Analogy View Post
The ap2 is geared such that you can drive in 6th at the US national speed limit of 55 mph, without rpm dropping to where you 'need' to downshift (so as not to lug engine). The chart that shows rpm you need to downshift at, IIRC says downshift to 5th if speed drops belpw 53 mph.

That seems to have been a design decision to allow 6th on hwy in USA. With that decision, you get 18.5 mph per 1k rpm. 4k rpm = 74 mph, etc.

If they had added a 7th gear, or made 6th taller, you couldn't use top gear on all US hwy at legal speed.
Well, assuming that is the reason, here is one owner who would prefer a taller 6th gear even if it means I need to drop to 5th at 55. When you drive in a hilly or mountainous area you can't drive in 6th climbing a hill at 55 mph anyway. Not a big deal really, and i am not making modifications to change it. But, it is one very small complaint I have about the s2k.
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Old 03-22-2018, 05:37 AM
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I agree. But I think they were concerned a less aware driver might just drop it top gear and leave it there. But I feel like the vast majority of this cars owner base would not fall into that category, and a taller cruising gear would be appreciated.

You actually can leave it in 6th and go up a hill at a steady 55. They chose the minimum speeds per gear based on ability to still climb a reasonable grade.

Of course, if you want to accelerate, or if its a really steep hill, you need to downshift.
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Old 03-22-2018, 08:59 PM
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Originally Posted by Car Analogy View Post
The ap2 is geared such that you can drive in 6th at the US national speed limit of 55 mph, without rpm dropping to where you 'need' to downshift (so as not to lug engine). The chart that shows rpm you need to downshift at, IIRC says downshift to 5th if speed drops belpw 53 mph.

That seems to have been a design decision to allow 6th on hwy in USA. With that decision, you get 18.5 mph per 1k rpm. 4k rpm = 74 mph, etc.

If they had added a 7th gear, or made 6th taller, you couldn't use top gear on all US hwy at legal speed.
Complete bollocks.

"US national speed limit of 55 mph" What? Did we somehow travel back in time to 1974?

States control speed limits. That is a completely made up reason for why the AP2 is geared the way it is.
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