Convert rpm to mph?
09-12-2002, 11:58 PM
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Convert rpm to mph?
Ok I need help from you math wizzards.
Last night I did my first full throttle run. I was able to pull 6200 rpm in fifth.
Problem is my spedo is unhooked......So how fast was I going?
Last night I did my first full throttle run. I was able to pull 6200 rpm in fifth.
Problem is my spedo is unhooked......So how fast was I going?
09-13-2002, 12:05 AM
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Just check out our man Danno's site! 
<a href="http://members.rennlist.org/951_racerx/" target="_blank">http://members.rennlist.org/951_racerx/</a>
The fifth gear ratio is 0.829:1 and the mph/1000 RPM he's got listed is 26.442. Then you multiply 26.442 by 6200/1000 and you get 163.94 mph!!
Biiiiiiill, watcha thinkin' foo'? At that speed you won't get a ticket...you'll get hauled to Big Bubba's stonewall pitstop with your wrists cuffed to your ankles!
<a href="http://members.rennlist.org/951_racerx/" target="_blank">http://members.rennlist.org/951_racerx/</a>
The fifth gear ratio is 0.829:1 and the mph/1000 RPM he's got listed is 26.442. Then you multiply 26.442 by 6200/1000 and you get 163.94 mph!!
Biiiiiiill, watcha thinkin' foo'? At that speed you won't get a ticket...you'll get hauled to Big Bubba's stonewall pitstop with your wrists cuffed to your ankles!
09-13-2002, 12:49 AM
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What kind/size of tires do you have on that car?
When I took a ride in your car a couple of months back, I remember thinking that your car seemed to accelerate no faster than mine, especially when looking at the speedometer. Later, I realized that your tires probably have a larger circumference than mine, thus resulting in a lower (closer to reality) speedometer reading but also slightly taller gearing.
A while back when I was curious about transmission swaps, I made a Excel spreadsheet which has input fields for max rpm and tire specs. Once this information is entered, there is a separate sheet for all iterations of 944, 951, and 968 gearboxes, including European versions. Each of these sheets allow you to see the following for each gear:
*Gear ratio
*Speed at max rpm
*Rpm after shift to next highest gear
*Rpm range from upshift to redline
*Mph gain from upshift to redline
You should be able to enter 6200 rpm into the spreadsheet, your tire size, and click on the US 951 gearbox and get a number.
Additionally, you can enter tire size with standard tire specs, measured circumference, or even revolutions per miles (as posted in Tire Rack specs). It does not account for any kind of tire stretch, which I am not sure even happens with radial tires. You will note that the calculated tire circumference from the listed tire size can differ from the calculated number listed in tire rack's revs per mile spec by two or more inches (2%+).
<a href="http://boards.rennlist.com/upload/944gearing.xls" target="_blank">944 gearing.xls (Right click and select save as).</a>
When I took a ride in your car a couple of months back, I remember thinking that your car seemed to accelerate no faster than mine, especially when looking at the speedometer. Later, I realized that your tires probably have a larger circumference than mine, thus resulting in a lower (closer to reality) speedometer reading but also slightly taller gearing.
A while back when I was curious about transmission swaps, I made a Excel spreadsheet which has input fields for max rpm and tire specs. Once this information is entered, there is a separate sheet for all iterations of 944, 951, and 968 gearboxes, including European versions. Each of these sheets allow you to see the following for each gear:
*Gear ratio
*Speed at max rpm
*Rpm after shift to next highest gear
*Rpm range from upshift to redline
*Mph gain from upshift to redline
You should be able to enter 6200 rpm into the spreadsheet, your tire size, and click on the US 951 gearbox and get a number.
Additionally, you can enter tire size with standard tire specs, measured circumference, or even revolutions per miles (as posted in Tire Rack specs). It does not account for any kind of tire stretch, which I am not sure even happens with radial tires. You will note that the calculated tire circumference from the listed tire size can differ from the calculated number listed in tire rack's revs per mile spec by two or more inches (2%+).
<a href="http://boards.rennlist.com/upload/944gearing.xls" target="_blank">944 gearing.xls (Right click and select save as).</a>
09-13-2002, 03:49 AM
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Here's a formula I always use- it's not PERFECT, but is pretty damn close...
RD= rolling diameter
FD= final drive
OR= overall ratio
constant= 2.96
/ = divided by...
RPM X RD X 2.96 / OR / 1000
SO, you have 3.38(FD), .83(5th), 24.86(RD)
3.38 X .83 = 2.804
6200 X 24.86 X 2.96 = 456230.72
456230.72 / 2.804 = 162707.1041 / 1000 = 162.7...
By this formula, the car will redline at 167.96
RD= rolling diameter
FD= final drive
OR= overall ratio
constant= 2.96
/ = divided by...
RPM X RD X 2.96 / OR / 1000
SO, you have 3.38(FD), .83(5th), 24.86(RD)
3.38 X .83 = 2.804
6200 X 24.86 X 2.96 = 456230.72
456230.72 / 2.804 = 162707.1041 / 1000 = 162.7...
By this formula, the car will redline at 167.96
09-14-2002, 04:11 AM
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[quote]Originally posted by Mark-87-951: So what does it mean if the car is at 6600 rpms and the speedo is reading only 150 while in 5th gear?<hr></blockquote>
It would mean- there's something wrong SOMEWHERE, and your's would be ONE IN A MILLION!
It would mean- there's something wrong SOMEWHERE, and your's would be ONE IN A MILLION!
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09-14-2002, 04:15 AM
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Serioulsy though, if the tach's correct, then the speedo's showing WAY low, BUT, you SHOULD have a rev limiter kicking in by 6500, unless you've removed it. Otherwise, the speedo's correct, but the tach's off... Do you ever go by any of those radar signs that show you your speed? If so, get going really fast (watch your *** though) and run by to see how close the speedo is- if you're over 100, it will start over at 0- at least that's the way they do around here. In otherwords, if you're running 110, it will say 10... This can give you an idea as to how accurate the speedo is- I say to run at least a little fast, as the speedo is usually off by more at the higher speeds...
09-14-2002, 04:16 AM
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[quote]Originally posted by Mark-87-951:
<strong>So what does it mean if the car is at 6600 rpms and the speedo is reading only 150 while in 5th gear?</strong><hr></blockquote>
This would make sense if you had a 924S, 944 w/o tall 5th gear, 944S, or a 951 gearbox with a S2/ROW ring and pinion?
In my car at 6,600 rpm in fifth gear, my speedometer would probably be somewhere around 190 mph (it's an '86 so it only has marks to 170 mph).
Otherwise, you are probably in 4th gear (150 mph is about what my speedo reads at that rpm in 4th gear). Since the mph sender reads off the differential somewhere, the gear ratio, final drive, and rpm have everything to do with what the speedometer says. It really doesn't make a difference if your tires are 25" in diameter or 50" in diameter, with a specific gear ratio and rpm, the speedometer will read the same. That is not to say that the speedometer is calibrated with any significant amount of accuracy when using standard sized (225/50zr16) tires in the rear.
If you happen to have a ROW 951 box, especially if it has that nasty on-off lsd thing, I recommend you get rid of it. In fact, for just the cost of shipping, I will trade you gearboxes. I have a very nice US 951 gearbox with oil cooler and none of that nasty scary limited slip differential stuff.
<strong>So what does it mean if the car is at 6600 rpms and the speedo is reading only 150 while in 5th gear?</strong><hr></blockquote>
This would make sense if you had a 924S, 944 w/o tall 5th gear, 944S, or a 951 gearbox with a S2/ROW ring and pinion?
In my car at 6,600 rpm in fifth gear, my speedometer would probably be somewhere around 190 mph (it's an '86 so it only has marks to 170 mph).
Otherwise, you are probably in 4th gear (150 mph is about what my speedo reads at that rpm in 4th gear). Since the mph sender reads off the differential somewhere, the gear ratio, final drive, and rpm have everything to do with what the speedometer says. It really doesn't make a difference if your tires are 25" in diameter or 50" in diameter, with a specific gear ratio and rpm, the speedometer will read the same. That is not to say that the speedometer is calibrated with any significant amount of accuracy when using standard sized (225/50zr16) tires in the rear.
If you happen to have a ROW 951 box, especially if it has that nasty on-off lsd thing, I recommend you get rid of it. In fact, for just the cost of shipping, I will trade you gearboxes. I have a very nice US 951 gearbox with oil cooler and none of that nasty scary limited slip differential stuff.
09-14-2002, 04:27 AM
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I think Steve nailed it. I would think the Huntley chips would remove the rev limiter. The car must have been in 4th. Boy do I feel stupid now. Of course this was just a hypothetical discussion and someday someone will have to try this on a racing course ;-)
09-14-2002, 04:32 AM
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[quote]Originally posted by Mark-87-951:
<strong>I think Steve nailed it. I would think the Huntley chips would remove the rev limiter. The car must have been in 4th. Boy do I feel stupid now.</strong><hr></blockquote>
Sorry, I edited my post. It is possible that you may have a non standard US 951 gearbox in your car. Or, as I first thought, you could have been in fourth.
[quote]Originally posted by Mark-87-951:
<strong>Of course this was just a hypothetical discussion and someday someone will have to try this on a racing course ;-)</strong><hr></blockquote>
Yeah, of course. That's why I make these spreadsheets. I wouldn't actually want to go that fast. Besides, I can probably figure out on paper that my 951 will accelerate evenly with a E46 M3 until his speed limiter hits at 150 mph.
<strong>I think Steve nailed it. I would think the Huntley chips would remove the rev limiter. The car must have been in 4th. Boy do I feel stupid now.</strong><hr></blockquote>
Sorry, I edited my post. It is possible that you may have a non standard US 951 gearbox in your car. Or, as I first thought, you could have been in fourth.
[quote]Originally posted by Mark-87-951:
<strong>Of course this was just a hypothetical discussion and someday someone will have to try this on a racing course ;-)</strong><hr></blockquote>
Yeah, of course. That's why I make these spreadsheets. I wouldn't actually want to go that fast. Besides, I can probably figure out on paper that my 951 will accelerate evenly with a E46 M3 until his speed limiter hits at 150 mph.
09-14-2002, 04:43 AM
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Well how can I determine which gearbox I have? There is nothing in the option codes about it. I think I recall the dealer service manager telling me the occasional thunk I heard was the LSD locking up but that was some time ago and I cant remember his exact comment.
Eventually Im going to have to replace it since the shifting feels rough, especially from 1st to 2nd and is only going to get worse with time.
Eventually Im going to have to replace it since the shifting feels rough, especially from 1st to 2nd and is only going to get worse with time.
09-14-2002, 05:26 PM
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[quote]Originally posted by Steve Lavigne:
<strong>In my car at 6,600 rpm in fifth gear, my speedometer would probably be somewhere around 190 mph (it's an '86 so it only has marks to 170 mph).</strong><hr></blockquote>
No way, we probably have the same gearboxes and my speedo indicated 170 mph at 6500 RPM in 5th gear. I've heard of a Turbo gearbox with an optional taller 5th gear but I haven't done any research on it. It's supposed to be good for ~182 mph at 6200 RPM, do you have this gearbox with a taller 5th gear?? For comparison, my 968 is turning about 3800 RPM at 100 mph in 5th gear.
<strong>In my car at 6,600 rpm in fifth gear, my speedometer would probably be somewhere around 190 mph (it's an '86 so it only has marks to 170 mph).</strong><hr></blockquote>
No way, we probably have the same gearboxes and my speedo indicated 170 mph at 6500 RPM in 5th gear. I've heard of a Turbo gearbox with an optional taller 5th gear but I haven't done any research on it. It's supposed to be good for ~182 mph at 6200 RPM, do you have this gearbox with a taller 5th gear?? For comparison, my 968 is turning about 3800 RPM at 100 mph in 5th gear.
09-14-2002, 06:49 PM
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WARNING: Long and boring post follows!
[quote]Originally posted by PorscheG96:
<strong>No way, we probably have the same gearboxes and my speedo indicated 170 mph at 6500 RPM in 5th gear. I've heard of a Turbo gearbox with an optional taller 5th gear but I haven't done any research on it. It's supposed to be good for ~182 mph at 6200 RPM, do you have this gearbox with a taller 5th gear?? For comparison, my 968 is turning about 3800 RPM at 100 mph in 5th gear.</strong><hr></blockquote>
Trevor, you need to read a little closer. I never said the car was actually going that fast, only that the gauge said that.
On another note, your 968 gets its speed information from the abs computer, which gets its information from the wheelspeed sensors. That is one of the problems when trying to put a 968 gearbox in a 951 which is designed to use a speed sender on the gearbox.
Using my nifty little spreadsheet that no one seems to care about, I get 167.9 at 6,600 rpm with my AVS-i 245/45-16 and the Tirerack revs per mile number of 843. If I calculate the size of the tires using 245/45-16, I get 173.2 mph.
I've never heard of a factory option for a taller 5th gear on a 951. If the 5th gear from a '87 944 gearbox would fit, it would go 196.7 at 6,600 rpm with 245/45-16 tires. You'd be at 4660 rpm once you shifted out of 4th at 6,600 rpm, rather than 5291 with the standard 6th gear.
Using a friend, a stopwatch, and mile markers months ago, we estimated the speedometer error to be about 1 mph optimistic per 10 mph. Thus, 88 mph equals 80 mph actual and 170 mph equals 153 actual. My estimate of 190 would be about 171 mph, which seems pretty close to my calculations.
Another note which is significant is the accuracy of the tachometer. Anyone out there ever really checked it with another device? I know I have seen more than 7,000 boucing off my chips 7,000 rpm rev limiter from wheelspin in first gear.
[quote]Originally posted by PorscheG96:
<strong>No way, we probably have the same gearboxes and my speedo indicated 170 mph at 6500 RPM in 5th gear. I've heard of a Turbo gearbox with an optional taller 5th gear but I haven't done any research on it. It's supposed to be good for ~182 mph at 6200 RPM, do you have this gearbox with a taller 5th gear?? For comparison, my 968 is turning about 3800 RPM at 100 mph in 5th gear.</strong><hr></blockquote>
Trevor, you need to read a little closer. I never said the car was actually going that fast, only that the gauge said that.
On another note, your 968 gets its speed information from the abs computer, which gets its information from the wheelspeed sensors. That is one of the problems when trying to put a 968 gearbox in a 951 which is designed to use a speed sender on the gearbox.
Using my nifty little spreadsheet that no one seems to care about, I get 167.9 at 6,600 rpm with my AVS-i 245/45-16 and the Tirerack revs per mile number of 843. If I calculate the size of the tires using 245/45-16, I get 173.2 mph.
I've never heard of a factory option for a taller 5th gear on a 951. If the 5th gear from a '87 944 gearbox would fit, it would go 196.7 at 6,600 rpm with 245/45-16 tires. You'd be at 4660 rpm once you shifted out of 4th at 6,600 rpm, rather than 5291 with the standard 6th gear.
Using a friend, a stopwatch, and mile markers months ago, we estimated the speedometer error to be about 1 mph optimistic per 10 mph. Thus, 88 mph equals 80 mph actual and 170 mph equals 153 actual. My estimate of 190 would be about 171 mph, which seems pretty close to my calculations.
Another note which is significant is the accuracy of the tachometer. Anyone out there ever really checked it with another device? I know I have seen more than 7,000 boucing off my chips 7,000 rpm rev limiter from wheelspin in first gear.