View Full Version : :: Top speed, please correct my math
:: Top speed, please correct my math
Having sparked interest from the RPM limiter thread, I did a quick calc.
Assuming 6800RPM with 235-45/17 tires, final drive of the e34 535i at 3.64, my theoretical top speed would be 140.9 MPH or 226.8 KPH. Clearing this is wrong as I have done 240 per the speedo before. Is my math wrong or stats?
EDIT !!!
Brain fart. Final drive is 0.81. I plugged in the diff instead. Vodka inhibiting rational judgement. So theoretically at 6500 (safe) RPM in 5th, it can reach 166 MPH or about 267 KPH. Ok. Of course the speedo only goes up to 260 ;)
rob101
09-20-2006, 09:03 PM
whats the final gear ratio? as in overdrive or 5th gear?
whats the final gear ratio? as in overdrive or 5th gear?
Oh silly me. That's right.
rob101
09-20-2006, 09:08 PM
USE THE SEARCH lol. kidding mate.
you've done 240? holy crap!
USE THE SEARCH lol. kidding mate.
you've done 240? holy crap!
Yeah I've done 240, but only briefly...
The final drive is 0.81. The vodka kept the diff ratio stuck in my head. ;)
rob101
09-20-2006, 09:21 PM
i haven't even made 100 mph, i came close (155 kph), but yeah still getting used to driving a car that can exceed the speed limit everywhere without feeling like i am going to die.
632 Regal
09-20-2006, 09:30 PM
dude, ever try to smoke vodka? not a great thing but you can use it in a bong, gives a whole new meaning to bong water...you can drink it!
Digita1 Ecstasy
09-20-2006, 09:58 PM
Okay, drinking bong water is gross as hell.
Water or liquor
Bong water smells really bad and usually has burned residues from the bowl you just smoked.
Come on man! Don't ruin the vodka, mix it with some juice instead...
Traian
09-20-2006, 10:12 PM
Just because the speedometer said 240 doesn't mean that you were DOING 240. I read in the newspaper that German cars are notorious for overestimating road speed, and that VW and BMW are the most innacurate. To keep you out of trouble I guess. Rant: People always claim that their fast cars can do more than the limiter (155 mph) but this is not so: only a calibrated testing device will tell you exactly how fast you're going (like car magazines use).
632 Regal
09-20-2006, 10:40 PM
seroiusly try it before defaming it, the vodka keeps the oily residu off the bong, dont spoil and dont smell like ****. try it and then post, ill wait. you can use it to clean used bongs but without heat it wont pull the crus off the sides.
Okay, drinking bong water is gross as hell.
Water or liquor
Bong water smells really bad and usually has burned residues from the bowl you just smoked.
Come on man! Don't ruin the vodka, mix it with some juice instead...
Jimbo
09-20-2006, 11:18 PM
Just because the speedometer said 240 doesn't mean that you were DOING 240. I read in the newspaper that German cars are notorious for overestimating road speed, and that VW and BMW are the most innacurate. To keep you out of trouble I guess. Rant: People always claim that their fast cars can do more than the limiter (155 mph) but this is not so: only a calibrated testing device will tell you exactly how fast you're going (like car magazines use).
I've had my car up to an indicated 143 MPH on two separate occasions. For obvious reasons I was not playing around with my GPS unit during these top speed runs, but at more sane speeds (eg. 75 MPH) I have tested my speedo against the velocity reading from my GPS unit and have found the speedo to be only about 1 MPH higher. This difference would obviously be amplified at higher speeds, but not by very much :D
rob101
09-20-2006, 11:43 PM
I've had my car up to an indicated 143 MPH on two separate occasions. For obvious reasons I was not playing around with my GPS unit during these top speed runs, but at more sane speeds (eg. 75 MPH) I have tested my speedo against the velocity reading from my GPS unit and have found the speedo to be only about 1 MPH higher. This difference would obviously be amplified at higher speeds, but not by very much :D
yeah because after all Civilian GPS for speed calculation has 0% error doesn't it?
no, it doesn't in fact.
angrypancake
09-21-2006, 12:27 AM
seroiusly try it before defaming it, the vodka keeps the oily residu off the bong, dont spoil and dont smell like ****. try it and then post, ill wait. you can use it to clean used bongs but without heat it wont pull the crus off the sides.
psh, for a challenge. take a bong rip, as much as you can, inhale as hard as you can. without exhaling, drink a full beer, then take a shot of any liquor. exhale. it knocks you on your ass.
as for the real root of this thread. how do you figure out your theoretical top speed, and moreso how can you use the GPS to indicate speed?!
BillionPa
09-21-2006, 02:06 AM
actually speed calculations on GPS recievers arent that innacurate
Jimbo
09-21-2006, 02:31 AM
yeah because after all Civilian GPS for speed calculation has 0% error doesn't it?
no, it doesn't in fact.
Whoa, settle down, I never claimed it did, but it's the only way I have of verifying the accuracy of my speedometer. Furthermore, I am not an expert in the field, but the way I understand it, any environmental effects which would cause inaccuracies in the ability of a GPS unit to calculate its absolute location, would not come in to play as much when comparing relative positions over a short period of time (such as when calculating velocity) because the variance causing effects would remain largely unchanged over short periods of time.
I also know by comparing my cruising speed to other cars when traveling with friends that when I am going an indicated 75 MPH, they are traveling about the same indicated speed. So if my GPS is indicating around 74MPH, then it is probably pretty accurate.
rob101
09-21-2006, 02:58 AM
Whoa, settle down, I never claimed it did, but it's the only way I have of verifying the accuracy of my speedometer. Furthermore, I am not an expert in the field, but the way I understand it, any environmental effects which would cause inaccuracies in the ability of a GPS unit to calculate its absolute location, would not come in to play as much when comparing relative positions over a short period of time (such as when calculating velocity) because the variance causing effects would remain largely unchanged over short periods of time.
I also know by comparing my cruising speed to other cars when traveling with friends that when I am going an indicated 75 MPH, they are traveling about the same indicated speed. So if my GPS is indicating around 74MPH, then it is probably pretty accurate.
yeah sorry mate i wasn't having a go at you directly. sorry if it appeared that way.
I know at least in this country speedos are allowed to be +/- 10% by law.
which is funny because in some states you can get booked for doing 3% over the speedlimit.
New requirement for licencing system, being innately able to detect your speed to 1% accuracy.
I also think that the error in the speedo actually increases because of the centrefugal force on the tyre increasing the rolling diameter.
Jimbo
09-21-2006, 03:18 AM
yeah sorry mate i wasn't having a go at you directly. sorry if it appeared that way.
I know at least in this country speedos are allowed to be +/- 10% by law.
which is funny because in some states you can get booked for doing 3% over the speedlimit.
New requirement for licencing system, being innately able to detect your speed to 1% accuracy.
I also think that the error in the speedo actually increases because of the centrefugal force on the tyre increasing the rolling diameter.
Hey, no problem, I probably shouldn't have taken it personally :)
+/- 10% seems excessive, I guess I don't know exactly how car speedos measure the speed, but I would imagine they just measure the speed of one of the axle shafts and have a built in multiplyer for the circumfrence of the stock tire diameter. I would think it would be easy to get a pretty accurate reading (the increased circumfrence caused by centrefugal force you mention being the only real variable, assuming tire size is stock) and I would hope auto makers could do better than +/- 10%.
BMW 5 SERIES
09-21-2006, 03:37 AM
http://www.bimmer.info/forum/vbpgimage.php?do=full&p=130
As testimony to Traian's claim I personally drove an E32 735i to an indicated 155 mph, I don't think so.
califblue
09-21-2006, 11:14 AM
with what BillionPA said "actually speed calculations on GPS recievers arent that innacurate"
I saw 156 MPH on my Mark D' Chipped 1992 535im and if you want more proofe sit in the back seat next time and take a photo:D
Just because the speedometer said 240 doesn't mean that you were DOING 240. I read in the newspaper that German cars are notorious for overestimating road speed, and that VW and BMW are the most innacurate. To keep you out of trouble I guess. Rant: People always claim that their fast cars can do more than the limiter (155 mph) but this is not so: only a calibrated testing device will tell you exactly how fast you're going (like car magazines use).
Actually, according to my Garmin streetpilot and etrek legend, the speedo is actually understating the speed ;)
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