working out speed from gear ratio, diff standard 2wd
27-11-2007, 09:54 PM
#1
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working out speed from gear ratio, diff standard 2wd
hi i dont know if anyone can help ive got a formula i use to work out speed from gear ratio, diff ratio and wheel revs.
i work it out like this 8000 rpm x 60 = 480000
wheel revolutions a minute 897, diff ratio 3.64, 5th gear .80.
so 897 x 3.64 x .80 = 2612.064
480000 / 2612.064 = 183.7 at 8k in fifth standard wheels.
but i have it on good authority that with 17s and 40mm tyres actual speed is 195 ish.
so i tried to work out wheel revolution a mile by calculating circumference of larger wheels and it works out at 165 mph
now the only thing i didnt check for myself in the above equation is the wheel revs which i was told is 897 per mile on standard wheels.
does anyone know what im doing wrong
i work it out like this 8000 rpm x 60 = 480000
wheel revolutions a minute 897, diff ratio 3.64, 5th gear .80.
so 897 x 3.64 x .80 = 2612.064
480000 / 2612.064 = 183.7 at 8k in fifth standard wheels.
but i have it on good authority that with 17s and 40mm tyres actual speed is 195 ish.
so i tried to work out wheel revolution a mile by calculating circumference of larger wheels and it works out at 165 mph
now the only thing i didnt check for myself in the above equation is the wheel revs which i was told is 897 per mile on standard wheels.
does anyone know what im doing wrong
27-11-2007, 09:57 PM
#2
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28-11-2007, 07:13 AM
#10
don't forget that this is a purely basic ratio calculation and it doesn't take ito account tyre swelling the faster you go which can account for an extra bit of speed
the maclaren F1 was geared to hit the limiter at 225 mph but their first run out saw it doing 231, so there was some serious head screatching going on there, because you aren't supposed to be making that sort of mistake when you build a car that expensive
the maclaren F1 was geared to hit the limiter at 225 mph but their first run out saw it doing 231, so there was some serious head screatching going on there, because you aren't supposed to be making that sort of mistake when you build a car that expensive
28-11-2007, 08:50 AM
#11
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Originally Posted by dojj
don't forget that this is a purely basic ratio calculation and it doesn't take ito account tyre swelling the faster you go which can account for an extra bit of speed
the maclaren F1 was geared to hit the limiter at 225 mph but their first run out saw it doing 231, so there was some serious head screatching going on there, because you aren't supposed to be making that sort of mistake when you build a car that expensive
the maclaren F1 was geared to hit the limiter at 225 mph but their first run out saw it doing 231, so there was some serious head screatching going on there, because you aren't supposed to be making that sort of mistake when you build a car that expensive
28-11-2007, 09:19 AM
#13
Measure your tyre again in mm's.
Then divide... 1 609 344 millimeters x the number of mm's your tyre measures. (my guess is it'll be around 820) thats your revolutions per mile.
Then do your equation again like you did at the top of this page
ps.. its revolutions per MILE not per MINUTE
Then divide... 1 609 344 millimeters x the number of mm's your tyre measures. (my guess is it'll be around 820) thats your revolutions per mile.
Then do your equation again like you did at the top of this page
ps.. its revolutions per MILE not per MINUTE
28-11-2007, 12:25 PM
#15
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Originally Posted by TiB
Also... if you limiter is set too 7800 i would guess you have a soft cut limiter around 7600 which you will prob only feel in 5th.
28-11-2007, 12:31 PM
#16
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Originally Posted by TiB
Measure your tyre again in mm's.
Then divide... 1 609 344 millimeters x the number of mm's your tyre measures. (my guess is it'll be around 820) thats your revolutions per mile.
Then do your equation again like you did at the top of this page
ps.. its revolutions per MILE not per MINUTE
Then divide... 1 609 344 millimeters x the number of mm's your tyre measures. (my guess is it'll be around 820) thats your revolutions per mile.
Then do your equation again like you did at the top of this page
ps.. its revolutions per MILE not per MINUTE
im confused by the equation above ? 17 in rims and 235 40 tyres. can you show me the equation for working that into revolutions per mile please mate ?
nice 3 door to its
28-11-2007, 12:40 PM
#18
Testing the future
you will never be totally accurate unless you accurately measure the actual diameter or better still circumference of your tyre as it deforms and goes along. using standard tyre measurements will not be accurate as there can be a big difference between new and worn tyres, tyre pressure etc
28-11-2007, 01:02 PM
#19
As a rough guestimate (going by a fixed* tyre circumference of 1947mm for a 235/40x17):
7800rpm = 194.45mph on 0.80 fifth gear and 3.64 final drive.
7700rpm = 191.96
7600rpm = 189.46
7500rpm = 186.97
7400rpm = 184.48
7300rpm = 181.98
7200rpm = 179.49
7100rpm = 177.00
7000rpm = 174.51
*due to centrifugal force, the diameter actually grows slightly, so you may achieve fractionally higher speeds than quoted, or as Nick points out, the above circumference is based on a new tyre and an almost fully worn item could be up to 1.5% less speed.
7800rpm = 194.45mph on 0.80 fifth gear and 3.64 final drive.
7700rpm = 191.96
7600rpm = 189.46
7500rpm = 186.97
7400rpm = 184.48
7300rpm = 181.98
7200rpm = 179.49
7100rpm = 177.00
7000rpm = 174.51
*due to centrifugal force, the diameter actually grows slightly, so you may achieve fractionally higher speeds than quoted, or as Nick points out, the above circumference is based on a new tyre and an almost fully worn item could be up to 1.5% less speed.
28-11-2007, 03:16 PM
#20
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Originally Posted by Mike Rainbird
As a rough guestimate (going by a fixed* tyre circumference of 1947mm for a 235/40x17):
7800rpm = 194.45mph on 0.80 fifth gear and 3.64 final drive.
7700rpm = 191.96
7600rpm = 189.46
7500rpm = 186.97
7400rpm = 184.48
7300rpm = 181.98
7200rpm = 179.49
7100rpm = 177.00
7000rpm = 174.51
*due to centrifugal force, the diameter actually grows slightly, so you may achieve fractionally higher speeds than quoted, or as Nick points out, the above circumference is based on a new tyre and an almost fully worn item could be up to 1.5% less speed.
7800rpm = 194.45mph on 0.80 fifth gear and 3.64 final drive.
7700rpm = 191.96
7600rpm = 189.46
7500rpm = 186.97
7400rpm = 184.48
7300rpm = 181.98
7200rpm = 179.49
7100rpm = 177.00
7000rpm = 174.51
*due to centrifugal force, the diameter actually grows slightly, so you may achieve fractionally higher speeds than quoted, or as Nick points out, the above circumference is based on a new tyre and an almost fully worn item could be up to 1.5% less speed.
28-11-2007, 03:20 PM
#21
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Originally Posted by TiB
Also... if you limiter is set too 7800 i would guess you have a soft cut limiter around 7600 which you will prob only feel in 5th.
maybe i am doing 150 on the limiter in 4th
cant check it with sat nav as id have to hold it on the limiter for a few seconds and i dont wana do that else il be
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