RPM versus MPH
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RPM versus MPH
An interesting conversation cropped up on https://www.rx8club.com/showthread.p...threadid=25338 and using the following information on the UK Website I went ahead and got the following numbers for our 6-speed and automatic models.
The above figures are based on stated Gear Ratios and information from HERE and Miata.net website that it takes 777.0 turns of the 225/45R18 Tires to travel one mile.
The numbers come close to real-life but I see a few MPH drop beginning at 3-4K due to drag. The numbers drop significantly as the car moves faster and drag builds up.
BASIS FOR CALCULATIONS:
As an example, for third gear on the 6 speed box, the Gear Ratio of 1.65 means that for each FULL revolution of the crank shaft (not rotor) there will be 1.65 times FEWER revolutions within the Gear Box. The 'Final Gear Ratio' then means that there will be 4.444 times FEWER revolutions of the Prop Shaft (and hence of the rear wheels). For 4000 rpm (and taking into account the conversion from minutes to hours), this gives an equation as follows:
Speed = (4000 x 60)/(1.65 x 4.444 x 777.0)
= 42.16 mph
The above figures are based on stated Gear Ratios and information from HERE and Miata.net website that it takes 777.0 turns of the 225/45R18 Tires to travel one mile.
The numbers come close to real-life but I see a few MPH drop beginning at 3-4K due to drag. The numbers drop significantly as the car moves faster and drag builds up.
BASIS FOR CALCULATIONS:
As an example, for third gear on the 6 speed box, the Gear Ratio of 1.65 means that for each FULL revolution of the crank shaft (not rotor) there will be 1.65 times FEWER revolutions within the Gear Box. The 'Final Gear Ratio' then means that there will be 4.444 times FEWER revolutions of the Prop Shaft (and hence of the rear wheels). For 4000 rpm (and taking into account the conversion from minutes to hours), this gives an equation as follows:
Speed = (4000 x 60)/(1.65 x 4.444 x 777.0)
= 42.16 mph
Last edited by brothervoodoo; 10-14-2004 at 05:47 PM.
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Nice Math
Has anyone attempted Max Speed on their RX. I've not had mine over 100 but we don't have a lot of straight/open roads here in S. MD. Thought maybe some of the RX owners out West may have tried it on some of those long straight highways?
Has anyone attempted Max Speed on their RX. I've not had mine over 100 but we don't have a lot of straight/open roads here in S. MD. Thought maybe some of the RX owners out West may have tried it on some of those long straight highways?
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i actually had mine uo 125 screwing around with this guy on the interstate late one night....not sure what the RPM's were...but i am pretty sure i was in 4th gear to keep the RPM's up at the throttle response high
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I have some problems with Brothervoodoo's figures.
First, to dispose of a detail, the auto second gear in Mazda's book, p. 49, is 1.543, which doesn't round to 1.55.
What's important is to take a "stock wheel" number from a Miata site and use if for a particular tire on the RX8. The result is to get figures considerably different than reported in respected sources. To be specirfic, Road & Track says the RX8 turns 3,100 revs at 60 in 6th. Do the math (60 divided by 3.1 thousand) and you get 19.4 mph per 1,000 rev. I have seen a 20 mph figure in other mags, but never more.
I don't put 100% trust in any figure, because there are different versions, but I have a lot more confidence in Road & Track'stechnical ability, applied to the exact car, than in some generic table.
First, to dispose of a detail, the auto second gear in Mazda's book, p. 49, is 1.543, which doesn't round to 1.55.
What's important is to take a "stock wheel" number from a Miata site and use if for a particular tire on the RX8. The result is to get figures considerably different than reported in respected sources. To be specirfic, Road & Track says the RX8 turns 3,100 revs at 60 in 6th. Do the math (60 divided by 3.1 thousand) and you get 19.4 mph per 1,000 rev. I have seen a 20 mph figure in other mags, but never more.
I don't put 100% trust in any figure, because there are different versions, but I have a lot more confidence in Road & Track'stechnical ability, applied to the exact car, than in some generic table.
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I linked and quoted the sources I found, the numbers can be reworked. "URL" me to some better sources and it can be re-computed in Excel in less than 10 seconds.
#9
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Re: RPM versus MPH
Originally posted by brothervoodoo
The numbers come close to real-life but I see a few MPH drop beginning at 3-4K due to drag. The numbers drop significantly as the car moves faster and drag builds up.
The numbers come close to real-life but I see a few MPH drop beginning at 3-4K due to drag. The numbers drop significantly as the car moves faster and drag builds up.
Otherwise, nice writeup.
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Re: Re: RPM versus MPH
Originally posted by Spin9k
Why does drag have any effect on speed in gears? The gears and wheels go 'round no matter what the drag at any particular speed don't they?
Why does drag have any effect on speed in gears? The gears and wheels go 'round no matter what the drag at any particular speed don't they?
If anybody can offer and point me to some better data regarding the gear ratios, or how many revolutions the stock tires make in a mile, let me know. I don't see how using the rotation formula from the miata link is contradictory. If you look at the second half of that page they are doing basic calculations for any tire size which has nothing to do with the miata.
#11
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Re: Re: Re: RPM versus MPH
Originally posted by brothervoodoo
If the back wheels are just "spinning" with no forward movement or if you drove in outerspace where no friction is involved, they would put down the "speed" indicated on the chart. In real life, as you move forward, you have to push the wind out of the way and that resistance slows you down. >>>>If you had a 100mph headwind you would go slower still at any specific RPM range. <<<<<
If the back wheels are just "spinning" with no forward movement or if you drove in outerspace where no friction is involved, they would put down the "speed" indicated on the chart. In real life, as you move forward, you have to push the wind out of the way and that resistance slows you down. >>>>If you had a 100mph headwind you would go slower still at any specific RPM range. <<<<<
IOW Unless the TIRES ****slip**** on the road at speed then the engine RPM is constant (at any particular speed) wheather you are on the moon (no air friction) or on earth (w/air resistance).
I am either totally missing your point (on this one subject) or I don't understand how it is possible to misunderstand this. Sorry but I think you are not correct here - certainly no offense - I just being logical (I believe).
Last edited by Spin9k; 04-11-2004 at 08:28 PM.
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The tyres deform under load (increases with speed -> drag force) so the effective radius becomes smaller than the theorythical at high speeds.
I'm not sure about the scale of this but could account for a few mph around top speed.
I'm not sure about the scale of this but could account for a few mph around top speed.
#15
Bummed, but bring on OU!
Anyone know the coefficient of drag for the 8?
My real world #'s are (4AT):
1st: 45mph
2nd: 80mph
3rd: 120-122mph didn't see the exact #
4th: dunno, don't want to pay that ticket, but i do know that even if 189 mph were possible, it would take a week to get there :D
My real world #'s are (4AT):
1st: 45mph
2nd: 80mph
3rd: 120-122mph didn't see the exact #
4th: dunno, don't want to pay that ticket, but i do know that even if 189 mph were possible, it would take a week to get there :D
#16
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Originally posted by therm8
Anyone know the coefficient of drag for the 8?
My real world #'s are (4AT):
1st: 45mph
2nd: 80mph
3rd: 120-122mph didn't see the exact #
4th: dunno, don't want to pay that ticket, but i do know that even if 189 mph were possible, it would take a week to get there :D
Anyone know the coefficient of drag for the 8?
My real world #'s are (4AT):
1st: 45mph
2nd: 80mph
3rd: 120-122mph didn't see the exact #
4th: dunno, don't want to pay that ticket, but i do know that even if 189 mph were possible, it would take a week to get there :D
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there would be slippage in the torque converter, unless it has a lockup ... i've really read nothing about the automatic on the 8.
so the automatic #'s wouldn't pan out the same way as the manual numbers
so the automatic #'s wouldn't pan out the same way as the manual numbers
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all automatics since the 80's have a lock-up torque converter...
Drag is a force, it effects acceleration. It acts in the direction opposite the direction you travel. It has no impact on RPM vs MPH. If your car goes 120mph in 4th gear at 7,000 rpms, its will always be going at 120mph at 7,000 rpms in the real world or a dragless world as long as your wheels are not spinning. Its a mechanical interlock.
Drag is a force, it effects acceleration. It acts in the direction opposite the direction you travel. It has no impact on RPM vs MPH. If your car goes 120mph in 4th gear at 7,000 rpms, its will always be going at 120mph at 7,000 rpms in the real world or a dragless world as long as your wheels are not spinning. Its a mechanical interlock.
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So then the chart formula must be flawed because I don't see a correlation when in upper revs (say above 4-5K) the numbers just don't match up. They do at lower revs though.
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I have to agree with Spin9K and Pr0ber - RPM vs MPH is the same whether you are driving in hurricane gale force tail winds or not. MPG, max MPH and numerous other abreviations would definitely be effected though.
Did get mine over 105 easily on a recent roadtrip - I think I need some countermeasures though before trying for max speed.
I thought it handled well at the higher speeds - felt pretty comfortable.
Man this is a great car.
Did get mine over 105 easily on a recent roadtrip - I think I need some countermeasures though before trying for max speed.
I thought it handled well at the higher speeds - felt pretty comfortable.
Man this is a great car.
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Re: RPM versus MPH
Originally posted by brothervoodoo
...means that for each FULL revolution of the crank shaft (not rotor) there will be 1.65 times FEWER revolutions within the Gear Box...
...means that for each FULL revolution of the crank shaft (not rotor) there will be 1.65 times FEWER revolutions within the Gear Box...