T-Series & Prewar Forum
RPM verses Final Drive Ratio
Posted by Paul J
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Topic Creator (OP)
Apr 30, 2016 07:45 AM
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Checking my speedo against the tach.
At 4000 rpm, what speed should be expected with a 4.1 final ratio? What speed at 3000 rpms? 165/15 tires. PJ
Just was told 4000 rpms gives 72 mph, would also like to know what 3000 gives.
Edited 1 time(s). Last edit at 2016-04-30 08:04 AM by Paul J.
At 4000 rpm, what speed should be expected with a 4.1 final ratio? What speed at 3000 rpms? 165/15 tires. PJ
Just was told 4000 rpms gives 72 mph, would also like to know what 3000 gives.
Edited 1 time(s). Last edit at 2016-04-30 08:04 AM by Paul J.
crankjournal
Len Fanelli
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Apr 30, 2016 08:56 AM
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18 MPH / 1000 RPM
3000 = 54 MPH
3000 = 54 MPH
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Topic Creator (OP)
Apr 30, 2016 09:04 AM
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Topic Creator (OP)
May 5, 2016 08:57 AM
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crankjournal
Len Fanelli
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May 5, 2016 10:24 AM
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Paul, you can pull the needle off & reset 11 MPH faster.
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MG XPAG Crane camshafts, roller lifter & flat lifter
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Len Fanelli Abingdon Performance Ltd. laf48@aol.com 914 420 8699
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Topic Creator (OP)
May 5, 2016 05:26 PM
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crankjournal
Len Fanelli
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May 6, 2016 06:50 AM
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Paul, you will need to disassemble the speedo, hold the drum that moves the needle, gently pull of the needle, & reposition as needed.
Member Services:
MG XPAG Crane camshafts, roller lifter & flat lifter
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Len Fanelli Abingdon Performance Ltd. laf48@aol.com 914 420 8699
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Topic Creator (OP)
May 6, 2016 08:14 AM
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May 9, 2016 07:34 PM
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If I remember correctly, the speedo drives off of the transmission. So if the speedo gear in the trans was originally for a different-than-4.1 differential, it will read incorrectly. The speedo really measures transmission output shaft speed, not car speed.
mgyowner
Roland Taras
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May 9, 2016 10:00 PM
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Topic Creator (OP)
May 10, 2016 06:16 AM
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Thanks fellas, I'm still wondering if the difference between the original 550/15 tire size and the 165/15s on the car now would make that much difference in the speedometer readings. The speedometer is set for a 4.8 rear and 11 MPH slow with the 4.1, not sure how much the tire size affects this. PJ
May 10, 2016 07:08 PM
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If you can measure yourself or have internet calculate from tire sizes, then input information to a conversion program such as for this link, one can determine what changes different diameter tires give to the equation. As you know the different rear ratio's.
http://www.advanced-ev.com/Calculators/TireSize/
http://www.advanced-ev.com/Calculators/TireSize/
May 10, 2016 09:05 PM
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Paul,
I dug up some info on tire sizes a while back. Here is what I found--I don't remember the sources, but I think these numbers are close.
Coker 5.60-15 Outside Dia = 25.88"
Firestone 5.60-15 = 25.79
Firestone 165R15 = 25.43 (this is their "F560" series)
tire rack 165/80 15 = 25.40
The circumference of the tire varies linearly with the dia (C = 3.14 X D) so speed will vary linearly with tire dia (at a given RPM). The extreme spread from the 25.4 to 25.88 dias of the tires above represents a 1.9% error, with the smaller dia tire making the speedo read faster than you are really going. The differential gear, if the speedo gears are set up for the 4.875 and you have a 4.1, will introduce a 16% error, and will make the speedo read slower than you are really going. 16% will result in an error at 60 mph of about 9.5mph, or 11.5 mph error at 72 mph, so I would say this is the source of your error. Depending upon what your tire size is exactly, the +16% combined with the -1.9% could give a error of 11 mph at 72 just about exactly. I took a quick look at Moss, and they show no availability of any speedo gears. I don't know what the available differential gears were for the TF, but if the 4.1 was available, there should be a proper speedo gear out there somewhere. Otherwise you can just go by the tach. If 4000 rpm is 72mph, 1000 = 18, 2000=36, 3000=54, 4000=72, 5000=90.
I dug up some info on tire sizes a while back. Here is what I found--I don't remember the sources, but I think these numbers are close.
Coker 5.60-15 Outside Dia = 25.88"
Firestone 5.60-15 = 25.79
Firestone 165R15 = 25.43 (this is their "F560" series)
tire rack 165/80 15 = 25.40
The circumference of the tire varies linearly with the dia (C = 3.14 X D) so speed will vary linearly with tire dia (at a given RPM). The extreme spread from the 25.4 to 25.88 dias of the tires above represents a 1.9% error, with the smaller dia tire making the speedo read faster than you are really going. The differential gear, if the speedo gears are set up for the 4.875 and you have a 4.1, will introduce a 16% error, and will make the speedo read slower than you are really going. 16% will result in an error at 60 mph of about 9.5mph, or 11.5 mph error at 72 mph, so I would say this is the source of your error. Depending upon what your tire size is exactly, the +16% combined with the -1.9% could give a error of 11 mph at 72 just about exactly. I took a quick look at Moss, and they show no availability of any speedo gears. I don't know what the available differential gears were for the TF, but if the 4.1 was available, there should be a proper speedo gear out there somewhere. Otherwise you can just go by the tach. If 4000 rpm is 72mph, 1000 = 18, 2000=36, 3000=54, 4000=72, 5000=90.
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Topic Creator (OP)
May 11, 2016 06:12 AM
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Thanks Robert, quite a bit of info there. I'm running the 4.1 and 165R/15s (Firestones) and have no issues, other than of course the speedo is off by 11 MPH. I'm using the tach for reference now, 3500 RPMs gives around 63 MPH, speed limit here is 65. Someone has a converter that goes between the transmission and the cable to correct this situation. Looking into it. Thanks again for the info, much appreciated. PJ
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May 11, 2016 10:10 AM
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