MGExp

T-Series & Prewar Forum

RPM verses Final Drive Ratio

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

RPM verses Final Drive Ratio
#1
  This topic is about my 1955 MG TF 1500
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
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.

Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
crankjournal Avatar
crankjournal Platinum AdvertiserAdvertiser Len Fanelli
Yonkers, NY, USA   USA
Sign in to contact
1953 MG TD
18 MPH / 1000 RPM
3000 = 54 MPH


Member Services:
MG XPAG Crane camshafts, roller lifter & flat lifter CSI ignition distributors, Roller rocker arms Swirl polished MG T valves, Positive stop Teflon valve stem seals. Safety Faster! Len Fanelli Abingdon Performance Ltd. laf48@aol.com 914 420 8699
Was this post helpful or interesting?
Yes No Thank
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
Thanks again Len. The 18mph per 1000rpm works perfectly for checking the range on the speedo. thumbs up PJ

Was this post helpful or interesting?
Yes No Thank
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
Well, Found out my speedo indicates 11 mph slow with the 4.1 rear. thumbs down PJ

Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
crankjournal Avatar
crankjournal Platinum AdvertiserAdvertiser Len Fanelli
Yonkers, NY, USA   USA
Sign in to contact
1953 MG TD
Paul, you can pull the needle off & reset 11 MPH faster.


Member Services:
MG XPAG Crane camshafts, roller lifter & flat lifter CSI ignition distributors, Roller rocker arms Swirl polished MG T valves, Positive stop Teflon valve stem seals. Safety Faster! Len Fanelli Abingdon Performance Ltd. laf48@aol.com 914 420 8699
Was this post helpful or interesting?
Yes No Thank
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
Thanks Len, Never gave that a thought. eye rolling smiley PJ

Was this post helpful or interesting?
Yes No Thank
crankjournal Avatar
crankjournal Platinum AdvertiserAdvertiser Len Fanelli
Yonkers, NY, USA   USA
Sign in to contact
1953 MG TD
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 CSI ignition distributors, Roller rocker arms Swirl polished MG T valves, Positive stop Teflon valve stem seals. Safety Faster! Len Fanelli Abingdon Performance Ltd. laf48@aol.com 914 420 8699
Was this post helpful or interesting?
Yes No Thank
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
Thanks Len, I figured that as I had it apart one time before to put a couple drops of clock oil in it and clean the glass and face. PJ

Was this post helpful or interesting?
Yes No Thank
Robert H Gold Member Robert Harvey
Wichita, KS, USA   USA
Sign in to contact
1951 MG TD "Millicent Grace (Millie)"
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.

Was this post helpful or interesting?
Yes No Thank
mgyowner Roland Taras
Hemet, CA, USA   USA
Sign in to contact
Hello Paul. I've attached the page from Carl's book. Take a look.

Butch


Attachments:
road speed vs ratio001 smr.JPG    40.7 KB
road speed vs ratio001 smr.JPG

Was this post helpful or interesting?
Yes No Thank
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
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

Was this post helpful or interesting?
Yes No Thank
Valgood Avatar
Valgood Enberg R (Disabled)
Disabled Account, Antarctica   ATA
Sign in to contact
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/

Was this post helpful or interesting?
Yes No Thank
Robert H Gold Member Robert Harvey
Wichita, KS, USA   USA
Sign in to contact
1951 MG TD "Millicent Grace (Millie)"
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.

Was this post helpful or interesting?
Yes No Thank
Paul J Avatar
Locust Grove, OK, USA   USA
Sign in to contact
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

Was this post helpful or interesting?
Yes No Thank
Declan Burns Avatar
Duesseldorf, NRW, Germany   DEU
Sign in to contact
Paul,
I make corrector gearboxes that sit on the back of the speedo. No need for any re-calibrating speedos.
Regards
Declan


Attachments:
002_Speedo_correction_gearboxes.pdf    399.5 KB

Was this post helpful or interesting?
Yes No Thank

To reply or ask your own question:

or

Registration is FREE and takes less than a minute

Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster





Join The Club
Sign in to ask questions, share photos, and access all website features
Your Cars
1976 MG MGB GT
Text Size
Larger Smaller
Reset Save