Speedo TPM for 2.88 diff.?

[Bluebottle]

Hi,
Can anyone please tell me the correct TPM firgure for a 1963 3.8 with standard Moss box and a 2.88 diff. on 185-15 tyres, please?
Many thanks,
John

[DWW]

Possibly some answers here. Unless you looking at an electronic GPS speedo.

viewtopic.php?f=3&t=16459#p135182

[bitsobrits]

Not sure what you mean by TPM? If it's tire revs/mile, the formula is below, shown as it would be input into Excel where DIA is the diameter in inches of the tire in question. Alternatively you could use rolling radius x2 for the diameter input, as some tire manufactures prefer to supply that number. Note that tire diameters vary from manufacturer, even if they are of the same nominal size, so you can't just use a generic "185/70-15" tire if you are after accuracy.

Tire revs/mile=(5280/((PI()*(DIA*0.968))/12))

[abowie]

What he's after is the specific speedo number eg 1080 for a 2.88 diff.

I've suggested on Jag Lovers he contact Speedograph Richfields as it may be that there's not actually an E type speedo that is geared correctly.

[Simonpfhc]

For 2.88 the TPM is 1058.

[mgcjag]

The TPM for the speedo refers to the turns per mile of the speedo drive cable....calculated by the rolling radius of the wheel/tyre.....rolling radius is from center of the wheel to ground...obviously tyre make and pressure will affect this.....for a correctly calibrated speedo you would need to roll the car a measured distance for a number of cable turns...the speedo is then calibrated to this info....Steve

[Heuer]

Series 1 3.8 - All fitted with 6.40x15 RS5 tyres

Number; TPM; Diff; Calibration

SN 6322/00 ; 1216; 3.31; mph
SN 6322/01 ; 760; 3.31; kph
SN 6322/02 ; 760; 3.31; kph (Germany/Italy)
SN 6322/03 ;1312; 3.54; mph
SN 6322/04 ; 820; 3.54; kph
SN 6322/05 ; 820; 3.54; kph (Germany/Italy)
SN 6322/06 ;1086; 2.93 ; mph
SN 6322/07 ; 678; 2.93 ; kph
SN 6322/08 ; 678; 2.93 ; kph (Germany/Italy)
SN 6322/09 ;1120; 3.07; mph
SN 6322/10 ; 700; 3.07; kph
SN 6322/11 ; 700; 3.07; kph (Germany/Italy)

Series 1 4.2 - various tyres as specified

Number; TPM; Diff; Calibration;Tyre

SN 6322/00A; 1216; 3.31; mph; 6.40x15 RS5
SN 6322/02A; 760; 3.31; kph; 6.40x15 RS5
SN 6322/03A; xxxx; 3.31; mph; 6.40x15 SP41
SN 6322/05A; xxxx;3.54; kph; 6.40x15 SP41HR
SN 6322/09A; 1120; 3.07; mph; 6.40x15 RS5
SN 6322/11A; 700; 3.07; kph; 6.40x15 RS5
SN 6322/13; xxxx; 3.31; kph; 6.40x15 SP41
SN 6322/14; xxxx; 3.54; mph; 6.40x15 SP41HR
SN 6322/27; xxxx; 3.31; mph; 6.40x15 SP41HR
SN 6322/28; xxxx; 3.31; kph; 6.40x15 SP41HR
SN 6322/29; xxxx; 3.54; mph; 6.40x15
SN 6322/30; xxxx; 3.54; kph; 6.40x15
SN 6322/31; 1020; 2.88; mph; 6.40x15
SN 6322/32; 638; 2.88; kph; 6.40x15
SN 6322/33; xxxx; 3.07; mph; 6.40x15 SP41
SN 6322/34; xxxx; 3.07; kph; 6.40x15 SP41
SN 6322/35; xxxx; 3.31; mph; 185x15 SP41
SN 6322/36; xxxx; 3.31; kph; 185x15 SP41
SN 6322/37; xxxx; 3.54; mph; 185x15 SP41
SN 6322/38; xxxx; 3.54; kph; 185x15 SP41
SN 6322/39; xxxx; 2.88; mph; 185x15 SP41HR
SN 6322/40; xxxx; 2.88; kph; 185x15 SP41HR
SN 6322/41; xxxx; 3.07; mph; 185x15 SP41
SN 6322/42; xxxx; 3.07; kph; 185x15 SP41

If you want to work out the expected speed from different axle ratios, gearboxes and tyre sizes, 5Speeds.com has a neat downloadable calculator: http://www.5speeds.com/rpm.htm which is fun to play with! Another online calculator is available here: http://wahiduddin.net/calc/calc_speed_rpm.htm

If you fancy a DIY approach to speedometer calibration Mike Frank has an excellent write up on his site: http://www.westnet.com/~mfrank/Speedometer/Smiths.html