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

[Bluebottle]

My new speedo has now arrived and was fitted this morning, so I thought I would post a follow-up.
Because it was not only malfunctioning (needle swinging wildly all over the place), but also the wrong TPM for my 2.88 axle, and a little bit tatty around the edges, I opted to get a new one rather than having mine repaired.
Having processed all of the comments and information kindly provided here and on the Jag Lovers forum, and statistics from the workshop manual, I concluded that the correct TPM for a 2.88 axle / 185 section tyres combination, might be as low as 1060.
The lowest TPM available from Caerbont (who remake the original Smiths instruments) is 1070, which would give an error of less than 1%, which is neither here nor there, so I went for one of those.
They state 6 weeks lead time from order, but mine came in 4 weeks.
My preliminary observations suggest that the new speedo is pretty accurate. However, this is from the short drive home from my mechanic’s workshop. Also, I have been using a Cycplus GPS speedo for bicycles while I had no working speedo, so I used this for the comparison - I think it is pretty accurate, notwithstanding its modest cost, but it is designed for cycling speeds and does not always respond as quickly as it might at car speeds, so I shall need to make further readings with another GPS instrument, on a stretch of road where I can drive at constant speed, before I am sure.
The new speedo is pretty true to the original. Visually it is identical bar two small differences. First, the digits in the trip window are different from those in the total mileage window (and, therefore, from the original). Second, the lettering below the warning lights are slightly larger / thicker than the originals.
Internally, there is an issue with the lighting. The original has opaque filters masking the bulbs, so that they are not too bright. The new one does not have these (apparently they are unable to reproduce them). Instead, they supply bulbs with opaque glass, i.e. effectively the filters are moved from the instrument to the bulbs. All very well unless, like me, you have fitted LED bulbs (green ones in my case) in your instruments – you then end up with either a speedo that is twice as bright as the rev counter, or a different colour, depending on whether you stick with the LED bulbs, or use the ones that came with the speedo. I shall have to look into how to address this.
Once again, thanks to all who contributed.

[mgcjag]

Thanks for the update....this small cheap gps speedo works very well and accurate when checked out in daily driver...Steve https://www.ebay.co.uk/itm/374991756451 ... media=COPY

[Philk]

I found the GPS Odometer App hugely useful for double checking the calibration of my speedo over about 20 miles of distance. The App can display real-time speed as well as distance covered.

[angelw]

Phil Wrote:

The App can display real-time speed as well as distance covered.

The distance traveled would only apply if the car had the correct diff ratio for the speedo being used,and calibrating the speedo to eliminate a small indicated speed error was the exercise. Calibrating the indicated speed by the speedo doesn't correct the odometer function if the diff ratio is different to the ratio of the diff that the speedo TPM was supposed to match.

Regards,

Bill

[Philk]

Phil Wrote:

The App can display real-time speed as well as distance covered.

The distance traveled would only apply if the car had the correct diff ratio for the speedo being used,and calibrating the speedo to eliminate a small indicated speed error was the exercise. Calibrating the indicated speed by the speedo doesn't correct the odometer function if the diff ratio is different to the ratio of the diff that the speedo TPM was supposed to match.

Regards,

Bill

Our speedometer was reading fast following it being refurbished a couple of years ago. The repair company requested we did a GPS distance check over 20 miles versus what the odometer was reading. We were pleased to see that the odometer and GPS distance covered were identical which showed the gearing within the speedometer from the gearbox cable drive was correct. That simply then meant an adjustment was required to the spinning magnet strength which drives the needle on the dial

[angelw]

We were pleased to see that the odometer and GPS distance covered were identical which showed the gearing within the speedometer from the gearbox cable drive was correct.

If a different ratio differential is being swapped in and the Speedo retained that has the Turns Per Mile to match the diff ratio that was replaced, the odometer wont be correct unless the odometer gearing in the Speedo is changed, or the ratio of the right angle drive out of the gearbox is changed.