TurbineHeli wrote:Sorry if this subject has been beaten to death before, but the search feature does not seem to be particularly intuitive...
2. If not, is it possible to have the speedometer instrument re-calibrated?
Thanks,
AMA
Short answer is yes, and might be preferable...
Here is link to a document I found helpful. Might be more detail than you need...
http://home.comcast.net/~rhodes/speedo.pdfI had my speedo done by Nisonger due to a diff & transmission change. They are located in Mamaroneck, NY...
http://www.nisonger.com/index.htmBasically their are two issues to sort out. You sort out the two issues for the odometer reading first, then the speedo reading is checked and calibrated on the bench using a known input rotational speed and the overall TPM.
1. The turns per mile (TPM) of the cable specific to your transmission speedo gear, diff, and tires. You can measure this by pushing the car a known distance or number of tire revolutions & counting the number of speedo cable rotations. Like John mentions above, each repair shop seems to do this a bit differently, but they will all provide you with what they want ahead of time. Some of the different shops I saw want 4 tire rotations, 6 tire rotations, 52.8 feet or 100th of a mile of tire travel, etc., but they all effectively do the same thing which is provide the shop with the actual TPM output at the cable for your present set-up. I opted to just give them the TPM I wanted (see below to determine this).
2. The TPM of the speedo head. For my Plus 2 it is 940 TPM stock; yours may be different. You can see the stock number on the speedo face just below the odometer to the right in small white numbers.
For my MT75 transmission, I opted to leave the gear in the transmission alone (actually I have no idea what it is) and get the speedo head done. I simply calculated the error/difference driving along the Interstate for about 15 or 20 mile posts, comparing to the odometer reading, and then calculating the required TPM from the known 940 TPM on my speedo face. I provided the required TPM number to Nisonger's (1000 TPM in my case) with the speedo head packaging, and they re-calibrated the odometer with an internal gear change and then adjusted the speedometer reading as required for age and wear. Turn around on the job was about six or eight weeks, and cost about $200 IIRC; I think this is their busy time of year.
David has done an excellent spreadsheet for calculating revs per mile etc. for any diff & tire combo. I used this and GPS speed readings to factor in the impact of future tire size changes and to determine that the speedometer and odometer were out of whack with one another, so I knew I needed the speedo head done along with the tachometer. You can use this spreadsheet to see what the impact of the transmission gear change will be ahead of time and see if you will get close to your speedo head TPM with your particular tire, diff & existing/modified transmission speedo gear.
Been a while, but I think the document above provides a pretty complete list of the gear combo's that are readily available for the Smiths speedo's. I made sure I was asking Nisonger for a TPM figure I figured they could easily build up and was within a reasonable percentage error to my custom set-up.
If your tire, diff & transmission speedo gear combo does not work out to a readily available speedo head TPM, they can apparently build up a transmission deal that mounts on the back of the speedo head. I was worried the Lotus would be tight for room for this approach. IIRC the transmission is about 3" long.
Sorry for length; probably more detail than you need. Sounds like you can either try a revised transmission gear or calculate the impact of a new gear and see if you can get close to the TPM requirement on your existing speedo head. Other approach is to leave the gear you have and calibrate the head TPM as required; you can check if this will work for you first with the info above. You may not have to do both, and if you find the speedo & odometer are out of whack you may want to just get it calibrated & serviced (or venture into doing it yourself as per Mr. Rhodes) and leave your existing gear in place.
HTH
PS I just remembered my speedo cable or angle drive broke on my last trip, so I need to fix that up this winter.