graham cooling Posted February 21, 2005 Report Share Posted February 21, 2005 Help please! have a 1981tr7 v8 convertible and speedo seems to read low, Tyre size is 205/55/15 and rear axle ratio3.08 standard 5 speed box and original speedo. Any ideas wrong speedo drive? Thanks Graham Quote Link to post Share on other sites
tr7jim Posted February 21, 2005 Report Share Posted February 21, 2005 Graham, This is due to the tyre size fitted, which as a greater circumference on your tyre from originals fitted. This causes your speedo to be low in your case. This is not a big problem to solve and suggest you talk to S + S preparation on the correct colour speedo drive you need to change on your gearbox. I believe this is not a big job to do. Good luck Jim Johnson:) Quote Link to post Share on other sites
mikehardwick Posted February 22, 2005 Report Share Posted February 22, 2005 Graham Who did the V8 conversion? Perhaps the change to the 3.03:1 conversion was not accompanied by the appropriate speedo drive, which would result in a fairly large speedo error. Though Jim is right about the tyre size (should be 205/50 on a 15" rim to match gearing of 185/70x13 original) I would not have though a 5% difference in aspect ratio would make a significant difference to the speedo. Mike Quote Link to post Share on other sites
graham cooling Posted February 22, 2005 Author Report Share Posted February 22, 2005 Jim/Mike, Thanks for your replies Jim, took your advice phoned s&s parts ordered now looking forward to a weekend on my back!! Mike,havent worked out the real differences between the two circumfirences but i supposes the ratio change adds to the error as well Again thanks for your advice what a freindly place this is Graham Quote Link to post Share on other sites
mikehardwick Posted February 22, 2005 Report Share Posted February 22, 2005 Graham Further to my last reply, I've done a little digging on the web with the following results - 185/70x13 at 60mph speedo reads 60mph 205/55x15 at 60mph speedo reads 58mph a mere 2.9% error - I'm sure your problem rests with the speedo drive, you wouldn't notice a 2 mph discrepancy - even if the speedo on our cars was that accurate! Good luck with the problem - let us know the outcome. Mike Quote Link to post Share on other sites
graham cooling Posted February 22, 2005 Author Report Share Posted February 22, 2005 A quick bit of Maths says that the speedo is 23% off!. Try this...... Tyre size> 205/55-15 Diameter = (15*25.4)+(205*.55*2) = 606.5 Circumference = 1905.376 Diff ratio> 3.08:1 mm/rev = 1905.376/3.08 = 618.629 (A) ---------------------------------------------- Tyre size> 185/70-13 Diameter = 589.2 Circumference = 1851.026 Diff ratio> 3.9:1 mm/rev = 474.622 ( ---------------------------------------------- Percentage difference = (A-B)/A x 100 = 23.278% I think that's right anyway. Sound right to you guys? Apparently, S&S have said I (He!... confusing....) need a red speedo drive. Mike (Son of Graham....!) Quote Link to post Share on other sites
mikehardwick Posted February 22, 2005 Report Share Posted February 22, 2005 Mike (son of Graham) - ouch! My head hurts. The circumference is spot on for the 185 - 1851mm/72.9ins but for the 205 it should be - 1905mm/75ins. I'm no maths genius - check out www.powerdog.com, and click on tire calculator! Mike Quote Link to post Share on other sites
Peter Vernon Posted February 26, 2005 Report Share Posted February 26, 2005 This is a car which I converted. Moss provided the "correct" speedo drive. After a conversation with Graham yesterday and some research this morning, S&S are correct, the spedo drive should be red. Looks like another supply/advise error from Moss has caused this problem. I am sure it was not the first and I am sure it is not the last that will happen! Sorry Graham hope it will not be too much work, glad the car is on song though! Quote Link to post Share on other sites
graham cooling Posted March 1, 2005 Author Report Share Posted March 1, 2005 We tracked the problem down to a different worn gear drive off of the main shaft. It seems there are two types in the same gearbox. Typical BL!. Although I can assume that a 22teeth gear would solve the problem with "the other" worm gear, it wont fix it with the drive fitted. So short of taking down the gearbox, the solution we did was to....... Take a run out in the car with my "other car's" GPS system, and map out where the errors are. Then scan in the existing dials, and recalibrate!. Saves both pulling the G-box apart, and having the speedo recalibrated. OK, it means the odometer won't be 100% accurate, but at least no speeding tickets!. It also seems that, as Peter says, the car had the correct pinion, but 'cause we're cheating anyway, we won't change it!. Had enough of getting my hands dirty lately anyhow! Oh, and if anyone wants the scans....... Thanks for the help all. Mike (Son of Graham) Quote Link to post Share on other sites
graham cooling Posted March 2, 2005 Author Report Share Posted March 2, 2005 After all the questions raised by our speedo error caused by changing diff to 3.08:1 ive come to the following conclusions. Following a V8 conversion people either use the existing TR7 5 speed box or a SD1 Rover box The difference between the 2 boxes is that the speedo worm gear is different (TKC1273 for TR7 & TKC1274 for SD1) When a SD1 box is fitted a change to 219003 red speedo drive gear will give correct speedo reading With a TR7 5 speed box the change from 3.9 to 3.08 diff will give a 23% error, fitting a 219003 red gear will still give a error of 20% The closest speedo drive gear that can be fitted is a 219001 20 tooth from a auto although this will still give a 12% error this could be nullified by fitting tyre size 205/35/15 if such a thing existed and you could afford it Simplest way around all off this is to leave boxes alone and get speedo re-calibrated by Speedogragh Richfield Is there anyone out there that agrees with the above? As my son Mike said we found it easier to re-print speedo dials which let us add some graphics as well and 150mph max! Has anyone else come up with any different solutions? Have i become a anorak on this subject (DONT answer that son) Quote Link to post Share on other sites
malcolm Posted March 6, 2005 Report Share Posted March 6, 2005 I figured S&S would have this one sorted out by now, having been doing TR7V8 conversions for many years. This was Steve's reply (thanks Steve). Malcolm 219001 20 teeth = orange 219002 21 teeth = green 219003 22 teeth = red 219004 23 teeth = black 219005 24 teeth =blue Graham is right about the fact that there is indeed 2 different drive gears for inside the gearbox. It is almost impossible to get the speedo working accurately by just swapping the external drive gear.There are to many factors involved to achieve this. If you change to a 3.08:1 ratio and fit the red speedo drive to get an accurate reading you must change the internal drive,this for most people is not worth the effort.You must run 13 inch wheels with 70 profile tyres. If you change to a 3.08:1 ratio and use a SD1 box and speedo drives it still won't read right because the SD1 had 14 inch wheels with 70 profile tyres.You are not going to replicate this onto a TR7. There are so many factors involved in this,final drive ratio,wheel & tyre combo's & speedo drives. The only way to get a completly accurate speedo is,as Graham suggests,to leave everything alone and have the speedo recalibrated.The only drawback with this,is it takes around 4/6 weeks to have them done.If you can live with a discrepancy then just change the speedo drive to the one that is closest. Hope this helps Steve Quote Link to post Share on other sites
stevew2 Posted March 6, 2005 Report Share Posted March 6, 2005 I knew I could rely on Malcolm to cross post that for me . Steve Quote Link to post Share on other sites
martynharrison Posted March 28, 2005 Report Share Posted March 28, 2005 Just to add a comment on the DIY calibrating, the GPS technique gives an accurate speed figure only when on the level and at a constant speed. It'll read less when accelerating and going up/down hills, around sweeping bends, etc. It'll normally be very accurate so long as you are going straight and level, though, much more so than the speedos which tend to read about 10% - 15% high even when they are set up with the right bits and pieces. Quote Link to post Share on other sites
Guest jeffremj Posted April 28, 2005 Report Share Posted April 28, 2005 Has anyone else come up with any different solutions? Yes, I use the TR7 gearbox and inner speedo drive and the 20 toothed outer and I only under-read by about 1%. The maths is simple: 3.08 divided by 3.9 equals the teeth you need divided by 24. This gives 19! As 20 is the lowest available, you will only be 5% out with 185/70x13 tyres. Fit 205/50x15 and this drops to around 4% under-read (4mph at 100mph!). BTW, I had my speedo checked at a new rolling road a month or so ago. Note that you may find a 23 tooth in your original TR7, this is because BL set up the speedo to over-read by 5% - the 24 tooth is the most accurate for the TR7 on a 3.9 axle! HTHs. Quote Link to post Share on other sites
graham cooling Posted April 30, 2005 Author Report Share Posted April 30, 2005 jeff, Yes, probably the easiest way if your gearbox is easily accessible (as in not attached to the car!). I think you'll find its still an over-read though (i.e. your doing 104 when it says 100). I for one don't fancy those cameras getting my nice clean licence..... HOWEVER, it must be made very clear to suppliers which gearbox you have. S&S correctly pointed us to a 22 tooth gear, only for us to find our gearbox type assumption was wrong. Also now, with my new dials, I'm 0% out! Quote Link to post Share on other sites
Guest jeffremj Posted May 1, 2005 Report Share Posted May 1, 2005 Graham, You say: > I think you'll find its still an over-read though (i.e. your doing 104 when it says 100). This is the 'usual case' - don't forget it is only under-reading by 2 mph at 50mph. > I for one don't fancy those cameras getting my nice clean licence..... I don't think you have a leg to stand on at 100mph, unless you are a Police Officer on the way to a meeting - or you are ferrying a Home Secretary. BTW, my speedo was only under-reading by 0.5mph at 70 mph - checked on a high-tech rolling road. IMO, if you are strapped for cash, fit a 20 tooth on a standard TR7 5-speed and then use 195/50x15 tyres - this will close the gap by another 2% or so. You really don't want to be opening a gearbox unless it is broken. Quote Link to post Share on other sites
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