Howdy All I have a question on the speedo gear. I have a 1970 Buick GSX (tribute car) 455 Stage 2, TB 400 SP, 3.42 posi. The speedo is showing around 10 MPH faster then I am going, going 40 the speedo is showing 50, it is even more when I'm at highway speed (70/75). I checked this with my Garmin GPS, it shows what speed you are going. My question is does anyone know what gear I should put into the trans to make my speedo read correctly and where can I get it ??? Thanks David
You need to know 3 things. 1, rear tire size (tire height) 2, rear gear ratio 3, the number of teeth on the speedometer DRIVE gear. The drive gear is on the out put shaft of the transmission. If you remove the DRIVEN gear sleeve, you will be able to see the DRIVE gear. By rotating the drive shaft (trans in neutral), you will be able to count the teeth. It is a helical gear, so it isn't easy, but it can be done. Do it several times to make sure you are accurate with the count. It should be 16, 17, or 18 teeth. Once you know that. It's a simple calculation to figure out what DRIVEN gear you need. The gears are still available from GM.
You can calculate gears needed to get close from axle ratio and tire size. But often its necessary to fine tune these with road measurements to be right on. Since you already know (via GPS) almost exactly how much your setup is off, all you need is GPS info and the trans speedometer gears. Do make a very careful record of this. if you are reading 50 at 40, the speedo shaft must be slowed down by 40/50. Count trans speedo gear teeth as was described, and calculate the ratio. If 18 teeth are driving 39 teeth, the ratio is 39/18 or 2.167 . Increase that by 50/40 to about 2.708 . With your 18 tooth drive gear thats 48.75 teeth driven, I don't think they go that high. Reducing the drive to 17 gives 46.04 teeth driven, 16 drive gives 43.33 driven, 15 gives 40.62. My recollection is 400 drive gears cover 15 to 20 teeth, driven 34 to 45. If you can't find a set of gears within those available, you could mount an external gearbox on the trans speedo output. This can can add another multiplying ratio, to shift into a range the trans can reach. I use them to get closer than simply the "nearest" number of teeth. Bruce Roe
Thanks guys. Yes I know it's a gear driven system, I've changed many of them. I was just hoping someone would have an idea of what tooth count I would need, sorry I should have said that, it was 4 AM and I wasn't thinking right ou: . As far as tires the rears are P275-60R-15 and I'm running 3.42 gears. I think I'm just going to have to buy a few and try them out one at a time, I was just hoping to get in the right range so I didn't have to buy a million gears to get the right one. I guess I can call TCI, but, I was hoping someone here would have an idea of what tooth count I will need. TCI has a good page were they have gears http://www.tciauto.com/tc/transmission-accessories/speedometer-gears/general-motors.html , I'm going to poke around there and see if they have a chart were I can put my numbers in to come up with the gear I need. Thanks for the answers so far. David
OK I new I could find it http://www.tciauto.com/tc/speedo-info Know I just have to take the gear cover off so I can find out what the Drive Teeth number is so I can finsh the chart. Here's what I came up with for now: <TABLE border=0 cellSpacing=0 width=407><TBODY><TR><TD height=35 vAlign=bottom width=297># Drive Teeth X Axle Ratio X Tire Rev. per Mile</TD><TD height=35 rowSpan=2 width=102 align=left>= # Driven Teeth</TD></TR><TR><TD height=35 vAlign=top width=297 align=middle>1001 </TD></TR></TBODY></TABLE> Use the following formula to calculate the Tire Rev. per Mile for theabove formula.<TABLE border=0 cellSpacing=0 cellPadding=2 width=330><TBODY><TR><TD height=28 rowSpan=2 width="42%">Tire Rev. per Mile =</TD><TD height=28 vAlign=bottom width="58%">. 20168 .</TD></TR><TR><TD height=28 vAlign=top width="58%">Tire Diameter (inches)</TD></TR></TBODY></TABLE> Try this handy custom calculator. It will do all the work for you. If you don't know your tire diameter, calculate it below using the P-metric sidewall designation.<!-- Speedometer Driven Gear Calculator --><FORM><TABLE border=1 cellSpacing=0 borderColorLight=#7f0a15 borderColorDark=#7f0a15 cellPadding=3 width=595 align=center><TBODY><TR><TD bgColor=#ffff00 width=247 align=left># of Drive Teeth: <INPUT size=5 name=dt> Axle Ratio: <INPUT value=3.42 size=5 name=ar> Tire Diameter: <INPUT value=27.99 size=5 name=td></TD><TD bgColor=#ff0000 vAlign=top width=338 align=middle># of Driven Teeth: <INPUT name=dn> <INPUT onclick=compute(this.form) value=Calculate type=button> <INPUT onclick=clear(this.form) value=Clear type=reset></TD></TR></TBODY></TABLE> </FORM><!-- Tire Height Calculator --><FORM id=tire name=tire><TABLE border=0 bgColor=#ffff00><TBODY><TR><TD align=left><INPUT onclick="clearform(this.form, 0)" value=Clear type=button></TD><TD colSpan=2 align=middle>Tire Diameter Calculator</TD><TD> </TD><TD align=right><INPUT onclick="update(this.form, 0); return false;" value=Calculate type=button></TD></TR><TR><TD align=middle>Tire Width (mm)</TD><TD align=middle>Aspect Ratio (1-100)</TD><TD align=middle>Wheel Diameter (in)</TD><TD> </TD><TD align=middle>Tire Diameter (in)</TD></TR><TR><TD align=middle><INPUT value=60 size=7 name=widthmm0></TD><TD align=middle><INPUT value=275 size=7 name=aspect0></TD><TD align=middle><INPUT value=15 size=7 name=diameter0></TD><TD> </TD><TD align=middle><INPUT value=27.99 size=7 name=height1></TD></TR><TR><TD colSpan=5 align=middle>Enter the tire width in millimeters, the aspect ratio, the wheel diameter in inches, and </TD></TR></TBODY></TABLE></FORM> Here's the gear break down: TH400 Speedometer Gears <TABLE border=1 cellSpacing=0 borderColorLight=#7f0a15 borderColorDark=#7f0a15 cellPadding=3 width=396><TBODY><TR><TD bgColor=#c0c0c0 width="47%" align=middle>Drive Gears </TD><TD bgColor=#c0c0c0 width="53%" align=middle>Driven Gears </TD></TR><TR><TD width="47%">7 teeth - red</TD><TD width="53%">16 teeth - (truck)</TD></TR><TR><TD width="47%">8 teeth - black</TD><TD width="53%">17 teeth - (truck)</TD></TR><TR><TD width="47%">15 teeth - brown or gray*</TD><TD width="53%">18 teeth - (truck) code H</TD></TR><TR><TD width="47%">16 teeth - orange</TD><TD width="53%">19 teeth - (truck) code J</TD></TR><TR><TD width="47%">17 teeth - red</TD><TD width="53%">20 teeth - (truck) code K</TD></TR><TR><TD width="47%">18 teeth - yellow or blue</TD><TD width="53%">21 teeth - (truck) code L</TD></TR><TR><TD width="47%">19 teeth - yellow or white</TD><TD width="53%">22 teeth - (truck) code M</TD></TR><TR><TD width="47%">20 teeth - green</TD><TD width="53%">34 teeth - light green</TD></TR><TR><TD width="47%">21 teeth - black</TD><TD width="53%">35 teeth - pink</TD></TR><TR><TD width="47%"> </TD><TD width="53%">36 teeth - white</TD></TR><TR><TD bgColor=#ffff00 width="47%"></TD><TD width="53%">37 teeth - red</TD></TR><TR><TD width="47%"> </TD><TD width="53%">38 teeth - blue</TD></TR><TR><TD width="47%"> </TD><TD width="53%">39 teeth - brown</TD></TR><TR><TD width="47%"> </TD><TD width="53%">40 teeth - black</TD></TR><TR><TD width="47%"> </TD><TD width="53%">41 teeth - yellow</TD></TR><TR><TD width="47%"> </TD><TD width="53%">42 teeth - green</TD></TR><TR><TD width="47%"> </TD><TD width="53%">43 teeth - purple</TD></TR><TR><TD width="47%"> </TD><TD width="53%">44 teeth - dark gray</TD></TR><TR><TD width="47%"> </TD><TD width="53%">45 teeth - light blue</TD></TR></TBODY></TABLE> Notes:<!--msthemelist--><TABLE border=0 cellSpacing=0 cellPadding=0 width="100%" sizcache="0" sizset="10"><!--msthemelist--><TBODY sizcache="0" sizset="10"><TR sizcache="0" sizset="10"><TD vAlign=baseline width=42 sizcache="0" sizset="10"></TD><TD vAlign=top width="100%">Two aluminum housings are available for driven gears with 34 or more teeth. One fits gears from 34 - 39 teeth and the second one fits gears from 40 - 45 teeth. <!--msthemelist--><TR sizcache="0" sizset="11"><TD vAlign=baseline width=42 sizcache="0" sizset="11"></TD><TD vAlign=top width="100%">The driven gear housings go in the case just forward of the tailhousing.<!--msthemelist--></TD></TR><!--msthemelist--></TBODY></TABLE> * - This gear is shipped in all TCI StreetFighter TH400 transmissions. Thanks again folks, I'll keep you up dated.<!--msthemeseparator-->
If you do what I said, you will get it perfect the first time, without buying any extra gears. Those handy calculators are for people with a new setup that hasn't been driven. They will only give an approximate answer, because the actual rolling dia is not the same as the outer dia you see, and it is not even exactly the same between different tires of the same size numbers. Notice, a tire mounted on a car is FLAT on the bottom, and this makes a difference. Bruce Roe
AH OK, if I understanding you correctly you are saying to replace both gears, is that what you mean ??? I'll try to get the numbers tomorrow off of both gears and I'll let you know what they are so we can go from there as to what gear(s) I need to get. Thanks Again David
I disagree. Yes there is a flat spot at the bottom of a radial tire. But a tire that is spinning can actually grow a bit from centrifugal forces. I realize that the TCI calculator isn't exact, and you must go to a whole number to get a driven gear, but I've always had excellent results with it.
I think we pretty much agree. The calculator gets close, close enough for many people. The only question mark is the tire. But a GPS measurement will eliminate that variable, giving exactly the error remaining. From there, the old gear ratio can be adjusted to give a setup just as accurate as the GPS measurement. If that data already exists, why go backwards? The problem of nearest tooth count can be a couple percent error. Changing BOTH the drive and driven geasr sometimes improves this. If that doesn't work, an eternal gearbox can be added, to move the solution to a 10X improvement. This will mostly show up on the odometer, as the speed is not readable to much less than 1 mph. An example of this with the math is on my PHOTOBUCKET site, sub album SPEEDOMETERS & GAUGES (make those gauges as accurate as possible), pictures 21,22, & 23. http://s93.photobucket.com/albums/l71/bcroe/SPEEDOMETERS and GAUGES/ This is overkill for most. But i bought an 80 Olds (currently 320,000 miles) with oversize tires, and by chance the odometer was perfect. 50 miles on the toll way, and I never gained or lost on the mile markers. I never knew if it was slightly high or low, but I could tell individual mile markers varied but averaged out overall. My first surprise was, the accuracy didn't seem to be affected by tire wear. My second surprise was, a different brand tire of the same size DID affect it slightly. Eventually I realized, the ROLLING circumference of a tire was tied primarily to that inner (steel) belt, not the apparent perfect round tire on its side, not the varying tread thickness, not the big depression where the flat area sits on the pavement. That belt doesn't change with wear, but it can vary slightly with brand/construction. The mfrs acknowledge this issue, but until they publish rolling circumference, a from scratch calculator is subject to this error. NOT GOOD ENOUGH for me, so I MEASURE results and then fine tune the solution. I really need to add some figures with details of this above. Bruce Roe
Just a quick note. If your using GPS data, make sure it is accurate. Your figures may not jive, if your doing 40 and the speedo reads 50 that is 25% fast, but if your doing 55 and it reads 70 then it is 27.3% fast, or if it is reading 75 then it is 36.3% fast. I would double check the readings, the faster readings are most accurate too, as the room for error at slower speeds is much greater. Also, if you don't know what your drive gear is on the output shaft, you can just add or subtract the percentage from the number of driven gear teeth. Example (extreme figures are only for ease of understanding, I know you can't get that many more teeth), if your speedo is 200% fast, you need twice as many driven teeth to make it run 1/2 the speed. But you may need to change the drive gear depending on what tooth count is available sometimes.
Yes I know my GPS is on, so I have that to go by. I also have a bunch of those road side "you are doing this speed" set ups around here. I have matched my GPS numbers to those and those are showing the same as the GPS. So in short the road side ones are showing Im doing 40, my GPS says Im going 40 but my speedo is saying 50.
+++++++++ Then just find out your speedo gear driven to drive ratio, increase the ratio by 50/40, and you are good. Bruce Roe
I have to do some work to her this week so while I'm under there I'm going to pop the housing off and see what I have for gears in there right now, I'm sure that will help to figure out what I need to get. I'm thinking the same way you are, seeing that it is off by so much, I'm going to have to change both gears to get it to read correctly. I'll let you know what I have and what I come up with for new gears.
