Does anyone know where I can get one of those inline gizmos for adjusting your speedometer reading. I think Stewart Warner used to make them, but I don't remember for sure. The one that I'm thinking of is just a little box with 2 interchangeable gears inside. I even checked J. C. Whitney, they used to sell them, no luck.
You might find there are more than two gears inside! I've collected a few of the boxes, mostly from the junkyard or cars/trucks I took apart, and pressed the gears apart and put pieces of several of them back together into one with the correct ratio for what I needed. they show up on ebay every now and then, some go for some $$$, and you probably won't find the ratio you need.
try a search on e-bay for speedometer adapters.....i have seen several on there but the trick is getting the right ratio as squirrel said, mike
Just how far off are you ... most can be adjusted without the " little boxes " unless you really have a LOT of rear gear ratio ... I can always get my speedometer correct with simple changes to the output shaft gear and the driven gear ... Simple math really. A speedometer should turn 1000 RPMs in one mile. It is very easy to figure ... How far are you off and what type car and transmission are we dealing with ???
'54 Chevy. 261 inline with a Saginaw 4 speed (Monza/ Vega style). 10 bolt GM rear with 3.08:1 gears. Right now I'm about 10 mph slow, at 65mph I'm reading 55 mph. If memory serves me, my driven gear is either pink or yellow. I know it's the closest I can get without changing the drive gear.
This page has calibration info, and the company supplies both drive gears and ratio conversion boxes: http://www.speedometershop.com/rep-pag.htm
This post intrigues me because I want to use my stock Ford gauges. Is there any formula that you can use BEFORE assembling the car? If possible, I would like to try and get mine close while the tranny is being overhauled.
my formula for figuring the gears is driven gear teeth = 20.2 X driving gear teeth X rear gear ratio / tire diameter for example: driven gear teeth = 20.2 X 8 X 3.08 / 28 = 17.6 which you would round to 18 teeth and give that a try. Figuring out a good number for tire diameter can be tricky.
You want to use standard speed vs RPM formulas to figure out driveshaft RPM at exactly 60 MPH, then choose gears to produce 1,000RPM of the distributor cable at that speed. Nearly all normal regular American speedos are calibrated to 1,000 RPM=60, 1,000 turns puts a mile on odometer. Both early Ford catalogs and 1960's-70's Chevy catalogs have a table specifying speedo gears by rear end ratio and tire size, and if you can correlate your tire diameter to something that is cataloged you can use that chart to get close enough for most purposes. On the original speedo, try driving it from a power drill or such at a known RPM; if drill RPM is reasonably close to spec, you can make at least a reasonable flying guess from indicated speed whether speedo is working correctly, and cross check that against odometer movement in one minute.
The formula I gave above is based on the 1000 turns/ mile standard. When I'm building a car, I use the formula to get figure out which gears to put in the trans. Then when the car is running I'll drive it on a highway with mile markers, preferably over a 5 or 10 mile stretch, and find the error in the odometer. If it's within a few percent, then I'll leave it alone, if it's more I'll change the driven gear to get it as close as possible. Then I'll check the speedo, again over several miles, driving at 60 mph and seeing how many seconds more or less than a minute it takes to go each mile. If the speedo is within a couple mph I'll leave it alone, but if it's off by quite a bit (and the odometer is accurate), I'll play with the spring on the speedo, or reset the needle posistion if it's also off by the same number of mph at low speeds.
if you want to figures out what gears to use in your tranny , use TCI's claculator: http://www.tciauto.com/tech_info/speedo_gears.htm
Wow, this is gonna take some thought. I haven't even made up my mind what rear end gear I am gonna run. I was leaning towards a 3.50:1 ratio.
Wow! Thanks for the help. Now I just need to figure out what drive and driven gears I have, and I'll be all set.TCI's web site is the cat's pajamas as far as speedo gear info goes! I'll let you know what I figure out. Thanks again.
O.K. my driven gear has 18 teeth and it's kind of a brown color with an orange band on the end. When I look in the speedo gear hole, I can see 6 teeth across the bottom of the drive gear, and it's kind of a brownish orange too. If I plug that into TCI's calculator with a tire diameter of 28.287", It gives me a driven gear of 13.162. Now, just because I can see 6 teeth at the bottom of the gear, does that mean that the gear only has 6 teeth? Is there a web site that lists manual trans. gear specs? How do you determine what drive and driven gear you need if there isn't one made (with 13 teeth)?
IF you have a 18 on the driven ... you need a 8 on the output shaft. Your tires turn 714 times in a mile ... 28.287 X 3.1417 and a mile is 63360 inches . With a 3.08 gear ... the driveshaft turns 2199 times in a mile. ( 714 X 3.08 = 2199 ) The speedometer cable needs to turn 1000 times in a mile ... SO ... 2199 divided by 1000 is 2.199 That is the ratio you need for the speedometer to be right. You have a 18 ... so 18 divided by 2.2 = 8.18 ... IF you have a 6 ... and want to change the easy one ... 6 X 2.2 = 13.2 or you need a 13 ...
I have an old Chevy parts book, it lists the part numbers for the Vega trans drive gears, but no color or teeth info. Put a mark on one tooth using white paint, turn the driveshaft slowly as you look in the hole, and see if you can count the teeth on the drive gear. or get that $62 adapter. or do like some other guys and take out the cable and don't worry about it.
I'll give it a shot. That is the only way to know the # of drive teeth, for sure. With a little luck, I'll be able to get this fixed without having to pull the tail shaft. Not that it's that big a deal. In my car that would involve romoving the crossmember, shifter, brake lines, drive shaft, etc...
On my both 32's ... it is easy ... to get the tailshaft housing off ... but then ... I planned it that way
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