Hello all, I have a '73 Grabber with a C4 automatic and 3.55 gears with 235/60R14 rear tires. Currently my speedometer is way off, at least 10-15 mph, and I would like to get the right gear to fix it, but I don't know which size to get. Could anyone inform me of the proper size because I haven't been able to find it myself. Thanks, Ford Guy
You need to find out exactly how far off it is, and calculate the percent of error. Look at the existing gear on the end of your speedo cable, count the number of teeth, add the percent of error to figure out how many teeth you need to be accurate. You didn't say if the speedo is reading high or low. Add teeth to slow it down, subtract teeth to speed it up. Those roadside radar things the cops set up are great for that. Or if you have a tach calculate it with this http://www.slowgt.com/Calc1.htm#SpdInGear
THere was an excellent article in Mustang and FOrds awhile back that showed you how to calculate which gear you needed. It also gave you the part numbers. I will see if I can find it this evening.
Speedometer drive teeth X Axle ratio X Tire revs per mile divided by 1000. Drive teeth on a C4 is 7. A speedo gear from a 3.40 geared car would be close. I can count the teeth on mine later this evening and tell you how many teeth that gear has, to be accurate you'd only need to add about 1 tooth above it.
Unless somebody put the wrong output shaft in it when they rebuilt it! One of these days, my speedometer will be right... oh it will.
Thank you all for your help. I ordered that issue of Mustang and Fords and I will try using the other methods as well. Thanks, Ford Guy
Do you have to change the gear in the trany as well. If not how will the gears match up if you put a new one on the spedo cable.
The gear in the tranny is part of the output shaft, you can't change it without changing the output shaft, and generally, you don't need to. My guy just grabbed one off the shelf and didn't count the drive gear teeth. All the cable gears are compatible with that's in the tranny.
You don't usually need to change the tranny gear unless you make a really big change in gear ratio. The gear in the tranny is held on to the shaft by a snap ring.
My speedometer was off by almost 18 mph @ 60mph. After doing the math in the article and picking up the correct gear, its within 1 mph now.
Not on a C4. You've been dealing with manual trannies. http://www.tciauto.com/Products/TechInfo/speedo_gears.asp#FORD
Not quite, the manual and auto's use different cable gears(the "driven" gear). The to complicate thngs more, the gear also depends on what side of the tailshaft housing the cable plugs into. These use "backwards" gears from those that go into the other side.
I know not all the driven gears are same, I was talking specifically about C4 drive/driven gears. I was answering Dean's question about matching the # of teeth.. You don't need to change the gear in the trans to replace the driven gear on the speedo cable. Also, not all the manual trans speedometer gears are backwards from the C4 gears. My factory 3-speed speedometer cable was on the same side of the tranny as on my C4, so the teeth on the driven gear were cut the same direction. I am using it with my C4 now.
