My speedo is running 5 MPH too slow. I have a 350 Turbo ******. Is it true that you can make small adjustments by just changing driven gear, avoiding having to pull tail shaft?
fords use different color plastic gears at the ****** and of the speedo cable... different color for different ratios... changing tire size can affect your speedo.
That's true as long as you're not at either end of the available 'range' of driven gears paired with your drive gear. You'll need to go up one tooth and keep in mind it's not an exact thing. Calculate how far off you are as a percentage (5 mph off at what speed) then try to increase the gear tooth count by the same percentage.
We had an old time parts guy, Billy RIP. Who was a pro at dialing in speedos and making fresh speedo cables. He knew exactly which gear to grab when we needed one.
Both the drive and driven gears are available with different tooth counts and are color coded. Remove your driven gear and note the color and tooth count. You can then calculate if the next available driven gear will get you where you want to be. Look inside the trans case with the driven gear out and note the color of the drive gear so you can look up the tooth count of the drive gear.
The speedometer might be off relative to the odometer, also. I like to get the miles reading correct by changing the drive gears in the transmission, then adjust the speedometer mechanism to get it to read the speed correctly (by adjusting the spring tension, and the needle position). If the speedo is off by 5mph when it's displaying 25 mph, then it's off by quite a bit, percentage wise. But if it's off by 5 mph when it's displaying 75 mph, then it's not so far off. So you might want to figure out how many percent off it is, then you can change the gear by the same percentage, and it might be right on. Maybe.
Officer: You were going really fast. Driver: I was just trying to keep up with traffic. Officer: But there is no one else in sight. Driver: I know. That shows how far I am behind.
As they said you can by figuring out the ratio of the drive and driven gears you have now and swapping gears. This chart shows drive and driven gears for a turbo 350. That comes from this page that you can use to figure out what change you need to make. https://www.tciauto.com/speedometer-gear-calculator Gears can be found here https://www.tciauto.com/transmission-accessories/speedometer-gears.html But you can usually get them at a transmission shop that has a parts department or the dealer once you know what you are looking for. PACT is my usual go to for figuring out what speedometer gear I need no matter where I buy the gear. https://transmissioncenter.net/shop/speedometer-gear-calculator/ If you know exactly how much it is off you can figure the percentage and what direction and go from there after you figure out what gears are in it now.
I'm running 50mph on speedo when I'm really doing 55. I have a gray 22 tooth driven. I'm thinking, and just a guess, could I go down to a 21 driven and be a little closer to correct speed and not change the driver in tail shaft. YES! I am lazy and ready to ride. I've been almost two years on build.
5 mph off at 55 is about 10 percent...so you'd want to change the transmission gear 10 percent, also, that would be 2 teeth, not just one. So try a 20 tooth. beware that there are two different sizes of driven gears, it might be that you have the "large" one now, and the tooth count you need is a "small" gear, and they two won't mesh properly, because you need to change the drive gear on the output shaft. But I'm just guessing here, do some more research! and you can drive with the speedo reading wrong, for years...no need to delay your long awaited drive for that.
When I got my A roadster together, I took it for a shake down spin. Seemed to be going awfully fast compared to the speedo reading. Couldn't get the cable drive Mopar cruise control that I adapted to work either.C/C wouldn't work until above 40? mph. Had to change drive and driven gear to get almost correct reading. It's not hard to change driven gear if you have to. Lift rear of car so fluid in transmission goes forward, remove driveshaft and tailhousing, and you'll see it. Little clip holds it in place.
Have had success adjusting speedometer readings in a Muncie 4 speed by going up or down tooth count and always kept a few different colors on hand.
You can also get the little gear boxes that change the ratio. A lot of the larger older trucks have them. Different gears are available to help. This may be easier than changing out the driven gear.
I've been thinking about this issue for about as long as I've been building my '29AA, and figure I'll need to do more than just change the drive and driven gears. You see I've got a 5.14:1 Rockwell differential and even though the 8.0-by-17.5 tires are 31.5 inches tall and the AOD I'll be running overdrives down to .66, it will still be singing mezzo soprano at 75 mph. I'm hoping I can find a ring-and-pinion set around 4.11 or so to change out, and that would probably bring down the speedo drive/driven gears to something that parts houses have in stock.
Wrong way round..... He needs to go down one tooth [on the driven gear] so it spins slightly faster. His speedo is presently reading too low. @Willywash GM's also use the same principles [they are color coded] @osage orange Most speedometers need 1000 revolutions per mile. So it is reasonably easy to do the math
It is correct that speedometer cables should turn one thousand Revolutions in a mile which is 5280 ft. Unhook your speedometer cable from the speedometer and put a paperclip or a piece of tape on the end of the cable and push or drive the car 52.8 ft and the cable should turn 10 times. More or less is your e r r o r. I have no gauges in my model A, I just look out the windshield!
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