speedometer running 12 mph too fast .1941 chevy big block with TH350 ,a 12 bolt rear from a 1967 chevy c10.any thoughts?thanks
You probably have shorter rear tires than were originally matched with the speedo and/or other driveline components.
There are two different ways to correct the speedometer. Change the driven or drive and driven gears for the speedometer in the transmission or use a a calibration box that is basically a tiny quick change that fastens to the speedometer drive housing and the cable fastens to it. Summits calculator that requires knowing tire diameter, drive gear gear count and rear gear rato Book1 (version 1).xlsb (summitracing.com) Transmission center's speedo calibration page and once you figure out what change you need you go to the Turbo 350 page and they show the gears. You can often get the gears locally at a good transmisson shop or even over a dealer parts counter If GM still sells them for a turbo 350. The gears are color coded so you should be able to pop the gear housing out and look in and see the color of the drive gear and the color of the driven gear or get the teeth count off of it so you can do your calculations before you pull the tail shaft off (if you have to pull the tail shaft off to change the drive gear) with that am thinking that 12 mph difference at a set speed is going to require a drive gear change. Somewhere there is a "these are used together chart telling what drive gears work with what driven gears.
You see that a lot with the local lowrider brigade with their 5.20 13 tires on their Impalas. running down the road at 45 mph with the speedo reading 70. Remembering the 67 C 10 that I had wiith the 327 and automatic it had a powerglide rather than turbo in it. Rear gears with the automatic were 3.70 though. Looking at the 67 option list you could get a turbo 350 with a 250 six but could only get a powerglide with the 327 gas drinker. At least mine was. That thing went though gas like a barfly goes though free drinks. 67 also had optional 3.08 or 4.11 gears available The issue is probably mixed, what the 350/350 left the factory in the 3.73 gears and the height of the tires.
I used to do speedometer calibrations all the time when I ran a ch***is dyno. Usually one tooth change will alter the speedometer reading by approximately 2 1/2 mph. If you need to change the speed by 12 miles an hour, you just might have to change the drive gear on the output shaft. If you have a GM trans, they use nylon gears attached to the output shaft. If you have a Ford product, you could have a problem because the drive gear is cut into the output shaft and not changeable. I remember that I had a 77 LTD II that was the sales managers demo that would not get anywhere near the right speed. It turns out that it had a truck output shaft in it from the factory. We sent it to the trans department and they put the right output shaft in a and THEN I was able to calibrate it correctly. As an aside, I got lots of people coming it to have me calibrate their speedometer AFTER they got a speeding ticket. They all thought they could beat the ticket because their speedometer was off. I used to laugh when they said that, because all the feedback I got from others that argued that before the judge was him saying, "Well, now you won't get another one will you? Pay the bailiff, next case!".
Philly used to have one guy only, that was certified: Billy The Speedometer Man. A work order from him would make the charge be dissmissed. You Know,, How I Know
If you know the tire height and rear end ratio the calculators mentioned above work great...but...on the last build I didn't have the same success (I didn't accurately determine the rear axle gearing) and changed both drive gears and driven gears a couple of times from gears I already had. I was closer but not good enough. I ended up buying an ***ortment, which I wish I'd have done earlier, here is the vendor I used. https://speedometergear.store/shop/ols/products/th350-chevy-speedometer-gear-kit
Use one of the online calculators to find the set you need. I like TCI's, posted above. Should pull the driven gear housing to see what color the drive gear is, that way you can plug that in to the calculator. Also, early Turbo 350's used the smaller "bullet" housing, later ones used the 2" housing, which is like the 700R4's. But, it is easy to do, even if you have to pull the drive gear, just slide the tailhousing off, and you have access to the drive gear and clip.
I've changed gears before to get the reading right. I also have a reducer box like Mr48Chev mentioned on one vehicle and hate it. In fact I had one break in half while driving once. Figure out what gears you have, figure out what gears you need, and change them. If you can do that, it's the best way to go. Lots of good information was provided on how to do that.
Check the odometer, its gear to gear, no spring or magnet drive like the speedometer needle. Adjust transmission gears so odometer is correct. If speedometer is still not correct, the drive may need cleaned. Use the milage markers on the interstate, 10 straight miles will give you a good percentage to work with.
If you called the parts department at 100 random GM dealers today, I bet you less than 5 would have someone that could help you with this inquiry. It's not functional knowledge for dealerships anymore. Speedometers have been electronically controlled for more than 2 decades, and their bread and ****er is supporting new models, not old stuff.
I have 20 yrs in GM parts and I'm a 'newbie' in my dept (4 of my coworkers have more time than I do). I've been fortunate to learn from a few really great parts guys. And you're right....very few of us know the old stuff anymore.
I took the Evelyn Woodhead spedridden course and now my compression is much unproved. You tell your friends you heard it first right here... on Roller Derby.
I knew some really great parts guys too. Their recall of part numbers and body of knowledge was unbelievable. Seems like there's almost no value in knowing the old stuff today if you work for a dealer, Napa, etc. The money is made on the newer stuff.
Just a bit of clarification. Your speedo is off by a percentage, which might be equivalent to 12 mph fast (72 mph reading) at 60 mph for example. But it should be off approx 6 mph (36 mph indicated) at 30 mph. Or 18 mph (108 mph indicated if it goes that high) at 90 mph. To fix your problem, as stated you need a different speedo gear since youa re running a mechanical speedometer with your T350 trans. Take whatever number of teeth on your current gear and add 20% to that number; you need more teeth to slow the gear down. For example, if your current speedo gear has 20 teeth, then you need a 24 teeth gear. Or whatever is close to the required percent correction. The 20% comes from the fact you are reading 20% high if you indicate 72 mph when actual speed is 60 mph: 12/60 = 0.20 = 20%
I've had one of those little gearbox units on my trans for about 20 years with no problem. Easy to figure the ratio needed and easy to install.
The only caveat to that being that you're playing in a finite range for the number of teeth on each gear. If you have the small TH350 gear, your drive gear is generally 7-10 teeth and the driven is usually 17-22. At least in what is generally available now. So for instance if you're already at a 20 driven and need 20% more, you definitely need to change the drive gear to one or two less teeth. That's where a calculator like this one from TCI can come in handy. If you know your rear gear and tire diameter, you can play around and figure out what drive/driven setup would work best for your car. Keep in mind that if it calls for 19.5 gear teeth, for example, you'll have a bit of error.
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