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Picking the right Speedo

76zimmer

76zimmer

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Looking to dial in my speedo on the Vette T400
I know there are a couple different driven gear housings and many different driven gears.
Is there a way to figure out exactly what combo of gear and housing I need for it?
 
Frankin5 beat me to it. Saved that site when I was researching for Eileen's Corvette 700r4 upgrade.
 
My old GM parts catalog for 10-30 series trucks from 1946-72 has listings to look up which speedometer gear you'd need by using a chart that has the tire sizes and gear ratios in the catalog..maybe you can find an old GM parts catalog for the year of your car online ?..
 
76zimmer said:
great I was hoping there was something out there like that...
so having a 3.90 gear and 26.75 tires I'm guessing at a 43 tooth gear, which I already have the housing for.
thanks guys!
Click to expand...

Do you know how far off it currently is? IMO it's easier to take your current gear count and figure out how far off it is, that will tell you where you should be.
 
I know its way off...like more than 10mph @ 70
 
evidently I need to change the drive gear on the tailshaft...it is 18 tooth, and figuring a 26.5 tire w/ 3,90 gears it is off the charts
 
Are you sure the drive gear is 18 tooth? What is the current driven gear?

With 15 teeth driven, 44 tooth gives 1002 revs/mile with 3.90 and 26.75". At 18 teeth, you need 53!
 
I counted the teeth on the drive gear @ 18, the driven gear I took out was 42. I can't recall how much its off, but is significant like +10 @ 70mph.
I can't remember what this T400 came out of, but if I change to the 15 tooth drive gear I think it'll be much closer....I have to pull the driveshaft, crossmember, tailshaft to get to it though.
 
I thought the drive gear was the big one on the output shaft and the driven gear was on the cable output.

But I am old and don't remember so good no more.
 
18 and 42 is 1260 rev/mile or 26% error, so if you're going 70MPH it should read 88. I've never used a ratio adapter, but that would be an option: https://www.amazon.com/PATC-speedometer-ratio-adapter-decrease/dp/B075MHV7TN.

Given both options, I agree with campfire that it's easier to adjust from your known reading and known speed than use gear ratio, tire size, etc. Mechanical speedometers aren't perfect devices and you probably don't know the rolling circumference of the tire exactly. Plus exact speed is easy to get now with any smartphone, so the days of freeway mile markers and stopwatches are gone. Either method will probably give close enough results. You can only use the discrete tooth counts available anyway.


76zimmer said:
I counted the teeth on the drive gear @ 18, the driven gear I took out was 42. I can't recall how much its off, but is significant like +10 @ 70mph.
I can't remember what this T400 came out of, but if I change to the 15 tooth drive gear I think it'll be much closer....I have to pull the driveshaft, crossmember, tailshaft to get to it though.
Click to expand...
 
ktmoutfront said:
I thought the drive gear was the big one on the output shaft and the driven gear was on the cable output.

But I am old and don't remember so good no more.
Click to expand...
you are correct Wade...the drive gear is steel in this trans and is on the output shaft w 18 teeth...the driven gear is easily interchangeable within a 5 tooth window of the particular gear housing options.

Blue85 said:
18 and 42 is 1260 rev/mile or 26% error, so if you're going 70MPH it should read 88. I've never used a ratio adapter, but that would be an option: https://www.amazon.com/PATC-speedometer-ratio-adapter-decrease/dp/B075MHV7TN.

Given both options, I agree with campfire that it's easier to adjust from your known reading and known speed than use gear ratio, tire size, etc. Mechanical speedometers aren't perfect devices and you probably don't know the rolling circumference of the tire exactly. Plus exact speed is easy to get now with any smartphone, so the days of freeway mile markers and stopwatches are gone. Either method will probably give close enough results. You can only use the discrete tooth counts available anyway.
Click to expand...
I will wait until I have a much better idea of the amount its off. I changed it last time I was in the trans as before that with 4.10 gears it was almost double what the actual speed was, so I'm zeroing in on it..by the way is there and "app" for the speedo?
 
76zimmer said:
you are correct Wade...the drive gear is steel in this trans and is on the output shaft w 18 teeth...the driven gear is easily interchangeable within a 5 tooth window of the particular gear housing options.


I will wait until I have a much better idea of the amount its off. I changed it last time I was in the trans as before that with 4.10 gears it was almost double what the actual speed was, so I'm zeroing in on it..by the way is there and "app" for the speedo?
Click to expand...

There are lots. But any good GPS device or app should have a speedometer function.

Figure out exactly how far off you are at a couple of different speeds (to make sure you're not also dealing with an offset error).

Also, did we figure out what exact gear combination you have? 18 and something? :dunno:
 
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