Ok what is the best way to fix the 15 mph too fast on his speedo. 70-GS 455 with a turbo 400 trans I believe from a 69 stage 1 car. 373 rear gears in a 12 bolt.
The speedo gears inside the transmission were from something with numerically lower gears,like a 3:42. You just need the correct speedo gear.
15 mph too fast at what speed? Reading +15 at 25 mph is a lot different than +15 at 75 mph! Use a smartphone with a gps speedometer to give you the real speed. Changing the driven gear one tooth changes the speedo reading about 3 mph @ 60 mph.... so about a 5 tooth change. That's a lot. If the gear is even avail, you might have to change the gear housing too. They are made for a specific gear range. Another method is to add a ratio adapter. 75mph/60mph= 0.8 , so a 0.80 ratio is about right. Here's a random page with gears, housings, and ratio adapters: https://transmissioncenter.net/product-tag/speedometer-gears/
You may be able to change the DRIVEN gear to correct it. To find out, you need 3 numbers, rear tire height, rear gear ratio, and the number of teeth on the DRIVE gear. That gear is visible when you pull the speedo gear sleeve out. With the rear tires jacked up off the ground and the trans in neutral, you can rotate the driveshaft and count the number of teeth on the gear. It is usually 16, 17, or 18. Once you have those 3 numbers, you can plug it into a calculator and get the number of teeth necessary for the DRIVEN gear. Sometimes you need to change the DRIVE gear as well, or you can get the ratio adapter Walt linked. http://www.tciauto.com/tc/speedometer-gear-calculator There are also 2 different sleeves depending on the DRIVEN gear you need.
OK, you need to reduce the speedo by 25%. A 0.75 ratio adapter would do it. OR Take the driven gear out and count how many teeth it has, add 25%, and that's how many teeth you need for the new driven gear. Try the link Larry posted, but it's difficult counting teeth on the drive gear..... at least see what color it is, might be able to ID it that way.
