I recently installed a Turbo 350 in my 1959 Chevy Apache. I connected it to the stock (new) speedometer on the dash. I took a short test drive and noticed the speedometer was way off. I was probably going 15 to 20 mph...the speedometer was showing like...60 MPH!. Does any know if I have to change the gear out and if so...to what? Any help is appreciated. Thanks! SJR
depending on the year of the tail housing...there are small opening tail housings where the insert is about 1" in diameter..there are large opening tail housings that accept a speedo drive support that is about 2 inches in diameter...inside the rear housing is a gear that is retained to the output shaft by a small clip...the large opening rear speedo gear supports have 2 different versions each has a slight offset.. one is for driven gears over a tooth count of 50 as i recall.. the other is for 49 and below..there are a bunch of different combos depending on rear tire height/rollout.. and rear end gear ratio... as you need 1,000 turns per mile of the speedo cable...there are a few ways to calculate this..mark the tire at the bottom exact center on the edge of the tread... place a mark on a piece of tape on the pavement. roll the truck forward until the mark on the tire is at the bottom center again.. place another mark on the pavement.. measure this out in inches.. this gives you the tire roll out.. from there you can calculate how many inches are in a mile.. (63,360" inches per mile). this and the gear ratio will get you to the proper calculations to perhaps attempt to get the speedo close..there are also little gear boxes that can be installed right at the side of the transmission with gear sets to correct the cable speed...you can do either or both.. depending on what's in there now..post the roll out measurement.. post your gear ratio...and if you have a 2 " speedo adaptor or a 1' diameter speedo adaptor into the tail housing.. if you don't have posi.. jack up one rear tire.. mark one point on the tire... mark the drive shaft.. turn the tire TWICE Exactly while counting the number of turns of the drive shaft . and partial turns..the amount of turns when ONE tire is spun twice is the gear ratio when you count the driveshaft..if the drive shaft turns 3 3/4 turns.. its 3:73
Hey thanks for the help...I will give this a try...I appreciate your time! SJR
http://www.bgsoflex.com/speedo1.htmlwith the small speedo adaptor..the gear on the output shaft could be 7, 8, 9 or 10 tooth..the driven gear in the speedo adaptor.. could be 17, 18, 19, 20, 21, 22.
Hey thanks again...I will chech this out and see what gear(s) I need...almost done with a full off frame resto...getting exciting to finish it up! Thanks again, SJR