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Mod | | | Forums66 Topics128,036 Posts1,055,450 Members48,422 | | Most Online14,466
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| | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | SM420 transmission 1:1 final with 3:38.1 ring pinion 27" dia. Tires 1954 3100 PU. At 35mph on speedometer actual speed 40 mph about 5/6 mph difference. We need a ratio adapter for the transmission any suggestions or model numbers. Don't want to fool around with the speedometer itself a ratio adapter would be an easy fix and easy to remove.
Thanks! | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | Some people just use a GPS for their new and accurate Speedometer. Jim Carter used to provide ratio adapters - he might still do that. The Speedometer Shop is where I have obtained ratio adapters - they seem to be out of business. Bob's Speedometer Shop supposedly makes them ($!50). A Google search of "speedometer ratio adapter" gives these results. Maybe someone who has obtained one in the last year will post information? | | | | | Joined: May 2005 Posts: 1,624 Shop Shark | | Shop Shark Joined: May 2005 Posts: 1,624 | Would a one tooth change in the driven gear from the tranny, less than $15, not work?
Six volt guy living in a twelve volt world
| | | | | Joined: Feb 2000 Posts: 5,195 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 5,195 | You will need to check it at 60 mph for the best reading. Most all of the cable driven speedometer are calibrated at 60 mph, 1000 rpm of the cable is 60 miles per hour. Here is a good link to really check yours. https://www.hotrodhotline.com/calibrating-speedometer#.V9vrrjtT5TcTrans gears are a easy to change, but you still need to know how far off it is at 60. | | | | | Joined: May 2009 Posts: 11 New Guy | | New Guy Joined: May 2009 Posts: 11 | I got a ratio adapter custom made from PATC Transmission Center in Louisiana. I figured out my percentage with a GPS and multiple test drives and then called them up and it works great | | | | | Joined: Jun 2011 Posts: 5,096 Crusing in the Passing Lane | | Crusing in the Passing Lane Joined: Jun 2011 Posts: 5,096 | I'm sure there is a speedo shop in Redding, even Sacto where
you can get an adapter, even look for one in a wrecking yard, speedo shop should be able to fix you up.
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
| | | | | Joined: Dec 2006 Posts: 1,154 Shop Shark | | Shop Shark Joined: Dec 2006 Posts: 1,154 | I wrote to Novak transmission and gave them my numbers. Turns out I'm about 11% off the mph. They directed me to an ebay supplier for said ratio adapter. Novak excels at after sale customer service.
Thanks to you all! | | | | | Joined: Sep 2010 Posts: 100 'Bolter | | 'Bolter Joined: Sep 2010 Posts: 100 | I affirm "delwrench"s comments! Works great on my 1946 Chevrolet truck original speedo. | | | | | Joined: Sep 2001 Posts: 29,269 Bubba - Curmudgeon | | Bubba - Curmudgeon Joined: Sep 2001 Posts: 29,269 | | | |
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