JIPJOBXX Posted April 23, 2009 Report Posted April 23, 2009 As you guys know I installed a set of radials on my 42 and the size are 205-15-R. While I getting set up to install these tires someone here stated they had the same size tire. I know my 6:00-16 are about 2" taller and I was wondering how much my speedometer change in speed reading when I switched over. I do noticed that the old 100 hp engine is struggling a little bit more when I'm doing 50 mph. I think its oft about five miles per hour. Jon PS. I really don't run my car much faster than 50 most of the time anyway as thats what that engine was built for back when the old Dodge was first sold. Quote
greg g Posted April 23, 2009 Report Posted April 23, 2009 Mine ran about 6 mph slower than the indicated speed with the 205 75's. if your are running 70's it mught be a bit more. Quote
desoto1939 Posted April 23, 2009 Report Posted April 23, 2009 If your car is setup with 12v then hook up a gps. Then set the spped indicator to a specific setting say 45. Bring your car up to this speed. the unit should then tell you taht you have gone over the speed setting. If the unit does not inform you then continue increasing your speed until then unit signals your setting. The GPS will give you the correct speed since its is going against a triangulated system of satillites. I have done this with my modern car to determine if my speedometer is correct. Our modern cars can also be off by a few miles. This will also help if you want to contest a speeding ticket. Just thougth that might help. rich desoto1939@aol.com Quote
JIPJOBXX Posted April 23, 2009 Author Report Posted April 23, 2009 Thanks I do have a GPS and I can hook it up to my portable battery. Jon Quote
Niel Hoback Posted April 24, 2009 Report Posted April 24, 2009 Most GPS will run a good 3 hours on their internal battery. I have 205/70 15's on my 48 and the speedo shows exactly 8 mph fast. Quote
JohnS48plm Posted April 24, 2009 Report Posted April 24, 2009 There's a company that makes a small gearbox that speeds up or slows down your speedometer reading. It goes on the back of your speedometer and the speedometer cable attaches to it. They used to advertise in the Street Rodder magazine. JohnS Quote
Savoy Boy Posted April 27, 2009 Report Posted April 27, 2009 I have the same problem with our 55 Savoy. I intend to hange the diff to a 3.5 ratio to bring things back into line and restore the cruising speed. Quote
dndrodshop Posted April 27, 2009 Report Posted April 27, 2009 http://www.csgnetwork.com/speedocalibcalc.html Quote
dndrodshop Posted April 27, 2009 Report Posted April 27, 2009 http://www.apexgarage.com/tech/gear_ratios.shtml Quote
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