pocobill7t1 Posted January 17, 2013 Share Posted January 17, 2013 Anyone know what the issue is with my speedo being out 10mph? When i am doing 50mph the speedo shows 60mph, I had my GPS on and this is how i know it is out . thanks Bill Quote Link to comment Share on other sites More sharing options...
Leghome Posted January 17, 2013 Share Posted January 17, 2013 What size of tires are you running. I once had a car that called for 7:35 14's and I was running 8:00 14's and mine was off by about that much. Quote Link to comment Share on other sites More sharing options...
MCfan Posted January 17, 2013 Share Posted January 17, 2013 You're either running a significantly UNDER-sized rear tire (over-sided would cause your actual/gps speed to be greater than your speedometer shows) OR your rear gear ratio has been changed to a higher numerical ratio and the speedometer gear has not been changed to it to match the engine rpm to final drive relationship. Quote Link to comment Share on other sites More sharing options...
pocobill7t1 Posted January 17, 2013 Author Share Posted January 17, 2013 good points all around, i am running 235 60 15's on all corners.when i am griving at 55 mph the tach says 2100 rpm's Quote Link to comment Share on other sites More sharing options...
Murphy Posted January 17, 2013 Share Posted January 17, 2013 Maybe it's a canadian translation thing like the currency exchange rate........ Quote Link to comment Share on other sites More sharing options...
stangeba Posted January 17, 2013 Share Posted January 17, 2013 What has changed from when it was built? You mention tires, what about rear axle ratio? Is the original trans still in the car? Many things can change in over 40 years. A simple gear change of the trans speedometer driven gear by two more teeth could fix it but knowing why it's off, to me, is important. Let the search begin! Bruce Quote Link to comment Share on other sites More sharing options...
CKZ03 Posted January 17, 2013 Share Posted January 17, 2013 You should be able to correct your problem using this calculator/formula. If you get lucky you may only need to change one gear at speedo. There is a drive gear & a driven gear. You must know your rear end gear ratio for this to work properly. Then verify the gears at speedo drive in your trans tail stock housing. http://www.tciauto.com/tc/speedometer-gear-calculator/ Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted January 18, 2013 Share Posted January 18, 2013 you will need to know your tire diameter too Quote Link to comment Share on other sites More sharing options...
MCfan Posted January 18, 2013 Share Posted January 18, 2013 good points all around, i am running 235 60 15's on all corners.when i am griving at 55 mph the tach says 2100 rpm's Bill, here's a link to one of several tire size and height (diameter) charts: Tire Size and Height Chart As you can see, your 235/60-15s are only 26.1" tall (diameter). Your Monte was very likely shipped with G78-15s which is close to a 205/75-15 tire that is27.11" tall and that is probably what your speedometer is set up for (in addition to the rear axle ratio your Monte was built with). With a full 1" of difference in tire diameter, each revolution of your tire covers 3.147" LESS distance (Circumference = Diameter X 3.14172) than an original Monte tire. That translates to a 9.6% underdriven error between your actual speed and what is showing on your speedometer. At 60 mph on your speedometer, you actual speed would be no more than 54 mph. Many of us have the opposite problem. For example a 275/60-15 is 28" tall which translates into a 3.3% overdriven error between actual speed and speedometer readings (again, assuming factory speedometer gears and rear gear ratios. So we are always actually going faster than our speedometer shows - not good for avoiding speed traps. Quote Link to comment Share on other sites More sharing options...
MGD72Monte Posted January 18, 2013 Share Posted January 18, 2013 Yes, 215/75R15 is the right size for G78-15s but it is a dang skinny tire. The chart I have lists the diameter for both of these at 27.7" and as mentioned yours is 26.1", quite a difference. I use to have 275/60R15 (28.0") at the back and they seemed to be pretty accurate, my 225/70R15 (27.4") are a little off. If I had to get replacements I would get 235/70R15 (28.0"). Quote Link to comment Share on other sites More sharing options...
cny first gen 71 Posted January 18, 2013 Share Posted January 18, 2013 I am very far out of wack too speed o is the least of my problems Quote Link to comment Share on other sites More sharing options...
cudax Posted January 21, 2013 Share Posted January 21, 2013 These guys can help you out. http://www.transmissioncenter.net/speedometer_calibration_______va.htm Quote Link to comment Share on other sites More sharing options...
Leghome Posted January 21, 2013 Share Posted January 21, 2013 These are the guys who got my Speedo to match the 700R4, 3:73 gears and the the 235/15 tires. It matches my GPS dead on. Quote Link to comment Share on other sites More sharing options...
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