Tony Dombrowicki Posted April 27, 2018 Report Share Posted April 27, 2018 Everything I have read says the 48 Plymouth can standard with 15 inch tires. So when restoring the car I put 15 inch rims back on the car, now using a speedometer app on my phone I find that the speedometer has to register 80 miles to actually be going 60 miles. I can't believe that the drive gear in the transmission would make that much of a difference. I would really like to get it to register closer to the actual speed. Any ideas on how to fix or any adjustments in the speedo head that would help correct the problem. Thanks Quote Link to comment Share on other sites More sharing options...
Don Coatney Posted April 27, 2018 Report Share Posted April 27, 2018 Google speedometer speed correction adaptor. What differential ratio are you using? Quote Link to comment Share on other sites More sharing options...
BobT-47P15 Posted April 27, 2018 Report Share Posted April 27, 2018 My speedo reads about 15 mph faster than I'm going. (If I want to go 40, then it reads 55. etc) My differential is close to original......engine and trans are original type items. Quote Link to comment Share on other sites More sharing options...
dpollo Posted April 27, 2018 Report Share Posted April 27, 2018 The most common Speedo drives are 16 tooth 17 tooth and 18 tooth. on the era of cars we have These ratios usually accommodate the axle ratio and tire size but perhaps the instrument itself is at fault. Check the odometer against a measured mile or travel at an indicated 60 or 30.... whatever is safe and with a stopwatch see how much distance the odometer indicates. Fast readings are almost the norm on certain makes of car but I have found all my Chrysler product's speedometers to be quite accurate . Quote Link to comment Share on other sites More sharing options...
DJ194950 Posted April 27, 2018 Report Share Posted April 27, 2018 On good ol Ebay see speedometer gear ratio adaptors. Some to raise the # some to lower. https://www.ebay.com/i/362017988462?chn=ps Just one of Many. DJ Quote Link to comment Share on other sites More sharing options...
DonaldSmith Posted April 28, 2018 Report Share Posted April 28, 2018 Your GPS will give you your actual speed. Then you can see how far you are off. Some time ago, I had the speedometer rebuilt. The rebuilder adjusted the speedometer to a set RPM. My reading was still 10 percent off, due to axle ratio, tire diameter, etc. I bought a 10 percent ratio changer on line, and installed it at the transmission. I reworked the transmission access cover to fit the projecting ratio changer. I should have put a short cable between the transmission and the ratio changer. (Why do I keep typing ration? Something subconscious?) Quote Link to comment Share on other sites More sharing options...
Niel Hoback Posted April 28, 2018 Report Share Posted April 28, 2018 Autocorrect makes you say things you didn't nintendo. 1 Quote Link to comment Share on other sites More sharing options...
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