Fellow Fox Body Fans,
I have a 5-speed manual transmission in my 1985 Ford Mustang GT. My speedometer was off by about 8-9 mph. When the speedo is showing 57, my GPS is showing me doing 65 mph. It gets a little more off as I go faster (as expected).
I pulled out the current speedo driven gear and it is 16T. The LMR.com calculator and CJPony Parts calculators stated I should be using a 17T. So, I bought a 17T and installed it. The speedo is off by a much larger margin now. At 55 mph on the speedo, the GPS is showing me at 65 mph.
I looked at the American Muscle chart and it shows I should be using an 18T. Will an 18T make it worse?
As far as I know, this is the stock T5 transmission for the car with the 3.08:1 rear end gear. My tires are the same size as stock and have about 870 revolutions per mile.
What is my next step? Speedometer cable? Speedometer gauge? Thanks.
JB
1985 Ford Mustang GT, 8.5K original miles
USAF, CMSgt (ret), 1985 - 2015
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