Simple Speedometer Correction

I figured I would share this since most of us have speedometer issues. I purchased Handkook 235 R15 tires. Even though these are the same size of the original tire, they are radial tires. Unlike the original hard nylon or Polly glass tires that came on it new. The radials give with weight making them actually smaller. At a reading of 70 mph my phone GPS showed 63 mph. So, it was off by 10%. I ordered a gear ratio adapter that was pretty much plug and play… I ordered the one that reduces speed by 10%. This pretty much corrected my speedometer.

Ratio is 0.900 (10%)
(Actual Speed)/(Displayed Speed) = Ratio

Here is the link where I got mine. You should be able to get this at other places or at a local speedometer shop. These are very common and mostly used on road trucks to correct for different transmissions.

https://transmissioncenter.net/shop/...ratio-adapter/

 

I got one of those for my 1970 Cutlass Supreme back in the 80s after I swapped the original 2.56 rearend for a 3.55 rearend. I worked at a NAPA store at the time and was able to order it from Ohio Valley Speedometer, which was half way across the country from me.

 

My '66 Toro's speedometer reads at least 10% high which is pretty much a universal observation per talking to others.
However, the odometer is right on the money based on clocking the miles on 338 miles of the PA Turnpike - I assume PennDot is pretty much accurate with their roadside mileposts, certainly not inaccurate to the point of 10%.
In the end, I just drive with mentally calculating what my approximate speed is, whereas any MPG calculations don't need any adjustments.