MG MGB and MGB GT Tech Talk
My 75 B currently has 175/60r14 tires and the speedometer in 4th (no OD) runs about 5 miles faster than the actual speed of vehicle. (eg. speedometer shows 50, car is running 45). I want to put a larger tire on the car. 195. Would going to a 195/65r14 correct the discrepancy in the speedometer or make it worse? The 60 or 65 refers to the hieght of the tire, right?
The aspect ratio of a tire is determined by dividing a tire's section height by its section
width when the tire is inflated to maximum air pressure, mounted on the approved measuring rim and under no load.
Thus, a 175 with 60 aspect ratio would have a height of 105 mm.
A 195 with 65 aspect ratio would have a height of 126 mm.
By replacing 175 60 with 195 65, you would increase the distance traveled each revolution by about 20%. The speedometer would therefore be showing a speed that is 20% less than now shown.
Edit: Oops! I went too fast in the analysis. I worked with the change in radius, not circumference. Using the charts referenced below works more accurately. Ron.
There are tire size calculators on the Net. However, if you have a factory speedometer you probably want either 185/65x14 (which will read 2% to 3% fast) or 185/70x14 which will read about 1% slow. In practice, these speedometers are off by a percent or two themselves - often by different amounts at different speeds.
This is the calculator that I use most often. Not claiming it's the best.
http://www.mintylamb.co.uk/?page=tyre.htm
Try putting in your 175/60 vs 175/70. 5.4% difference!
I've used both 195/60x14" and 195/60x15".
That is a nicer calculator than the one I've seen. However, all of the calculators do not allow you to specify 82 series tires - the original ratio of factory 155x14 radials in the early 1970s.
Mind you I don't think these speedos were ever all that accurate. The factory used 165/82x14 tires on the GT and, AFAIK, used the same speedo as the Roadster.
So the difference between 80 & 82 series is 2.5%. Be aware that some 175 tires will not measure 175 wide. And I don't suppose that the 60, 70, 80% is terribly precise, either. They're all (except rim diameter) nominal measurements! And I'm sure the Jaeger and Smiths gauges were not "calibrated", either. Except, perhaps, those supplied to the Police in the UK.
Road and Track used to publish the speedometer errors in their road reviews. The speedos of that day (regardless of manufacturer) were often off by several %.
Speedometer error is often not constant either (although it should be). I've checked mine with a GPS. At some speeds its a bit low, some a bit high. I've even seen the error change with temperature.
Air temperature or tire temperature? As the tire heat up, the pressure rises. This will affect the diameter. But I have no idea how much. I'm sure some engineer has written a paper about it, but not this one.
I know tirerack.com has a calibration calculator. That's the one I used when I bought my tires for the B last year. You can get very close with today's tires but not spot on. You should put the tires on that you want and then have the speedo calibration checked and adjusted if needed.
Shawn Escue
