|The MGA With An Attitude
LARGER TIRES To Fit the MGA - TI-101A
If you want to change to wider tires for appearance or better handling, try to keep the overall diameter about the same so it doesn't mess up the speedometer and odometer too much. In general, for each 10 points increase in width you should go about 5 points less in profile. Following that formula, if the following sizes were available they would be a fairly close match to the original diameter: 175-75-15, 185-70-15, or 195-65-15.
A friend of mine uses 185-65R15 (nearly 1" too small). This over-revs the odometer by nearly 4%, and makes the speedometer read about 2 mph high at highway speed. This size is also a little too wide for the stock rim width, so unless you also install wider wheels you may notice strange wear patterns on the tires, especially scrubbing off of the outside shoulder.
For my racing tires I use 195-60R15 which is about 1/4" smaller diameter yet, but this is (was) the narrowest tire size available in race rubber compound for a 15" wheel. If you're concerned about clearance for these larger tires, you can stop worrying. The MGA has lots of open space inside the fenders to accommodate wider tires. Do not be surprised if you start breaking wheels when using real race tires, because real race tires are really sticky. For more information about my race tires and special wheels, check out Mag's and Stickys.
However, when using wider tires you absolutely must have wheels with offset similar to the original wheels. Most currently available replacement wheels do not have enough offset, putting wide tires too far near the fenders. See the Wheel Tech section for more information on wheel offset and alternate wheels (multiple pages).
Addendum June 2011:
This is my car running 195-60-15 race tires on special order 15x4-1/2 modular wheels with original 35mm offset. This has original track width. Notice the space between tires and fenders.
This is Lyle Jacobson's car wearing the narrower 165-80-15 tires, but the wheels have considerably less offset. Notice the 165 tires are closer to the fenders than my 195 tires above.
This is Dennis White's car wearing 185-70-15 tires on 15x5-1/2" Saab Shelby-Ronal wheels with 40mm offset. The 40mm offset puts 5mm extra rubber on the outside closer to the fender, and 15mm extra rubber on the inside where you don't notice it so much. Measured tire circumference (new 185-70 tire) is 80-1/2" compared to 80-1/4 inches for the somewhat worn 165s. Perfect! These 185 tires fit better than the 165's in the prior picture due to the improved offset of the wheels.