[quicksilver97ta]

Anybody ever notice much discrepancy between advertised and actual specs? On the street my 68 wears BF Goodrich 255/60/15's out back which are advertised at 27.1" OD but I have some 275/60/15 MT ET Street Radials advertised at 28" OD used for the track. Every time I bolt on the MT's they look HUGE. I figured the shoulder, tread design, and maybe compound difference must give them that look. They looked bigger on or off the car. I finally decided to measure both tires and I suspected the MT's might be slightly larger but to my surprise the BFG's are at least an inch shorter than spec! The MT's were about as advertised. So I'm looking at a two inch tire height difference. I always thought the car was a little more sluggish with the MT's and thought they just gripped better. The rpms were noticeably lower cruising with traffic too.

[pfilean]

In tire terminology the 275 or 265 you have is the width of the widest place on the sidewall when the tire is mounted on a rim of some stated width. The rim width usually varies some with the stated tire width. Wider width - wider rim. The 275 or 265 is that width in mm. The 60 refers to the "aspect ratio" of the tire. That is, the height of the tire above the rim diameter (15 in your case). So as the width increases the height of the tire (hence the diameter) increases but multiplying the width times .6 (from the 60 in your case) and adding two of those heights to the stated rim diameter to get the tire diameter.

Manufacturers actual width and diameters do vary some from the stated value but for either BFG or MT I wouldn't expect much deviation. For non name brand tires it could be quite a bit different. And remember as the tire wears the diameter will decrease in proportion to the wear. So as you keep doing burnouts your effective gear ratio is changing some.

[goatless]

Take a look at this picture. The tire on the left is a BFG 225/70/14 with a listed 26.5" diameter. The middle tire is a repro Goodyear Polyglas G70-14 with a listed diameter of 26.8". Looking at them lined up side by side it's pretty clear that there is much more than a .3" difference in their height.

[quicksilver97ta]

Yup, I see. I bet the bfg is smaller than advertised. I went next door where my neighbor has an 81 vette on bfg 255/60/15's and they are clearly taller, about 3/8 " and it did not appear to be the tread. Oh well, now we see.