Have always used NAPA 25-24379 (Gates TR24379) fan belts on my TR3 with the stock fan and generator pulleys. Purchased a new NAPA 25-24379 several weeks ago and ran out of belt tension adjustment before the new belt was tensioned. The cardboard sleeve for the new belt gives the outside circumference as 978mm (38-1/2”) and the top width as 17mm (21/32”). I still had the cardboard sleeve from the old belt (don’t ask why…) and the sizes are listed as circumference 976mm (38-3/8”) and the top width as 19mm (3/4”). So the new belt is 2 mm larger in circumference and 2 mm smaller on the top width. The combination of the two dimensional changes on the new belt made it impossible for me to tension. I did purchase a NAPA 25-28386 (20mm x 996mm) belt but have not tried that option yet.
I did have a belt from The Roadster Factory, purchased several years ago, which was part # 58924 and is an Optibelt-VB and sized as 20 x 900. Only Superman could install that belt. If you are carrying the Optibelt belt as a spare in your boot (trunk) and don’t carry a 6 foot long crowbar to pry it over the pulleys you will be crying when you try to install it on that cold, moonless night when your belt breaks.
I did have a belt from The Roadster Factory, purchased several years ago, which was part # 58924 and is an Optibelt-VB and sized as 20 x 900. Only Superman could install that belt. If you are carrying the Optibelt belt as a spare in your boot (trunk) and don’t carry a 6 foot long crowbar to pry it over the pulleys you will be crying when you try to install it on that cold, moonless night when your belt breaks.