I did google it and see that you are right.. Did not know that the 51r battery was smaller years ago. My 21 HRV uses the 51r ...larger size and I wish that you could change it to a more powerful battery but they wont fit in the HRV. When I had my 07 Accord with the 51r there was a kit to switch to the larger battery used for the ones used with the v6...M1Accord's Fit came with the tiny 151R battery (340CC), not the larger 51R (500 CCA) battery. My suggestion was to go to the larger 51R battery which was OEM on the Fits built in Japan. I've never seen the tiny 151R battery at any WalMart and it is often more expensive than a comparable grade 51R size.