On the firewall of your Sunny should be a tag like the one in the picture below.





































This tag will tell you all that you need to know about what was originally fitted to the car & I expect that your tag will have the same mechanical details as this one.
The engine should be an A15, with a single [S] carb & it displaces 1488cc

The gearbox is a [F] floorshift [4] four speed [W] wide ratio [60] sixty series gearbox with the [L] long extention housing.

The differential is an H150b [code HB] in 3.7 to 1 ratio [37]
The H150 diff was used in Australian 120Y & Sunny models, however the ratio was 3.9 to 1 when used behind A12 & A14 powered models. H150's also came in a 3.54 to 1 ratio as well, but this seems not to have been used in any Australian models.

Only the last of the Australian Sunny Sportswagons got the A15 engines & the 3.7 to 1 diff ratio.
I have an owners manual for one of these A15 powered models & it says that the original wheels were 13" x 4-1/2" [steel] or 13" x 5" [alloy] & they came with either 6.15S-13 Bias ply tyres, [582mm OD] or 155SR-13 radials. [580mm OD]
What is the size of your 14" tyres? If they are larger in OD then this will raise the overall gearing resulting in a slightly reduced performance & a slow reading speedo. [you would be traveling faster than you think.]
The amount of performance reduction & speedo error will be dependant on just how much larger the tyre OD is compared to standard.