My ute still has the stock 12" rims with 155 tyres on them. These have an overall rolling diameter of 554mm. I was thinking of putting some 13's on it. The closest overall diameter tyre is a 185/60R13 but these cost twice as much as the 12's!!

A 165/65 has an overall rolling diameter of 544mm and a 165/70 will be 563mm.

Am I right in assuming that smaller tyres will make the car accelerate faster but rev higher at freeway speeds while a larger tyre will be slower to accelerate etc?

What wheel and tyre combinations do you all use?

Matty

i once had 175/70 x 13's on my 1200 sedan,they were slightly taller than stock,so increased the gearing,but not by much.the drop in diameter will affect the gearing,but you could always get a different diff ratio.if this is a h165 diff in a later ute housing,series 1 bluebirds had a 3.89:1 ratio,& maybe even a 3.7:1 ratio.if it's an early 1200 ute,you could always use the 3.9:1 from a 1200 sedan.

You got it right. Taller tires mean taller gearing ("highway gears"). Smaller tires give a steeper "take off" gear.

There are not many tires smaller than the stock 12s. If you do go smaller (like the 165/70-12s) it won't be much difference, but an overdrive 5-speed will drop your cruising revs back again. But unfortunately the 5-speed will also reduce 1st-gear torque (as first gear is taller). This is a good thing if you fit an A14 or A15, but the A12 needs all it can get.

I have a friend with a Toyota pickup with very tall tires. It takes off so slowly, but he bought it that way and doesn't realize how bad it is. If I had that truck, I'd be swapping gears or smaller tires to get the performance back.

I had Bridgestone 165/65-13's on my car for a while. There just about the same as 155/80-12's.
Now i have my spare's on, 185/75-13's out back 155/80-12's in front. There's not much of a difference taking off, on the street of couse. At the drag strip you will certainly notice a change on your time-slip.
I notice it more on the highway at speed. With the big tires, I can just barely push 5th gear. With the small tires 5th gear is no problem.
Try fiddling with this...calculations2.xls

In my 310 coupe I use a 4.111 diff so to get the same gearing as the speedo was calibrated for without having to find a rare 4.111 gearbox gear, I run 195/70 x 13 when cruising and drop on the 205/515x13 Bridgestones for the track.
Big difference in gearing as "tyres are gears"