G'day,
I have a '76 929, which I would like to make a bit more practical on the highway.
I have recently had the engine rebuilt and and put a set of 14'' rims and larger tires on her. I put a 5 speed on a few years back.
The problem been that the speedo is 10% out and that it is doing 4000 rpm at 110km/h which seems a little hight for cruising in 5th gear.

Was thinking of changing the diff ration.

Any help or info on whether this is normal or what my options might be would be great.

Cheers!

Get smaller tyres?

actually, it's the opposite. Go for a larger rolling diameter = larger wheel and tyre.

changing the diff ratio is probably the best modification you can do. Mazda gave their 4 cylinder cars pretty steep rear gears so that they moved off the line pretty quickly, but as you've found, they're screamers on the freeway.

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There were two questions in the post.. original size tyres would help the speedo , but diff ratio would help reduce the revs. Cant really do both at the same time without changing the speedo drive

What diff ratio are you currently running? What car did the 5sp gearbox come out of?
How accurate is your tacho? All my old 4 speed piston Mazdas used to rev between
3300 - 3500rpm at 110kmh The 3sp autos rev a bit harder - 3700rpm.

As a comparison, in my RX626 (to take advantage of the bridgeport), I took out the
3.6 centre and am now running a 4.3 centre, HB 5 speed box and at 110Kmh my engine revs at 3900rpm.
My speedo now up to 110kmh is correct, then after that its about 10K out!

I got the 5spd out of a '84 929 wagon.
It's got the original diff which is 3.909 I think, if the source is correct.
I got larger tires put on as I though that would be the simplest way to give it more legs.
The speedo issue is that when it tells me I am doing 100km/h I am actually only doing 90km/h.

Thanks for the responses.

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