Which rear axle ratio for cruising

[RV-4]

I have a 1969 Seven S 3. I would like opinions on the best rear axle ratio for cruising at lower RPM ?

I have a 3.9 and a 4.2.

Cheers

George

[Jonathan Kay]

Four speed box with direct top?

What wheels and tyres do you have/ would you like?

What engine speed would you like at 70 mph?

Thanks

Jonathan

[ScottR400D]

3.9 will give lower revs for a given speed over 4.2, other things equal. 

[Paul Richards]

3.9 is higher than 4.2 so will give you more mph per 1000 revs. 

[OldAndrewE]

I guess that has a Ford axle and a Xflow engine.  For touring I would certainly use the 3.9 out of the two you have.  What wheels and tyre size do you have?

Don't forget your speedo calibration will be different for the two ratios

[RV-4]

Thanks Guy's for your input

Cheers

George

[jbcollier]

I also have a '69 S3.  I'm happily running the 3.9.  I added a Quaife diff so it wouldn't just spin up one tire.  Now I can spin both in second under hard acceleration.  There's also 3.7x, etc, and even a 3.3x ratio available but I find the 3.9 to be just about right with a stock-ish crossflow and the stock gear box ratios.  Bit buzzy on the freeway but nothing  a short stroke engine can't handle with aplomb.

[Iamscotticus]

Consider using an air fuel ratio meter for checking your burn at those cruising speeds.

Short term rich / lean may not be an issue like distance crusing will be.

I sure would like to see pics of your car