Gearbox and diff

I have a 5 speed gearbox with a USA spec diff in my series 1.5. I’m told that in 5th it gives a similar gearing to a UK spec car with original 4 speed box in 4th. Does this stack up? Surely the point of having a 5 speed is to make higher cruising speeds more relaxed, so I’m wondering why the guy who restored it originally didn’t just change the diff to UK spec and keep the original box. The 5 speed cost him 4.5k pounds

Maybe his original gear box was toast.

Are you sure the original gearbox was a manual (not an automatic)?

I’d be very surprised if that was true unless it was built with very close ratio gears. 5Th is generally an over drive gear whereas 4th is 1:1 in the original box. What are the rpms at 70?

Not possible to know unless you can tell us the ratio of 5th on your gearbox. The difference between a 3.54 and 3.07 rear end is about 15%, and though a 5th speed might be as high as 0.85 to 1, typically it is around 0.70 to 0.75.

be greatful. with the 354 you get rocket ship acceleration and the overdrive 5th gives you the same cruising you would get with a 307. This is the best solution if you are willing to spend the $.

Plus it’s not all that easy to get someone to change over the 354 to 307. There are issues.

The original box is manual. Not sure on the differential except that its the original USA spec one from 1968. When I’m next out in it I will look at the rpm at different speeds/ gears. I was out in it last night and when accelerating hard I was changing up when it was at 6k rpm but it felt more like 4.5k rpm. I think the rev counter is misreading too. It has a megajolt electronic ignition which may be causing that. It does drive very well though. The gearbox was from E-type Fabs Ltd but no detail on the spec except 5 speed.

If you read the number on the tachometer, can you match it up to Bentleys to tell the ratio?

Or the diff may have its original tag showing the ratio:

does this help?