So I just determined my rear diff is a 3:54:1 ratio, which was standard for the US, while at the same time in 70 a 3.07 was standard for Britain. I also understand that 3.31 were used in Britain on the earlier 3.8Ls.
Since I'm rebuilding the whole rear end the question then is do I want to stay with the 3:54 or change and why? I'm thinking the change from 3.32 to 3.07 in Britain may have been driven by the high cost of petrol over there and to 3:54 for the US to compete with the US muscle cars of the day. Today with higher highway speeds here and gas prices going out of sight I have seen a lot of people talking about going to 3.07 for better hwy cruising, lower top end RPMs and fuel economy . I would appreciate any reflections on this and differences drivers may have seen by making a rear end ratio change. Thanks.

 

Definite vote for the 3.07. I've been up against 3.54 cars, and there is no difference really in off the line speed - the engine is so torquey it just pulls from low down. Big difference is in motorway cruising, where at 70mph'ish the 3.54 is too loud and buzzy. The 3.07 is much more relaxed. With the 3.07, 70 mph is 2850 revs. (According to tyre sizes etc) I'm not interested in economy - didn't get an E Type for that! But the 3.07 just seems far more suitable. Which is why Jag wanted it over here!

 

Thanks so much for that respective!!

 

Just a bit of advice. It's not as easy to change to a 3:07 as it sounds. Although the dif in an E-type is a Dana 44, the splines on the pinion are different than most other Dana 44 gear sets. So, you have three options that I know of. 1) Find a good 3:07 from a home market E-Type and have it shipped over. I tried this but could not find one that needed a new home. 2) Buy a new R&P from SNG Barratt. They say they have the appropriate parts that fit but they are over $1,600. 3) Buy a new yoke to match your new pinion. I believe that Moss has a kit with a ring, pinion, and yoke that work together to serve the purpose for about $500. Since it is a Dana 44 there might be other companies that are not Jaguar specific that can supply something similar.

Good luck. In the end it will be worth it to cruise the US highway system at under 3k RMP..