Robert, you got something mixed up. diesels have longer ratios than petrol versions.
if a petrol powered cars usually do 100kph at 3.000 rpm, at half of max revs (6.000rpm), diesel needs to do the same but at 2.000rpm, again half of max 4000rpm. also, if same car was available with diferent engines, bigger engine will always be coupled to longer diff ratio because it can maintain decent acceleration with longer ratio and higher top speed due to higher power and torque output. perhaps that volvo 740 (760?) had diff from some smaller engined model?
some examples:
ford granada was available over the years with engines from v4 1.7 to v6 3.0 diff ratios ranging from 4.11 to 3.22. a friend of mine has a '74 granada that was originally 2 liter v6 and we slapped in 2.9 v6 from 1999. scorpio leaving original 3.9/1 diferential. max speed is still same as it was, but acceleration is much better than originally and even better than scorpio.
http://www.youtube.com/watch?v=JcmBEFFytRcladas, talking about rear wheel driven models, were available with diff ratios ranging from 3.7/1 for 1.6 liter sedan down to 4.3/1 for 1.2 liter sedan and even 4.44/1 for 1,2 liter station wagon. when you couple 4.44/1 diff to a 1.6 liter engine you get acceleration one would expect from a modern 2 liter car. and when you couple 4.44 diff to a 2 liter fiat engine, you get acceleration of a 3 liter car.
http://www.youtube.com/watch?v=488sWbbhmkgrussians made diffs for their racing lada VFTS, 160hp high revving engine, up to 5.7/1
top speed 140kph but coupled to a 6 speed gearbox it has insane acceleration
http://www.youtube.com/watch?v=Rq2y8_YvVe4