Have a look at this site:
http://wallaceracing.com/calc-gear-tire-rpm-mph.php
I've tried it out. Based on a 215/45x17 tyre, if your 2.4 does 3500rpm at 80mph, then a JTD ratios (if the actual gears fit, or if you just fabricated new gears with the same ratio) would give you 2720rpm.
If your 2.4 does more than 3500rpm at 80mph, then I've used it wrong...
Ralf S.
http://wallaceracing.com/calc-gear-tire-rpm-mph.php
I've tried it out. Based on a 215/45x17 tyre, if your 2.4 does 3500rpm at 80mph, then a JTD ratios (if the actual gears fit, or if you just fabricated new gears with the same ratio) would give you 2720rpm.
If your 2.4 does more than 3500rpm at 80mph, then I've used it wrong...
Ralf S.