I have been doing the same calculation but for a different reason: I want
less sturm & drang from the engine room, not just better fuel economy.
Spending 17 hours at highway speeds with the"racket" reduced is a big
incentive for me.
I have driven to Midcoast, Maine the last two years to the annual "Tiger"
gathering at the Owl's Head Museum and both times got approx 25 miles to
Canadian gal. (~21 miles/U.S. gal.) for the 1500 mile return trip.
My car also has a 3:09 rear end, but I am running 205x50Rx15 rear tires
(23.07" dia) 901.6 revs/mile. Absolutely no clearance problems on stock,
unmodified rear wheel arches.
But I think you should consider a different size - try 195x70x13. Should
no problem, and you get 876 revs/mile, a significant difference: 6.45%.
Here's a great calculator to make this comparison easy:
Only problem is, tire manufacturers recommend you don't change tire size
more than 3% for fear of the risk of brake failure. I have no idea how
this danger might be, but I am running Wilwoods and vented discs up front,
with stock brakes in the rear, so I'm not too concerned. Do you have
One way or another - see you there....