What kind of MPG's are you getting???
I checked my speedometer against my phone and its accurate. A better check would be against my garmin gps. But thats not my concern.
I have 185-80-13 as is listed in the door.
With the 4 speed manual the Tach reads 3600 rpm at 65 mph.
Using rpm calculator It should be 3000 rpm.
(3.9 diff, 24.7" tire, .885 5th gear)
The truth is probably some where in the middle.
Where I live, My daily commute is flat. The speed limit is literally 70 mph. But you better be in the slow lane. 80 is acceptable.
75mph puts me at nearly 3500 rpm. Too high for cruising.
80mph = 3750 rpm
Last mpg check was 26 mpg. If i get the rpms down, I bet i could get close to 30. I do a lot of hwy.
Change the tire diameter to 27 = 300 rpm drop.
If only I could find some pug rims I have some 27 x 8.5 R14 in the back yard.
Once I get a .78 5th gear I could be cruising 80 at 3000 rpm and keep the turbo lightly spooled.