how short will first gear be??

87gtconv - just do the math to check it. 245/45/16 tire is
((245/25.4)X.45)X 2 + (16") = 24.7" tall. So, for each tire revolution you cover PI X 24.7" = 3.14159X24.7" = 77.5". Now we need to get to engine RPM (rev. per minute). So, 80 mph = 1.333 miles per minute. 1.3333 miles = 7040 ft/minute = 84480 inches/minute. Divide that by the number of inches you get with each tire rotation -- 84480/77.5" = 1090 tire rpms. The driveshaft is turning 3.73 times that amount -- so at 80 mph the driveshaft is turning 4065 rpm. With a .625 5th gear, the engine should be turning 4065X.625 = 2541 rpm.

So, 2600 rpm sounds just about right -- assuming your speedo and tach are accurate.

By the way you can check your odometer and your speedometer on the Interstate. Odo is easy - just compare over a 10-20 mile run the mileage on the Odo compared to the mile markers. Speedo takes another person in the car (you must trust me on this). Cruise control helps -- set it at 60 mph, and run 1 mile based on the mile markers while timing it. At 60 mph 1 mile should take exactly 1 minute. Of course the closer you can hold it at 60, and the longer a distance you can run by mile marker the more accurate your check will be. Just do the math if you want to check it at 70 or 80 mph. 70 mph should go by in (70 mph/60 min./hour) = 1.16666667 miles/minute. So, 1/1.666667 = .857 minutes to cover a mile; .857 X 60 sec/min = 51.5 seconds to cover a mile at 70 mph. Same math -- at 80 mph you'll cover a mile in 45 seconds. Your speedo error is the same ratio as the time error.

At 60 mph if it actually takes you 63 seconds to cover a mile -- then 63/60 = 1.05 --- your speedo is 5% slow. Same error calculation on the odo. If the mile markers show 20 miles, but your odo reads 19.5 miles -- your odo reads 2.5% low. On some vehicles (like my Volvo) the speedo and odo are calibrated separately -- so one could be accurate, but the other one isn't. Don't know if the Stang is that way or not. If they're not separately calibrated, then your speedo will be off the same % as your odo - and you can just run the mile marker test to determine both speedo and odo % error.

Clear as mud?

Michael makes good sense to me and I think I have the right speedo gear in for the 3.73 gears. BTW I use to test the speedo for my Dad just like you said of course that was in 1952. In Cali those mile posts are still on the blueprints but a lot have been lost on the highways.

That means somebody is even older than me....

Foxfan - '....deep for me....' What - multiplication and division?