JChase

So I scrapped my plan to do a T5 swap when a I got anAOD with all the conversion goodies out a 67 for 300 bucks. My question is about rear end gears(again). 3.55's or 3.80's in my 8 inch? Im looking for a decent launch from a stop, yet decent cruising RPM's at 70MPH. ALL opinions are welcome. Thanks

LCC

I'll step in before all the "conservitive guys". 3:55 are a good all round gear but 3:73 hands down for pep and fuel economy. I on the other hand love my 4:10 with the AOD, but then again I like the feel of power in every gear...Have never looked back for anything lower....

JChase

Where do your RPM's sit at 70MPH? Thanks

bodyputtyless

I'm about to swap out my FMX with an AOD from the 89-93 Mustang. I'll be just cruising the streets. I have 3:00 opens in my rear end now.
I also am wondering if I should swap out the stock converter for a 12" upgrade. IYO should I go with lock or non lock? Are my rear end gears what I need for cruising or will I be spinning hard and going no where?

LCC

I calculate 4.10 at 70MPH at about 2550RPM with 26.5 " diameter tires. If you go with the 3.73 it's down just over 2300RPM with the same tire size. 3.55 it's down again to 2200 RPM. Your tire size will differ, taller the tire the less RPM...

JChase

Whats the formula you use to calculate that? That would help me out alot!!!!

THUMPIN455

(gear ratio x 336) / (rpm x tire dia) = mph
or
(3.55 x 336 = 1193) / (2200 x 26 = 57200) = 47.95 mph.

By mixing the inputs around you can get different properties. Such as what gear at a certain rpm and tire size.

There is a book called something like 'Gearhead Math' and you should be able to find the formulas on the HOTROD or CARCRAFT sites. They have been printing the info for years.