help! how do i decifer the gear ratio of my rearend

kstones63 · Post by **kstones63** » July 9, 2007, 11:56 pm

If you go to your tire manufacturers website and get the specs for the tire you can figure the RPM with any gear ratio. Find the tire specs and get the tire revolutions per mile and multiply by the gear ratio to get the driveshaft RPM. If the final trans gear is 1:1 this will be your engine rpm @ 60 mph. Now you can change the gear numbers to get the RPM where you want it.
My 33's spec at 656 Revolution per mile.
656 X 3.73 = 2446.88 RPM in 3rd gear on my diesel which is accurate.

kstones63
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63 F100 Project
29 Ford Sedan Delivery
99 F250 SD, CC, 4x4 Diesel
95 F350 Dually Flatbed Diesel

6166 Junkyard Dog · Post by **6166 Junkyard Dog** » July 10, 2007, 6:36 am

Years ago when we ordered a new Pete or KW we order a truck with say a certain trans, rears and tires and would tell us top speed. Run low profile 24.5's say run 100mph or tall 24.5's run 120.00 on the road tractors by using this formula. Now many years ago on the Dump Trucks if it was geared to run 60mph and got a ticket for 70mph you could go to court and beat the ticket by showing the formula with tire, gear, trans, rpm range by proving how fast this truck can run, Don't know if that can be done in today's world

65Truck_Nut · Post by **65Truck_Nut** » July 12, 2007, 8:27 pm

Man you guys are awesome !! Its like standing around leaning on the bed of my truck having the most informative conversation ever. I think, I have talked myself into waiting till I can afford the 5 speed tranny. I have been paying close attention to the truck and i think the loud pipes are playing alot into my decision on the workload or over taching sound. because 70mph is 2,700 rpm now and that isnt too bad for a short stroke small block right?? and 65 is way too fast as far as JOHNY LAW is concerned here anyway. I drive a semi tractor trailer rig and i'm used to keeping the 500hp detroit wound up pretty good at times. but on the open road it purrrrrrssss at about 1,500 rpm. that may be having some affect on my judgment too. The fact of the matter is ,I still use my truck like a truck and pull a trailer loaded with scrap metal or hay once in a while and although I dream of cruseing it out on the interstate i live almost 100 miles from a 4 lane road. maybe by the time i get the $$$ and vacation time at the same time and get a chance to come to some of the events you guys get to enjoy .I will have the 5 speed. that way i can choose the gear and rpm for the application for that day. I am tempted to change out the rear gears with a 70 ltd with 28 spline count just to see what happens and change back if i dont like it. I just bought a new dual exaust from LMC (since I drive right by there every other day and beat the shipping charges by picking them up) the turbo muflers and 2 1/4 inch pipes out the back might make it sound better. I also have been driving my wifes 66 with a 352 (LONGER STROKE) and a 3 speed overdrive tranny Its Quite possable the only gear that needs changed is in my mind!! LOL

MadMaxetc · Post by **MadMaxetc** » July 12, 2007, 9:35 pm

banjopicker66 wrote:Since you live in hilly Arkansas, I would recommend AGAINST anything lower than a 3.25.
You will find that a 3.00 or lower will really dog your little 302 out, as it does best at higher RPMs than a 300 six.

3.25:1 is as low as I would go.