correct rear axle ratio
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Junior User
Joined: Jun 2011
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correct rear axle ratio
i have a problem. i have a 52 f3 with a 215 six. i have installed a t5 with a .77 overdrive 5th gear ratio. the truck also has a 9"ford rearend with a 3.25 ratio. running 235 15" tires (approx 28" high). the truck had trouble pulling 5th gear. (barely 2000 rpm at 64 mph.). by chance i tried two 215 r70 15" tires on the rear. the results were dramatic. the truck now will pull 2300 or so rpm at 64mph and will continue to pull strong and not bog. the only problem is that the 215s look pretty small inside those big balloon fenders. i figure i will have to change the ring and pinion gears. problem is i dont know what ratio to go with. i was figuring anywhere from 3.45 to 3.73. i wont be towing or hauling anything in the truck and i dont live in a hilly region. i would like to keep the cruising rpm in that 2300 range as the engine seems happy there. thanks for any help
Hotshot
Joined: Oct 2009
Posts: 16,173
Likes: 4,780
From: Burbank, WA
Here's the RPM calculator someone posted up here the other day. It's pretty handy and you can play with it and find the right combination that'll work for you. I'm guessing Vern's suggestion is pretty close. 
Calculate RPM
Calculate RPM
Fleet Owner
Joined: Jul 2004
Posts: 27,294
Likes: 1,055
From: NM
What height are the 215/70's? Take the ratio of the 235's divided by the 215's and multiply by 3.25. Just using quick numbers I get 3.80, assuming the 235's are a 75 profile.
Some T5's came with (I believe) .86 OD ratio, you "may" be able to switch the 5th gear pieces. 5th is in the tailhousing so it is potentially an easy swap. It would get you just about the same place.
Some T5's came with (I believe) .86 OD ratio, you "may" be able to switch the 5th gear pieces. 5th is in the tailhousing so it is potentially an easy swap. It would get you just about the same place.
Fleet Mechanic
Joined: Mar 2005
Posts: 1,511
Likes: 1
From: peyton, colorado
The 9" is pretty common and easy to swap ratio's. You could probably find the right pumpkin in a junk yard and just swap it. I run a 3.55 rear with my T5, 28" tall tires and a v8. I think I'd run around a 3.70 with a small six. It may turn alittle fast on the highway, but consider your mix of highway and town driving before making a decision.
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Junior User
Joined: Jun 2011
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rear axle ratio
i ran the rpm calculator and concluded that 4.10 would be the best ratio. the little 215 seems to be happiest around 2400 rpm. which would put me at 65 mph (fast enough to cruise with the old school suspension. thanks for the help
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