Thread: Which rear gears for Ford 302/T-5?

Tweet

As some of you know, I'm building a 37 Ford tudor sedan. I'm trying to determine which gears to put in my 9" Ford rearend. I have a '69 302 bored to 306 with custom cam, GT40P heads, Weiand Xcelerator intake, Holly 4150 carb (320hp) mated to a T-5 trans. Currently my rear tires are 28" tall but may switch to 29" if I go with 18" rims on the rear. I want the car to be very highway friendly for road trips but also have some punch around town. Here are the breakdown for rpms at 60 & 70 mph with both 28 & 29" tires in 5th gear with 3.50, 3.70 & 3.89 gears:

28" 60mph 70mph
3.50 1714 1999
3.70 1812 2113
3.89 1905 2222

29"
3.50 1655 1930
3.70 1749 2041
3.89 1839 2145


So, if cruising in 5th gear at 60mph/1655rpms, (with 3.50 gears & 29" tires) would the car totally dog out trying to accelerate to 70+ without downshifting? At 70+mph, what rpm level would give the best performance/economy?

I just want to gear the car for comfortable all around performance while still trying to optimize a little fuel economy, you know?

Any thoughts?

Thanks.
Randy

3:89's get my vote. If you weren't running that 5th overdrive I would say 3:70's, but that 5th will make a lot of difference. I ran 3:70's in my 27 (also a 302 Ford) with no overdrive and it was about perfect with 30 inch rear tires. It would go right down the interstate at 70-80 mph without overrevving. You also need to get that cam into it's operating range.

Of course, what do I know. I'm running 4:30's in the 27 now with no overdrive.

Don

What is the operating range of your cam? Overdrives can be nice ,but if they get rpms below the cam range performance and mileage will suffer,

I have 3.89 gears in my '32 with 30 inch rear tires matched to a 700R4 overdrive. It is perfect for what I like--cruising around town in drive and cruising in overdrive at 70-75 mph on the interstate all day long and never exceeding 2200 rpm.

Originally Posted by 53 Willys

What is the operating range of your cam? Overdrives can be nice ,but if they get rpms below the cam range performance and mileage will suffer,

Here's the spec sheet from the cam builder. I was originally going with a C4 auto & 3.0 rear gears but decided to switch to 5 speed. I'm not quite sure how to interpret these numbers.....

With those cam specs, I would say definitely 3.89's.

is 5th gear .68??? if so i'd go with 3:73's

Originally Posted by hotrodstude

is 5th gear .68??? if so i'd go with 3:73's

Thanks for the input, Stude. Yes, 5th gear is .68 but the rearend is a 9". I don't think 3.73 is an option for 9". only 8.8. For 9" I think it goes 3.50, 3.70, 3.75, 3.89 and up. I know a lot of later model Mustangs run 3.73s with the T-5 but they're usually running 26" tires. Since my tires are 28", the 3.89 would probably accomplish the same thing.