I m building a 46 Chevy pickup for a guy. He has a new blueprint 383 (435hp)with sniper fuel injection and a 700r4. Looking at getting a new 9” from speedway. Rear tires are 29” tall. I would like to choose a gear where a guy could run at 75/80 mph comfortably. We live in South Dakota and the interstate across the state is 80 mph.
3.50:1 is a good all around ratio. Had that in a 79 f100 with similar sized tires and it accelerated well(despite having a 302 lol) and cruised at a decent rpm.
https://www.project33.com/tiresize.cfm That’s the calculator I’ve used for the last 25 years lol. And he’s from Sioux Falls so it’s South Dakota calibrated lol Put in your variables and bob’s your uncle you’ve got your info
I would probably go a wee lower numerically than 3.50 if 80 mph was my target. You can do side by side comparisons with all kinds of variables with this calculator. https://www.grimmjeeper.com/gears.html
With your specs and my suggested 3.50 ratio. Too far below 2200 and you'll likely cause a lot of trans fluid heat from the I assume higher than stock stall in the converter. I guess if lockup function still works that'd be a non issue though
This should be the engine specs. BluePrint Engines GM 383 C.I.D. 436 HP Base Stroker Long Block Crate Engines BluePrint Engines GM 383 c.i.d. 436 hp base stroker long block crate engines will provide you a great foundation to return your vehicle to the performance level you desire. They feature a new one-piece rear main BluePrint engine block with 4-bolt mains and passenger side dipstick. Inside the block is a cast crankshaft and connecting rod rotating assembly, with 10.1:1 compression hypereutectic pistons hung on the end. The full roller camshaft provides .544 intake/555 exhaust lift with 230 intake/236 exhaust duration at .050 with a 110 degree lobe separation. The engines also include a new oil pan, a set of valve covers, and a timing cover to finish them off.
To get things moving well from a standing start you should be aiming for an overall driveline ratio of around 10:1. 700r4 has a first gear ratio of 3.06, so 10 divided by 3.06 = 3.268 rear end ratio. Because there is a fairly big jump between 1st and 2nd in a 700r4, you should probably go a bit more. As @The Chevy Pope said, 3.5:1 rear end ratio would be ok, but would be at 1640 rpm at 60 mph. A bit low. Step up to a 3.7:1 rear and you get 1849 rpm at 60 and 2465 rpm at 80...... and it would be quick off the line. If you are building it for the type of guy that is more interested in fuel economy than fun, go with the 3.5's.
With an overdrive transmission the minimum ratio would be 3.73 : 1. My off topic daily has 33" tires on it and the factory front and rear ratio is 3.73. It would probably drive better if the front and rear diffs were 4.11 instead, since the factory sized tires are the same diameter as yours. You don't want to bog your engine down, cruising at 2200 - 2500 RPM would be ideal.
I'd pick a 4.11 for those tires and the OD. You'd still only be turning 2600-2700 at 80 and it would accelerate pretty nicely.
A 3.25:1 rear end will have it cruising at 2100 rpm at 80 mph At 60 mph it is 1581 rpm but if you knock it out of Overdrive it will cruise at 2260 rpm The 700R4 has a deep 3.06:1 1st gear so the 700R4 with a 3.25 rear in 1st is the same overall ratio as a TH350 with a 3.94 rear in 1st 700R4 3.06 x 3.25 = 9.94:1 [1st] TH350 2.52 x 3.94 = 9.92:1 [1st]
I've done a pair of 3.5's and one 3.73. All with 29" tall tires. The 3.5 was right at 2 grand at 80mph. I would look at what your cam suggestions are before making a decision. However, the sniper will adapt to that cam without difficulty. A discussion with the owner about how responsive he wants it to be might be in order but with that being said, If the truck was going to pass another car, wouldn't it kick down a gear or 2 and haul???
My Henry J has a 327, 700R4, 29"tires and a 8.8 rear. I started with 4.10 gears and 1st gear was pretty much useless to me. I switched to 3.50 or 3.55 (I don't remember which) and I'm much happier. Gary
The 302 is happy around the 2100-2400 rpm range cruising if you have the right cam. The 3.50 gears would be my choice (correction: is what I have going under my 51 to start with but it is a big block). Give Quick performance a call and see how they compare with Speedway. I have a GM 10 bolt and a 9 inch from them and both are well built and can be tailored to exactly what you want.
A rule of thumb I've used is: 1st gear ratio x rear end ratio = somewhere between 9 and 10.5, while keeping an eye on the speed in the final gear at highway speeds. For example, I have a T5 with 2.95 1st gear and .63 in 5th. With the 3.00 rear ratio my thumb number is 8.95, just under the minimum recommendation and would work fine except for 5th gear would be something to behold. A better rear ratio for me would be 3.4 or 3.55 (8 inch Ford), which would give me thumb numbers of 10.03 and 10.47. I opted for 3.55 because of the .63 overdrive. IMO you would like a 3.50 gear and that would work well with your trans 1st gear.
There's "plenty..." of gear ratio charts in the web... pick one or two, experiment with ratios, pick...your own ratio. Do your own homework. You'll learn more that way !! Mike
This is what I use in my Ford. I have no O D. and I'm around 3200 RPMs at 65. Transmission is a Muncie M20, it had a 2.56 1st gear, now has a 2.20 after I blew it up a few years back. My issue was it lugging around town with the stock 3.56 rear. At 30 on the side streets i ran it in 3rd gear.
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