Ok, I have a 300 in my Bronco which I did a SAS in. I am running 38x12.50x15 tires. I think my running gear is 4.10s with a ZF5 and my OD gear is .76 ratio. I dont have my speedometer calibrated yet so i used gps from my phone to see how fast I am going.


I have figured I am 5mph faster than stock.

Anyways, at 55mph my rpm is 1800 and 65mph I am at 1900rpm. Is this good?

Will my rpm gauge be off to? I doubt it will since its hooked up via the engine.

 

A ZF5 never came stock in any Bronco, only the F250's and F350's, so did you swap in the transmission?

You've probably got the 8.8'' rear, so you can crawl under the truck and look at the tag that's on the rear end and see what it says. It could be anything from 3.08 to 4.10. If you can't find the tag, look up the axle code on the driver's side door jamb sticker and that should tell you what the ratio is.

 

I swapped it in and a 9 inch in rear and straight axle.

I think i had around a 4.10 gear.

 

The ZF and the 9'' are two of the greatest things Ford ever decided to put in a truck. Very nice combo.

From there, it's actually pretty easy. You can calculate your final drive ratio, then use this to calculate the speed at a given RPM and tire size.

Final drive ratio is just the multiplication of the total gears in sequence through the transmissions, transfer-case and the differential.

So at highway speeds, you'll have a 0.76 overdrive gear, you'll have a 1:1 gear with the transfer case in 2H, and you'll have 4.10 gears in the rear.

Multiplying a number less than 1 is the same thing as dividing.

0.76 x 1 x 4.10 = 3.11 FDR.

Then, using the FDR plug this into a calculator to figure the speed at a given RPM you're moving. I like to use Ring and Pinion's calculator.

Calculators

With a 3.11 gear, you're turning about 1512 RPM at 55 mph. This is about the ideal cruising speed at 55 for a 300 with a carb. The stock 300 puts out all of it's torque at 1400 rpm. The EFI 300 is a little higher up in the rpm band (closer to 1800), but it also produces a little more torque and has a flatter power curve.

At 65 MPH with a 3.11 FDR and 38'' tires, you're turning around 1787 RPM. At 75 you're turning around 2062 RPM. Past 62 or 65 MPH, the 80 to 96 series of trucks encounters significant wind drag, which will be emphasized with your tire size. This will have a huge impact on gas mileage. For best cruising, I'd recommend 60 or 62 mph on the interstate, and it should give you the best MPG and ease of driving at that speed. No downshifting, etc.

Bear in mind, when you use this, you can put in either the FDR or the rear axle ratio. I like to put in the FDR and look under manual for standard transmissions, as this will let you get more accurate and won't be forced to look at a 0.80 overdrive gear as opposed to a 0.76 overdrive like you've got.

 

Cool deal. Thanks. YeH, my 300 is EFI. Maybe im making 15mpg at least.

I plan to go to smaller tires one day; maybe 36s.