Gear Ratio Question

[Jbwalsh76]

See below pic...I just tried the string trick to try and estimate my gear ratio, but I'm not sure if I would count this as 3.5 or possible 4.11. You can see the blue tape is now at 12 o'clock, but started it in the 6 o'clock position.

I'm going to take it to a local shop later this week for a better estimate, but figured I'd bounce this off you all first.

I'm thinking I need to regear in order to get a bit better on the highway. Suggestions for proper gear ratio would be greatly appreciated

FYI - My rig is a '77 with a 302 with a C4 and Dana 20 behind it. Have a Dana 44 up front. My Marti report shows a LSD of 3.50.

My '77 is primarily just driven around town, but trying to get more highway friendly so I can get to some of the nearby trails without the 302 sounding like it is about to fly away the RPMs are so high (I don't have a tach...going by ear at this point)

 

[DirtDonk]

You really do need at least a temporary tach. Can tell a lot from that and an accurate speedometer.
Some hand-held tach device, along with a good GPS in your phone or known-good car pacing you, and with a known tire diameter you've got your gearing.
But while we're at all this suggestioneering, measure your tires exactly. Do it from the center of the hub to the ground, then double that. That's your rolling-diameter and is what the online calculators use to determine gearing/engine rpm and such.

So at least we know that it came with 3.50's originally, but no telling what's in there now. However, it's a good chance that whether it's 3.50's or 4.11's you are most definitely NOT over-revving it on the highway. Your 302 can handle even 3500 rpm all day long. And with 3.50's you're probably nowhere near that even with stock original tires.

For the tape trick, you're saying that was one single rotation of the tire? Looks more like 3.50 to me, but it's hard to tell.
Another method is to kill two furds with one stone. Remove the front diff cover and count the teeth. This 2-birds scenario is so you can say you changed the gear lube and inspected all the bits inside.
If you've never done it, then it's a pretty good bet that the PO did not either. And that means it's about time for a gear oil change.

Paul

 

[Nothing Special]

So that shows how far the driveshaft turned when the tires made one full rev? The string shows a little over 4 full revs, and to me it looks like the blue tape is at about 3:00. If it started at 12:00 that's closer to 3.23:1. But if your 12:00 to 6:00 is more accurate than what I'm seeing from the picture, then ~3.5:1 would be the answer.

But generally speaking if you're going by ear you are too worried about engine speed. 302s were built to rev, and will easily handle spinning faster than most people think when they go by ear. That said, if the noise is annoying that's a factor too. But if you slow the engine down enough to avoid that you'll likely be losing performance. And THAT said, I have 4.10 gears with an overdrive transmission and 33" tires. Everyone says I should have 4.56 gears for performance, but I wouldn't mind 3.54 gears for the noise. So I'm certainly sympathetic to your position.

 

[Jbwalsh76]

Thanks Guys! Yeah...that was one full rotation. string was taped at 6 o'clock to start.

I'm still pretty new to the Gen 1 Bronco, so I'm sure I am a bit too worried about the engine speed.

BTW...I am driving on 33x12.5x17 tires.

Any recommendations on a temporaty tach?

 

[Broncobowsher]

Start at 6, end at 12, I count 3.5 wraps of string.

A bit of error from gear backlash.

 

[thegreatjustino]

Start at 6, end at 12, I count 3.5 wraps of string.

My calculation as well

 

[Jbwalsh76]

So assuming I am working with a 3.50 gear ratio...should I be good to go with the below setup, or should I consider changing and do you think the change is worth the $$$$?

302, C4, Dana 20 TC, Dana 44 axle up front and 33" tires

Just pulled the trigger on a Tach. Hoping to have that up and running before end of the week...

Thanks again for the input.

 

[thegreatjustino]

Combining 33" tires with the factory C4 and 3.50 gears would lower the RPMs compared to the stock configuration while cruising on the freeway. You should already be running lower RPMs than it originally would have. Trying to lower the cruising RPMs any further with a gear change would reduce performance.

 

[Broncobowsher]

And severely loading the transmission, shortening its life.

 

[Jbwalsh76]

Got it. Appreciate the feedback.

This site is best around!

 

[jckkys]

I also have the original 3.50/3.54 gearing in my '77. The tires are 30.5" diameter. With this, the engine is only turning a little more than 2500 RPM at 65 MPH, my top speed, as the MPGs drop quickly cruising any faster. The 29" 235X78 OE tires would be turning a little more than 2600 RPM. They are both in the sweet spot in terms of highway cruising with the original type 289-302-351Ws.

 

[DirtDonk]

BTW...I am driving on 33x12.5x17 tires.

So assuming that your 33" tires are actually 32-ish (we don't know for sure until you measure them. Traditional tires sizes on 15" wheels were often as much as 1.5" short), and assuming that your speedometer has been corrected for the tire change (was it?) and assuming an extra 100-250 rpm for the C4 auto slippage, we can figure approx. rpm.

So if all of that holds true, then your engine rpm at 70mph is approx. 2700 or so. Perhaps not in the perfect sweet spot, but not high at all either in the scheme of things.

For other data points, engine rpm at 55mph would be 2100, 60mph would be 2300 and at 65 you'd be at roughly 2500 rpm.

See if this is how yours works out once you have the tach installed, then you can figure out if your speedo is correct. But check the tires too, so we have an exact measurement. If for no other reason than it would be good to know if tires for 17" wheels have the same discrepancies as those for 15" wheels.

Paul

 

[Jbwalsh76]

Tires are just shy of 33" and the Speedo was corrected for these tires.

Thanks for the info the approx rpms. That is very helpful. Will obviously know more once I get the Tach installed.

Appreciate it.

 

[Apogee]

You actually don't need a tach if you know everything else, as you can calculate RPM. I'd suggest having a tach, as that's easier than crunching numbers while you're driving, but for the purpose of sitting at a computer, something like the GrimmJeeper calculator should tell you engine RPM in all gears and at all speeds if you're willing to input the specs on your drivetrain (transmission, t-case, diff gears, tire diameter). Most calculators don't factor in torque converter slip, so you can assume that RPM's will be slightly higher than the calculated value.