gear ratio for 35-36" tires

[JMiller81]

I know this has been discussed a million times but I need an accurate answer.

I have an 81 Bronco with a 302 and a C6. The rear is a 9". I am currently running a half worn set of 36" Buckshot Mudders. The tires measure out with a tapemeasure to 34.9". I was planning on installing a set of 4.56's but now I have some concern that this might be to much gear for the street. I have been told that 4.11's would be better but it seems like the concensus on this site is to run 4.56's. JBG also suggested 4.56.

The truck is not used off road so I need something that isn't going to **** the truck if I am on the highway.

Thanks for your help.

Jim :-B

 

[Justshootme84]

I would recommend a 4.56:1 ratio

 

[Seabronc]

It mainly depends on what your normal cruise speed is. If this is only a street machine then you probably want it to run in the most ecconomical end of the RPM range. The acceptble cruise RPM for these engines is 2000 - 3000 RPM with the optimum range being between 2300 - 2600 RPM.

If your normal cruise is 65 MPH then you can calculate what the cruise RPM will be in 3rd gear without OD by multiplying the M.P.H. X Gear Ratio X 336 / Tire Diameter = RPM.

So in your case 4.11 gears would make it 65x4.11x336/36=2493RPM while 4.56 gears would make it 65x4.56x336/36=2766RPM.

I think if I were only going to run it on the street and wanted the best MPG I would even consider 3.54 gears which would yield 2147 RPM at 65. I know that would make the power end suffer a bit and it wouldn't be a rocket off the line, but it would definately help the economy end. And if I wanted to sacrifice MPG for more power off the line then I'd consider going to 4.11.

3.54 = 2147 RPM at 65

4.11 = 2493

4.56 = 2766

In fact that is the very reason I like my 3.08 with 31 inch tires. I normally cruise at 65 and it yields 2170 RPM which is much easier on the fuel economy and gives me a reasonable acceleration. I am little beyond the Rabbit starts from red lights age and can wait a couple more seconds to get up to 65 MPH.

If off roading were my big thing I'd go for 4.11 or 4.56 to move my engine up in the power range and sacrifice the MPG.

That's my .02 /emoticons/[email protected] 2x" width="20" height="20" /> . Hope that helps.

Good luck on your decision,

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[$100.00Bronco]

With a 302 I would not recommond 3.54 gears. You would be putting the pedal to the floor just trying to keep up with trafic. I have an 83 with a 351 and a C6, I am planning on 36x1250 swamper radials with 4.56 gears. You already know how it feels with the 4.10s so if you are satified with the power stick with what you have. if you want more power and think 4.56 might be to low you could try 4.29 or 4.30s.

Dennis

 

[$100.00Bronco]

With a 302 I would not recommond 3.54 gears. You would be putting the pedal to the floor just trying to keep up with trafic. I have an 83 with a 351 and a C6, I am planning on 36x1250 swamper radials with 4.56 gears. You already know how it feels with the 4.10s so if you are satified with the power stick with what you have. if you want more power and think 4.56 might be to low you could try 4.29 or 4.30s.

Dennis

Oops, sorry. I wasn't thinking about the D44 front. I don't think you can find anything to match 4.29 or 4.30s.

Dennis

 

[Seabronc]

With a 302 I would not recommond 3.54 gears. You would be putting the pedal to the floor just trying to keep up with trafic.

Actually that is not true, the lower the number the lower the RPM at 65 asnd the less you would have to press on the pedal to travel at 65. What you will experience is a loss of power off the line, like I previously posted.

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[$100.00Bronco]

I"m talking power wise. It takes a lot more power to turn 36" tires with 3.54s than with 4.10s or 4.56s. I had a 76 F250 Highboy with 4.10s and 35" BFGs. I had a built 390 that could smoke the 35s off the wheels if I wanted and get 14+MPGs on the highway. I broke a ring gear being dumb and thought I would try 3.73s to improve MPGs. My power went down and I lost about 2MPGs. I know the RPMs drop with higher gears but you need the power to turn the tires and that lower RPM. If my 300+ horse 390 struggled the 302 will really struggle.

 

[Seabronc]

I doubt that anyone who had your engine in it would be looking for economy in running. My point in the first post was to give the information that allows JMiller81 to make the choice between economy running as opposed to power running or something in between.

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[BLADE262US]

The bronco is in no way an economy vehicle thats not why we drive them they are as areodynamically incorrect as you can get but they look cool as **** . I run 35/15.5/15 super swamper TSL,s on my 89 it has a 5.8 with a C6 tranny . I put 4:10,s in it and it does pretty good but if I had to do it over it would have 4:56 or even 5:13 in it . The bigger tires affect the final ratio also . I had an 83 3/4 ton F250 5.8 with 3:55 and a 3 peed with creeper tranny . I put a set of 40 inch groundhawg,s on it after running 33,s and after getting up to 55in 3rd gear Id shift into 4th and it would start lugging the motor down because of the loss of mechanical advantage through gearing and would actually slow down and Id have to shift back to 3rd. It was a yard truck after all that so was no big deal. By not gearing low enough for the tire size and weight you will add alot of stress on the rest of the drivetrain such as the build up of heat . If you gear too low you take the motor out of its operating range and it will not perform to its potential . A 302 has a lack of power anyway so your depending on the mechanical advantage after all that entire ratio is dependant on a constant input speed from the motor once under load if there is no mechanical advantage it will not maintain that speed and will slow down . If it were mine 4:56 it would be . /emoticons/[email protected] 2x" width="20" height="20" />

 

[Seabronc]

I agree with Blade262us in general, what I said about economy running was in reference to a Bronco only 13+ MPG as opposed to 9 /emoticons/[email protected] 2x" width="20" height="20" /> . I am now getting just over 13 and when I get the time I'll be making a few more mods to get it up around 20+. The first will be to get rid of the mechanical water pump and add a variable speed electrical pump. Then put her on a diet and drop several hundred pounds, carb modifications to lean it out a little or perhaps a more efficient carb than a Holley, etc.

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