Tire size question

[AzBroncoGuy]

I'm finishing up the restoration of my 74 Bronco. Looking at 33 x 10.5/15 tires mounted on 15 x 8 rims with a 3.79" back space. Bronco has a 1" body lift and disc brake conversion (GM). The tire in the photo is a worn out 31 x 10.5/15.
Any information is greatly appreciated.

 

[bigmuddy]

What is the question?

 

[Chiclassics]

Assuming the question is what is the right tire size....
here is additional info needed
1) What is total lift , in addition to the 1" body lift is there a suspension lift?(looks like it from pics)
2) Is it cut or uncut rear fenders? if uncut you may be somewhat limited in tire diameter (and the lift plays a role)
3) Whats your gearing - this is really to determine you can have larger tires without affecting performance


Typically for a 3.5inch overall lift with stock 3.5 gearing - 33inch tires are good. I run a 12.5 for a wider stance but 10.5 is good as well for normal street. (assuming its cut). Lets see above answers and ill suggest you best size.

 

[AzBroncoGuy]

I'm wanting to know if the 33" tire will work for my application. Gearing and suspension are stock. Rear fenders are cut as well. I've done some research and measuring and it appears that it will be OK for clearance on the fenders/wheel wells. It does like there could be a rub on the left side radius arm at full lock

 

[Chiclassics]

How much clearance you have on rear fender with 33? the cut can vary so (post a pic), ideally you need to have more than 1inch clearance for it to not rub into fenders. And ya radius arm could be a slight rub with stock height. Ideally 31s are good with no lift /1inch lift.

 

[AzBroncoGuy]

Here's a photo of the rear tire (worn out 31 x 10.5/15). I should have about 2 1/2" clearance with 33's from the fender as it measures 4 1/2" now.

 

[Oldtimer]

3½ of clearance.
The tire radius increases by 1, from 15½ to 16½.

 

[Broncobowsher]

Yes, 33s will work. With the cut fenders like you have, don't even need the lift you are running.
The flip side, do you have enough gearing to step up to 33s?

You might fit even larger tires, but if you actually wheel the rig, the tire rub will be present and mild with 33s. And gets worse as you get larger, or start limiting travel. And Gearing gets worse with larger tires as well.

 

[Chiclassics]

First of all that looks like lot more than 41/2 inches right now assuming those are 31 rollers you have.

ya go with 33s. its a good balance between looks and stock gearing. Because your rear fenders are cut more than one would typically expect.

However note that you will lose some power with stock gearing of 3.5. Typically for a 33-inch tire a proper gear ratio is between 4.10:1 and 4.56:1. This will put the engine back into its proper power band for normal driving. Make sure you have good driveshafts if you keep stock gearing. But because you are going with leaner width of 10.5inch(instead of 12.5) you might be okay with 33s. If you go to 35 you need to seriously think about changing diff gears to higher gearing to keep engine performance and acceleration.

 

[AzBroncoGuy]

Thanks for the information. It make good sense. Both driveshafts have been rebuilt (along with most everything else), This Bronco will probably never see dirt other than a driveway now so power issues and tire rubs from rock climbing will be a thing of the past.

 

[Chiclassics]

One adjustment you could make to "trick" the system is to adjust your C4 modulator (one full turn in the modulator adjustment screw)to shift delayed so it stays in lower gear 1sec more than it would normally do, This will compensate the bigger tire size with stock diff gearing. Why I LOVE manual 3 on the Tree anytime hands down.

 

[Broncobowsher]

Thanks for the information. It make good sense. Both driveshafts have been rebuilt (along with most everything else), This Bronco will probably never see dirt other than a driveway now so power issues and tire rubs from rock climbing will be a thing of the past.

It isn't really a power issue if running too tall of a tire for the gearing. Besides being a slug off the line that you really have to bury the throttle just to keep up with traffic. There is the transmission cooking itself with being against the convertor all the time. The trick for adjusting the modulator won't do anything for fixing the slip at cruising speeds. All that work being up against the torque multiplication of the convertor isn't good for fuel economy. Getting torque multiplication through the convertor is an inefficient way of multiplying torque. Using real gears are way more efficient than using the convertor. As the torque convertor is way more efficient at turning fuel into waste heat, that is wasting fuel.

 

[DirtDonk]

Just out of curiosity, measure the distance between the top of the axle tube at the bottom of the frame rail, front and back. Post up the numbers you have and then you’ll know exactly what suspension lift is present.
The body lift is easy. Anything over 3/4 inch is a body lift.

 

[Chiclassics]

Agree @Broncobowsher getting torque from diff gearing is the right way than from transmission. But if modulator is set at normal set up without changing diff gears, the transmission will stay in lower gears at cruise if the vehicle momentum is not getting picked up. Hence the "trick" is a bandaid but not ideal. It will help get the needed initial motion from 0-15mph and staying in motion becomes easy. It will cause bad fuel economy and a "little" load on transmission.

 

[ntsqd]

In all known to me US autos the hot fluid in the cooler lines is coming directly from the converter. A non lock-up converter, like what a C4 has, normally slips about 3%-5% when cruising (AKA "stalled"), that is not what Broncobowsher is talking about. The converter will slip whatever amount is needed to get enough power to go that speed in those conditions. When the converter isn't stalled all of that slippage generates a HUGE amount of heat.

 

[AzBroncoGuy]

Just out of curiosity, measure the distance between the top of the axle tube at the bottom of the frame rail, front and back. Post up the numbers you have and then you’ll know exactly what suspension lift is present.
The body lift is easy. Anything over 3/4 inch is a body lift.

3 " on the front and 5" on the rear. The body had been lifted 1"

 

[DirtDonk]

Well, that doesn’t make sense when you take the pictures into account. Looks like you have a lot of lift, as others have stated already. But 3 inches in the front is 4 inches lower than stock. While 5 inches on the rear is 1 inch lower than stock.
Gets more interesting every day!

 

[Chiclassics]

I disagree(respectfully) some of it. The affect(lack of power, to be technically correct lack of torque ) you will see of bigger tires is at points of "acceleration" which is more at start when engine is trying to move vehicle from standstill to motion. (rotational mass) The issue is less at cruise, a body in motion tends to stay in motion. Especially when the gear diff is not noticeable it wont slip frequently at cruise speeds. A torque converter uses transmission fluid to compensate for the RPM differences between engine and transmission and has high stress when someone is trying to accelerate which causes max diff between RPMs. So a modulator adjustment will get the vehicle stay little bit more in lower gears of c4(2.4, 1.4) to get to speed and compensates for the gear loss. But if shift points are stock then you have to increase your throttle to compensate for power loss (to be technically right torque loss as power o/p is same at given engine rpm), that causes higher RPM differences between engine and transmission causing more heat. The effective gear ratio difference being not that big for a 33inch tires i.e 3.55 vs 4.1, your effective gear ratio is 3 meaning 0.5 overall gear ratio loss. its not noticeable on flat terrains. However if you go to 35 your effective gear ratio is 2.8 which is 0.7 difference/gear loss and will cause drivetrain damage over a period of time if gears are not changed . The reason engineering suggests gears to be changed at differential is two things 1)its the final stage of torque multiplier 2) its highly costly to change gears in transmission/change engine power to compensate for lost torque-diesel engines have higher torque curves. But in. principal what matters is overall gear ratio for wheels to turn which is transmission gear ratio x diff gear ratio.

All that said 33inch tires wont cause a noticeable difference on flat terrains if they are 10inch wide. 33 12.5s will cause some noticeable diff as you are fighting more rotational mass. 35s is when you really need to change gears imo.

 

[ntsqd]

You have testing data to support these assertions? Because your points are incomplete and don't totally support your position.

Never mind, I'll just disagree and move on.

 

[Chiclassics]

All good sir, ok to move on - trying to help.

Its super simple: overall drivetrain gear ratio- there is a reason we have transfer case to go from High to Low for better torque ratio for off-road(big tires). If that can do it its brother transmission can do it. It is not preferred in "automatic" due to involved fluid dynamics. Differential gearing change is easy and cost effective. But based on engineering principles gearing at transmission also is equally effective. Hence my tip -its the best bandaid. Give it a try and tell me if its not working magic.

I have masters in engineering(mechanical and electrical) and worked on diesel engines, jeeps, vws and broncos and even on train engines as a kid with my dad.