31x10.5 tires on Bronco/3.55 gear??

[Minh Lang]

I'm not sure if I posted to the right group. If not, I apologize.
I have a full-size Bronco which has 235x75R15 tires that I will
soon need to replace them. I'm wondering if I can use a bigger
size tires (31x10.5 R15) with the existing gear ratio
of 3.55? Will I get lower MPG and less power if I don't
change the gear ratio? It has an automatic transmission,
with V8 302 engine. I do occasional off-road driving, and
it's my daily commute vehicle (mostly highway).
I would appreciate any comments/help. Thank you very
much.

== Minh ==

+------------------------------------------------------------+
Minh Lang, Software Engineer - Jet Propulsion Laboratory
Instrumentation Systems Group - Instrumentation section 375
Note: My employer has nothing to do with what I said here...
+------------------------------------------------------------+

[Jim Chott]

In article <1992Oct22.1...@elroy.jpl.nasa.gov>, Minh Lang <mi...@inst-sun1.jpl.nasa.gov> writes:
>
> I'm not sure if I posted to the right group. If not, I apologize.
> I have a full-size Bronco which has 235x75R15 tires that I will
> soon need to replace them. I'm wondering if I can use a bigger
> size tires (31x10.5 R15) with the existing gear ratio
> of 3.55? Will I get lower MPG and less power if I don't
> change the gear ratio? It has an automatic transmission,
> with V8 302 engine. I do occasional off-road driving, and
> it's my daily commute vehicle (mostly highway).
> I would appreciate any comments/help. Thank you very
> much.

You will have less power with the bigger tires since they are taller.
The 31x10.5-15 is about 10-15% taller than stock tires which will
change the speedometer by the same amount. I've seen mileage change
both ways with the addition of moderately taller tires, depending
on if the engine rpm goes closer to the torque peak. The 302 should
have adequate power to compensate. I put 33" tires on my Toyota
(235/75R15 stock) and noticed about 15% error in speedometer and
somewhat less power. It's bearable though, even with the 4 banger.

Hope this helps, Jim Chott

[Minh Lang]

In article <1992Oct22....@newsgate.sps.mot.com> Jim Chott,

RZA...@email.sps.mot.com writes:
>You will have less power with the bigger tires since they are taller.
>The 31x10.5-15 is about 10-15% taller than stock tires which will
>change the speedometer by the same amount. I've seen mileage change
>both ways with the addition of moderately taller tires, depending
>on if the engine rpm goes closer to the torque peak. The 302 should
>have adequate power to compensate. I put 33" tires on my Toyota
>(235/75R15 stock) and noticed about 15% error in speedometer and
>somewhat less power. It's bearable though, even with the 4 banger.
>
>Hope this helps, Jim Chott

Thanks, Jim, for the message.
How did your 33" affect your mileage (comparing to your
stock 235/75R15)?

== Minh ==

[Tom Grancey]

In article <1992Oct22.1...@elroy.jpl.nasa.gov> mi...@inst-sun1.jpl.nasa.gov (Minh Lang) writes:
>I'm not sure if I posted to the right group. If not, I apologize.
>I have a full-size Bronco which has 235x75R15 tires that I will
>soon need to replace them. I'm wondering if I can use a bigger
>size tires (31x10.5 R15) with the existing gear ratio
>of 3.55? Will I get lower MPG and less power if I don't
>change the gear ratio? It has an automatic transmission,
>with V8 302 engine. I do occasional off-road driving, and
>it's my daily commute vehicle (mostly highway).
>I would appreciate any comments/help. Thank you very
>much.

The 31x10.50 tires are a factory option. They'll fit, no problem, and
the overall change in ratio would be insignificant. I had them on my
big Bronco, and towed a 4500 pound trailer through mountains with no
problem. There will be a slight miscalibration to your speedometer. Go
for it!

Tom

[Jim Chott]

In article <1992Oct23.1...@elroy.jpl.nasa.gov>, Minh Lang <mi...@inst-sun1.jpl.nasa.gov> writes:
>
> How did your 33" affect your mileage (comparing to your
> stock 235/75R15)?
>

I was getting 21-24 mpg with the stock tires and am now getting 20-22
with the 33's. I just dropped my normal highway gear from overdrive,
about 0.87:1 to fourth, 1:1.
I had a Ford fullsize with a diesel in it with 29" stock tires,
getting 17 mpg. I put 36" tires on and it went up to 18mpg. ???

Jim C.

[Minh Lang]

In article <1c996d...@gvgspd.gvg.tek.com> Tom Grancey,

t...@gvgdsd.GVG.TEK.COM writes:
>The 31x10.50 tires are a factory option. They'll fit, no problem, and
>the overall change in ratio would be insignificant. I had them on my
>big Bronco, and towed a 4500 pound trailer through mountains with no
>problem. There will be a slight miscalibration to your speedometer. Go
>for it!
>
>Tom

Thanks for the info. The factory-installed 31x10.5 option Bronco
has 4.10 gear. Definitely I'll want something bigger than
235/75R15, as they are not suitable for off-road driving,
even it's occasional :-)

== Minh ==

[Minh Lang]

In article <1992Oct23.2...@newsgate.sps.mot.com> Jim Chott,

RZA...@email.sps.mot.com writes:
>I was getting 21-24 mpg with the stock tires and am now getting 20-22
>with the 33's. I just dropped my normal highway gear from overdrive,
>about 0.87:1 to fourth, 1:1.

This is understandable... and...

>I had a Ford fullsize with a diesel in it with 29" stock tires,
>getting 17 mpg. I put 36" tires on and it went up to 18mpg. ???

this is interesting... :-)

== Minh ==

[Steve Waite]

I replaced the P235x75R15 Michelins my Dodge Ramcharger had as original
equipment, with 31x10.50R15 Bridgestones. The conversion gave me a
speedometer error of 11-12% and slightly increased freeway gas mileage.
The reduction in acceleration was small enough I never really noticed it.
There was a nice improvement in cornering stability. The change in tire
width was just enough that I also changed my rim width ( from 7 to 8 inches
I think ).

Steve

[Tom Grancey]

In article <1992Oct23.2...@elroy.jpl.nasa.gov> mi...@inst-sun1.jpl.nasa.gov (Minh Lang) writes:
>
>Thanks for the info. The factory-installed 31x10.5 option Bronco
>has 4.10 gear. Definitely I'll want something bigger than
>235/75R15, as they are not suitable for off-road driving,
>even it's occasional :-)
>
> == Minh ==

I believe the axle ratio is related more to engine option than it is
tire size. I've had two big Broncos in the past, both with 351 V8s,
both with 3:55 axle ratios. In thinking it through since my first
reply, the speedo change should be about 5%, after taking into
consideration the actual tire radius. That is what I had over a
measured mile course.

If you have a newer Bronco, 87 or later, up to 33x12.50s will fit, with
some rubbing on the radius arms, but no rubbing on fender wheels.

TG

[Bob Hagg]

In article <1992Oct22.1...@elroy.jpl.nasa.gov> Minh Lang <mi...@inst-sun1.jpl.nasa.gov> writes:
>I'm not sure if I posted to the right group. If not, I apologize.
>I have a full-size Bronco which has 235x75R15 tires that I will
>soon need to replace them. I'm wondering if I can use a bigger
>size tires (31x10.5 R15) with the existing gear ratio
>of 3.55? Will I get lower MPG and less power if I don't
>change the gear ratio? It has an automatic transmission,
>with V8 302 engine. I do occasional off-road driving, and
>it's my daily commute vehicle (mostly highway).
>I would appreciate any comments/help. Thank you very
>much.
>
> == Minh ==

I have a FORD F-150 4.9L and gear ratio of 3.55. that came stock with 235x75R15.
I replaced them sometime ago with 31x10.5R15 Dunlop RV's and now with 10R15's
by Goodrich. The 10R15's are just a little under 31". You may notice a small
decrease in gas mileage but nothing too noticeable. I have'nt noticed any
decrease in power and you shouldn't have any trouble at all with the 302, the
302 has alot more torque than the stright 6. I would warn against purchaseing
anything with a large tread (mudders) due to excesive road noise from running
on pavement.

If your interested in a "beaffyer" look. Goodrich or Dunlop makes a 31x11.5"
tire. I've seen stock suspension Bronco's and F150's running the 31x11.5 and
haven't notice and clearence problems in the front wheel wells from steering.

I personally wouldn't go any larger than the 31x10.5. I've driven in the
north east snows and now in the desert and soft beaches. If your dealing with
mud and snow your better off with a tall thin tire. Sand and soft terrain you
obviously want fat tires. For me the straight 10R15's by goodrich does the
trick. There harder to find since there not the yuppy thing to have but
functional. The tread stands up off the tire about an 1" and is narrower than
the side walls opposite of the 31x10.5 AT. The sidewalls balloon slightly thus
helping in soft sands. You will probably want to change your wheel size as
well. I'm running 8.5 in. steel wheels and haven't had any troubles at all.
If you go to 11.5 and above you'll probably want to run 10 in. wheels.

Good luck. And let us know what you decided on.
--

[Bob Hagg]

I agree that the 33" would show some speedometer error. I've run the 31x10.5
and 10R15's and haven't noticed any speedometer error with the old stop watch
and mile marker method of timing.

--

[Robert A Walker]

>If you have a newer Bronco, 87 or later, up to 33x12.50s will fit, with
>some rubbing on the radius arms, but no rubbing on fender wheels.

If you have an older bronco like a '78 or a '79, 33x12.5 will
fit on a stock suspension without rubbing anywere and there is enough
fender clearance in the rear to fit my FAT head between the fender and tire
and a little less up front.