Spare tire of a different size?

I posted a wanted add a while back for some 12.00r20 tires. I have a set of 6 of them on my singled out M275A2 but I don't have a spare. Simp5782 pointed out that the MRAP Maxxpro runs 12.00r20 duals in the rear and 395/85r20 on the front axle.

Does anyone know if the gear ratios are the same in these axles?

Is the loaded radius of a 12.00r20 on a skinny rim about the same as a 395 on a wide MRAP wheel?
(the diameters are 44.5" versus 46.5")

If this can be done without damaging anything can I run 11.00r20 tires as a spare (43.5" diameter). Seems to me that would be even closer, and a lot cheaper and easier to source. I also like the idea of running MRAP wheels on front with 395's and then that gives me two spare 12.00r20 tires I can throw on the bed.

Anybody have any experience with this? Do I just need to fork out the cash and get 12.00r20 spares? (they are around, just kind of expensive and none are close to me).

First picture shows the tire combo I'm interested in on the right. The second picture is my truck.

We run 445/65/22.5s and 425/65/22.5s on the front of M923 with 11.00R20 G177s on the rear.

You can run a 11.00 as a spare and if you have a front flat just pull the front drive flange. on the rear I would just chain it up to get it home if it was a flat. Or simply put the 11.00 on and pull the axle shaft and stuff a rag in it.

Thats pretty much what I do. I have 395s on my truck but have a 14.50r20 as a spare. Whatever axle it g9es on either geats a spline cap thats been machined or an axle that has been cut off.


You are not too far from me. I can give you a cut axle if youd like, but I have no spare de-splined caps.

Does your truck have a Sprag clutch in the TC? I found the TC in my 817 had different ratios in the gears (front and back) so the rear was always trying to overrun the front and would relieve some of the stress of having different tires. Sorry, forgot the ratio of front and rear. I'm not a deuce guy (except the A3 which is a different animal).

Oh yea...how about lock-outs on the front

Hey Shrapnel, my daughter lives in Plainfield (and wife's cousin in Mooresville). We go over to the antique mall west of town on a regular basis. I can't believe all the industry that has sprung up since the airport was re-done....wow.

My truck has the air actuated front drive transfer case, not a sprag. My truck is road legal now but I'm trying to get it ready for the fall Haspin rally (assuming we are having one, haven't seen a thread yet) and I need to get one or two spare tires before making any long trips or doing any trail runs.

Sounds like I have some good options here. I can spend big money and run 395's up front on MRAP wheels, or I can use some of my spare rims and put some 11.00R20's on.

Gimpy, maybe if you can make the fall rally (again, if one gets organized) I could grab a cut axle from you then. If not I'll have to make a trip down sometime. Thanks for the help.

The TC is the same front-rear ratio then. I thought some had the sprag.

Myself, I like the 11.00 on the rears and use 14.5 on the front on A3 rims. Revolutions per mile is near perfect.

I'm not a fan of MRAP wheels with the adapter...as much because I have the A3 as to keep weight down. The G177 would be a great tire, (available) but not for mudding, they would clog up. But the NDCC....that would seem to work really well.

I'd let you borrow a pair of 14.5 on the A3 if you want to check out the look.

Have you done caster angle on the front yet for the bigger tires?

Caster isnt really a "big tire thing" its more of a biasply vs radial thing.