Ok wheel and tire questions

Ok so i got the 20s on here i got them cheap from a friend and they look freaking awsome on there but i know its making my truck slower and when i throw them on for spring-fall my gas milage goes down. So heres what i was thinking and i got worked out. My buddy has a 73 mercury montego GT (1-300 left like this car) Hes got a set of cragers for it but has always talked about geting 20s (hes got factory steel rims too) but any way to the point

We're gonna work a trade and put the cragers on the mounty and 20s on his (same bolt pattern and back spacing) And i was wondering
first If having different sized tires in front and back is gonna be bad for the AWD
Second if putting a shorter sidewall then factory is gonna be bad besides making it lower (which i like) and messing up the speedo (which i don't care about)

 

[FROADER]

If your stock tires were 235/70/16, then your current tire is 0.7" taller, which really should make much difference.

Is that your stock tire size?

 

[Xan]

If your stock tires were 235/70/16, then your current tire is 0.7" taller, which really should make much difference.

Is that your stock tire size?

Its more about weight, the 20s weigh (full of air) a freaking load 82.5 pounds and they actualy weighed more before i put nitro in them, stock ones are pushing about 47 pounds and my buddy says his cragers are about 51 pounds. this is all on our shop scale which is really old

 

[FROADER]

That could affect it a little.

The different sizes front to rear is a bad idea for the AWD system, unless you do like a few others have and put the 4406 t-case in so you have 2wd, 4wd hi and 4wd lo.

 

[Xan]

That could affect it a little.

The different sizes front to rear is a bad idea for the AWD system, unless you do like a few others have and put the 4406 t-case in so you have 2wd, 4wd hi and 4wd lo.

Yeah kinda what i thought, the rears are 275-60-16s but i have a more used set of 275-60-16s siting in his garage on another set of tires and we were thinking we'd just throw those on there to even out the front and back

 

[Xan]

but if thats the only problem with this swap i think we're gonna test fit it them on both vehicles tommarow there will be pictures no worries