[CC69Rat]

Tire size ÷ 25.4 = width in inches.

Multiply that number by the aspect ratio. 35 series for example = x .35. 40 series would be .40

That gives you your sidewall height in inches

Multiply the sidewall height x2 (top of the tire, and bottom of the tire)

Add your wheel diameter

That's your total overall height. Try it.

For example. 345/30r19

345÷25.4 = 13 58 in wide
X .30 = sidewall is 4.07 in tall
X2
+19
Overall = 27.14

Adjust as necessary to find your match. I like to put a slightly larger sidewall height on the rear than on the front.. it just looks more proportional in my opinion. It is one thing to have a big wide tire on the rear but if the sidewall on the rear tires is shorter than the front it will be pretty obvious.

Also - if you have a 12" wheel in the rear, and an 8" wheel up front. I'd try to put a 4" narrower tire up front too.

245 is 9.64 wide. Prob what I'd go with.

245/40 has a 3.85 sidewall. 245/45 is 4.33. (Remember your rear sidewall is 4.07)

255/40 would be close too. 10.03 wide, 4.01" sidewall.

Then of course your wheel diameter.

255/40r19 is 27.03 in overall
245/40r19 is 26.71
245/45r19 is 27.68

I am posting from my phone so I can't promise my math is right.. so double check me. And of course there will be slight variations in size by tire brand. But this will give you a general idea. Hope this helps.