Gear vs. Tire size question

Hi, I'm a newbie to this forum, but not to building. I have an 85 AMC Eagle wagon that I'm straight axling with parts from an 86 Commanche pickup. It has a D30 front and an AMC20 rear with 4.10s, the dealer told me it was some oddball package they offered for heavy weight and towing, didn't even know till now they used anything other than the 35 or 44. Anyway, I'm trying to figure out what the best tire size to use with the 4.10s will be to use for pavement reasons, I plan on being able to drive it on the freeway at 75-85 comfotably without over-revving the engine. The engine for now is an AMC 258 with a TF904 behind it with a 231 out of the Commanche. I'm hoping with this combination that I can use 31" or 32" tires, if I have to I can go up to 33", but I'd rather the smaller sizes. I'm going to use YJ Wrangler front springs and the rears are funny enough, the same as XJs, so I'll use some XJ lift springs for the rear. Any thoughts about tire size?? I can't seem to find a reasonable gear chart to help. Thanks.

Try this for starters: http://www.4lo.com/calc/geartable.htm

Thanks, that helps a lot. I think I'll try a cheap set of 31s first and go from there.

the 904 has no overdrive, I think 33x10.50s would be the best tire for ya, with 31s your 80mph rpms will be well over 3500.

Thanks. The 258/904 combination is only going to be in a little while. I'm going to either a 304 or 360/727 combo a little bit down the road. The 258 has a little rattle coming from the front lifter area and has 160K on the clock, cheap excuse to v8 it. Also, any idea if the 231 would live up to v8 power and 33s very well? It will only see mild to moderate trail use, the hard stuff will be seen by my 81 J10 on 35s or my long wheelbase Samurai when I'm done with it. Thanks again for being helpful guys. I wish other boards were as helpful.

I did all that math a long time ago.

With 31s and 4.10s with no overdrive, the RPMs will be 3250 at 70 MPH.

With 32s, the RPMs will be 3170 at 70 MPH.

With 33s, the RPMs will be 3060 at 70 MPH.

It's good to be concerned about over-revving the engine, but remember that this engine family was introduced by AMC in 1964, long before they started using overdrive transmissions. They were typically geared at 24 MPH per 1,000 RPM, which meant 60 MPH was 2500 RPM and 3000 RPM was a 72 MPH cruise. Cruising at 3500 RPM sounds excessive to people who have only known overdrive transmissions, but it's not really going to hurt the engine.

My brother's old 1970 Gremlin was cruised at 80 MPH quite often, autocrosses competitively for two or three seasons, and lasted (last we knew, it was still running when we lost track of it) over 350,000 miles.

neat project. Ive seen a few writeups on solid axle and modded eagles. my dad had a sx-4 (?) hatchback, that was actually a cool car. I wish I remember more about it.