360/904/Ford 8.8 Rear Gear Question

Having an AMC machinist/engine builders put together a 360/904 combo to replace my worn out 199/BW in my 69 Rambler. We have talked extensively about my goals and uses for the car. It will be a fun cruiser, driven a few times per week. Here in San Diego we have a lot of good freeways and I want to be able to cruise comfortably are 65 or 70 without overtaxing the engine.

I do not want the RPM's to hit 3,000 while cruising those speeds. I just bought a Ford 8.8 rear end and made a decision on a high (low numeric) gear ratio of 2.73. Some have told me I should get a lower gear (high numerically). But I am getting ready to complete the engine and need to finalize my decision.

Last week I pulled the drive train in anticipation of proceeding with the swap.

My question: has anyone run a 2.73 gear in a Ford Explorer 8.8 with a 360/904 combo and what do you think of the 2.73 gear selection for a cruiser? 3.08 or lower gear seems like it would give me an uncomfortable RPM at the above mentioned speeds. Any thoughts? Thanks.

Matt

Edited by LakesideRamblin - May/02/2016 at 1:56am

Use the calculators at www.4lo.com to figure rpm at cruising speed. Other calculator sites work as good, that is just easy to remember and has everything, including a tire diameter calc.

As UnclJohn has pointed out in another post, you want around 32 mph per 1000 rpm, or 2000 rpm around 64-65 mph. You need at least 2000rpm to be in the engine's torque band enough to efficiently pull the vehicle along at speed. Some can get by with less with the right cam/weight (light!) combo, but 2000 minimum in high gear is a good rule of thumb. I run a 3.55 and am right there, but I have an OD auto. You wouldn't want to run 3.55 gears. 2.73 might not be bad with a 904, especially if it's not a lock-up converter model. The older model trannys had more slip in the converters, meaning they multiply torque more, typically at least 2:1 (engine with 200 ft/lbs@ 2000 rpm gets multiplied to 400 ft/lbs through converter), and will take off better with higher (low number) gears like a 2.73. More modern trannys like my AW4 use high efficiency converters with less slip (AW4 has a 1.4:1 multiplication ratio stock converter) and need a bit more gear, made up for with OD.

Tire diameter makes a bit of difference. A smaller diameter is like lowering the gear a little, larger diameter makes it higher (remember, lower gear is a bigger number... 3.08 is lower than 2.73).  There isn't a lot of difference in tire size, maybe 100 rpm one size up or down, but it helps. If you can go several sizes of course it has more effect. You will have to adjust your speedo gear also, but there are many available for the 904.

Was going for a 15 inch wheel with 205/65/R15 tire. My current wheel is a 14".

I run a 360/904 with a 205 65R/15 rear tire. Before I changed my gears, I had a 2.73 and in my opinion it was perfect for cruising.

I just ordered five 225 60 15 tires for my Javelin. They are replacing old rotten tires of the same size and the spare.

The suitable rim widths for such a tire are 6" to 8".

There is a super deal on these right now for USA. I am picking them up in ND and bringing them here. Even with $ exchange and 13% tax at the border they are about half price compared to here. PM me and I will let you know where to get them.