Picking a Rear Axle Ratio for a new OD Transmisson

Time to replace the old FMX and the original 3.00 rear on my '70 with a 351C-4V. Looking at Tremec T5 and a shorter rear. I want to be able to drive the highways at a reasonable engine speed. The current 3400 rpm on the Texas Interstates (85mph) is not so good. The general rule of thumb on 2000 - 2500 rpm for best efficiency seems high for highway cruising. Most of the OD’s are at 0.63 ratio but 0.72 is also available. It’s big step from 4th at 1.00 to 5th at 0.63. This is a street and highway car for going to classic car events or just some pleasure driving.

What’s the lowest cruising speed and engine rpm I could expect with 0.63 OD? Where does the engine bog down and necessitate a downshift for even slight acceleration?

I think you’ll be fine with the 3.00 and T5 personally, but Tremec has a great calculator for theorycrafting your build. https://www.tremec.com/calculadora.php

My White 70 XR-7 had 3.00 factory gears when I swapped in the T-5Z. The OD was nearly useless under 75mph. I went with 3,50 gears and love it. But a 3.25 might have been a little better on the freeways.

Lowest engine RPM and cruise will depend on a lot of factors but also personal preference to some degree. Here’s another calculator. I used this one when I was deciding gear ratio for my 6R80 automatic upgrade. Confirm your actual tire O.D. by measuring it. I have found online data to be inaccurate sometimes. Your driveshaft RPM will increase with a shorter (steeper) rear end gear ratio. So keep that in mind. Might be best to swap the trans and then decide a gear after driving with the 3.00 for some time.

Gearing Calculator | RPM and Tire Diameter to km/h - mph with Shift Tracer (blocklayer.com)

I was thinking 3.70 or higher but need to know if it will cruise below 2000 rpm.

I’ve done all the calculations but it’s a question of what the engine will do comfortably at cruising. What is the lowest rpm that will run smoothly? The rpm and cruising speed would tell me what ratio was running and give me a practical operating range. The 3.00 rear would be about 2100 rpm at 85mph. It may not be useful at 60mph and about 1400 rpm.

Ah! I understand now, my bad. I thought you wanted to go 85 without tacking out, I’ll read more carefully next time. :smile_cat: I don’t have experience with the Cleveland, as mine has a 289. Mine will cruise at 2000 rpm give or take, but generally under 2000 it starts lugging. I would keep it above 2000, probably like 2200-2500, although your Cleveland has way more torque. Hope this helped! :smiley_cat:

Thanks Felidae

1 Like

Badcatt, What’s minimum speed that you can cruise at with the 3.50 rear? Is it a .63 ratio overdrive? My calc’s suggest that you run about 1900 rpm at 65 mph. How much slower can you go with the OD and not bog down? With the 3.00 rear you were at about 1900 rpm at 75mph.

I did the T-5 conversion on a 68 XR-7 302 4V. Did some calculations and bought a set of 3.55 gears for it. I found that the RPMs are still higher than what I would like when cruising at 70 MPH (can’t remember what the RPMs are at that speed, but I would like them lower). I don’t need the car to be a drag race winner, as a matter of fact there’s a need to shift very quickly from first to second… it’s almost like driving a truck with low gears, when unloaded you can go straight to second and shift from there (almost). I am thinking that I would be better off changing to 3.25 to get slightly lower RPMs when cruising at 70.

Thanks Fordnutz, The calcs show you running just under 2100rpm at 70. What’s the lowest speed you can do with the OD?

Tom, I can run in OD on flat or slight inclines down to about 45mph. As Scott said it does run at a higher RPM then I had expected at 75MPH. But I don’t mind. I might if I took it on long freeway drives more. But I prefer back roads.

I put a 3.50 in my '70 351C-4V when I swapped in the T-5 with 0.63 OD. I don’t do a lot of highway driving but my recollection is 75 mph is about 2,100 RPM. I found the 3.50 recommendation from MDL to be spot on for this setup.

For everyone’s reference the AOD/4R70W transmissions installed in Crown Vics/Panthers from 1979-2011 behind 302’s and 4.6’s and a similar sized tire to the factory Cougar sizes (at least from 1979-98) came with 2.73 rear gears in base US spec, mountain areas/Canada/Basic Tow cars came with 3.08, and the high end tow package came with 3.55’s.

Mine has 3.08’s and it is a nice match for a cruiser, 3.55’s would be a bit buzzy for anything over 70mph on a short tire like this. And the 2.73’s would be gutless.

Thanks bmwhd1,
Depending on tire size it calculates out to 2100 - 2200 at 75 mph (2200 w/ my 235/60-14s). It may depend a little on who you talk to at MDL, my conversation leaned more to 3.70 because of the 4V’s liking for higher rpms. I’m still trying to get a sense of the of the low end speed for the OD. When do you shift to/from the OD? I got the impression from MDL that I’d need about 1900 rpm to cruise in OD (close to 65 mph w/ the 3.5 or 60 mph w/ the 3.7). How does that match your experience?

Thanks badcatt,
45 mph translates to cruising at about 1400 rpm with the 3.5 rear. That provides a lot more driving range than I was expecting, but it’s good news.
Have you got the 2V or 4V? The 2V is probably stronger at that rpm.

That Cougar is a H code 351W. I have an Edelbrock 351Wperformer and a 600cfm Hollie on it. And 1400 looks about correct at 45 in 5th.

I bought a restomodded '68 where the original 390 4V had been replaced with a Ford Racing 302 crate motor rated for about the same horsepower. It ran a Lentech Street Terminator AOD. The previous owner was thinking that performance would be the same with less weight and greater economy. However the torque curve for the 302 called for a higher stall speed (2800) rpm torque converter. The original 2.79 axle ratio was left in the car.

Over drive was really not useful until about 75 MPH and even then the torque converter was still not completely locked up. Torque converters make a lot of heat when they are slipping. So going up hill you could see the revs climb. The net effect was that it ran hot on the highway if you tried to use the over drive. To get a solid 2800 RPM in over drive would have required something like a 3.79 ratio.

I returned that car to the original driveline and it was fantastic on the highway with the rebuilt 390 more than capable of moving the car with 2.79 gears.

My take away was that engine tune and power band is critical to making the decision on axle ratio. Even the type of carb (annular down leg boosters?) you are using can have an effect on how much torque you are making at low RPM. Also tuning is a big deal, as you can get lean surge with wider throttle angles in a large (for the RPM range) carbs.

I have a T5 with a 3.00 rear gear. It’s drivable (and gets really good highway fuel mileage), but I think it needs more gear. I’m planning to swap for a 3.50 or 3.70 soon.

Thanks badcatt,
That tells me that I can probably count on about 1500 as the low end for the 351C-4V that likes higher rpm. Very useful information.