Gearing & highway rpm

Formula (spec-diameter form): RPM = MPH x axle ratio x top-gear ratio x 336.13 / tire diameter (in), where 336.13 = 63,360 in/mile / (60 min/hr x pi). Using the rounded 336 instead changes every result by 0.04% (under 1 rpm per 2,500), so the two forms agree to the printed precision.

Formula (measured form, preferred for real-world prediction): RPM = MPH x tire revolutions per mile x axle x top-gear / 60. Tire makers publish revs per mile measured under load.

Important correction to the spec-diameter tables (applies to every table in section 4 and the cross-checks in sections 06-10): the overall diameter printed on a spec sheet is the unloaded, inflated size. The same tire's published revs/mile imply a smaller, loaded rolling diameter:

So real rpm will run about 3% higher than any spec-diameter table. (Section 10 says 1-2%; the two tires with published revs/mi both show about 3%.) Sections 4A-4D use spec diameters, as requested, so the rows are comparable with the other sections. Section 5 (the recommended tire) shows both versions. Expect the measured version to match a GPS check.

Other assumptions

• Rounded to the nearest 10 rpm.
• Automatics are assumed fully locked (4L80E) or zero slip (TH400, which has no lockup). A non-lockup TH400 converter adds roughly 100-200 rpm at cruise (section 06 estimate), more with a loose converter. Gear Vendors (GV) is a mechanical overdrive behind the TH400; the TH400 converter still slips.
• 55 mph is added for in-town/arterial driving; 75 mph is headroom for passing and rural stretches.
• 4.56 is in the tables because it was requested; no sensible combination here uses it.

Conflict resolved by a web check: I half-remembered the Magnum as "2.66 close with a 0.74/0.50 overdrive pair". Mal Wood's Tremec table (malwoodauto.com.au↗) lists three current GM-style Magnum sets: 2.66/1.78/1.30/1.00/0.80/0.63, 2.97/2.10/1.46/1.00/0.80/0.63 and 2.97/2.10/1.46/1.00/0.74/0.50. No 2.66 with 0.74/0.50 appears, which matches section 08. The 2.97 wide box can also be had with the 0.63 sixth, a combination the owner's earlier sections did not mention. A separate Mal Wood fetch did not label which set is TUET11009 or TUET11012; Modern Driveline's header does say "TUET11009 2.66 first .80/.63 OD". Part-number-to-ratio mapping beyond that is as in section 08.

Rear gear ratios. The ratios requested (3.08, 3.42, 3.55, 3.73, 4.10, 4.56) are GM 12-bolt ratios; Yukon lists the truck 12-bolt from 3.08 to 4.56 (section 09). Section 09's pick is a Currie Ford 9-inch, which is not made in 3.42/3.55/3.73. Currie's catalog ratios are 3.00, 3.50, 3.70, 3.89 and 4.11 (section 09). Section 5C therefore repeats the tables in Ford ratios.

Tires used

• Best fit for the owner's goal (comfortable 65-70 mph, no spun-to-the-moon, with a lope cam that wants about 2,000+ rpm): a 4L80E/4L85E with 3.42-3.73 gears on the recommended 275/50R17 (about 2,230-2,430 rpm at 70 mph measured, locked). The center of that range is 3.55 (12-bolt) or 3.70 (Ford 9-inch). 3.73 is the most launch-friendly gear that still stays at or under about 2,450 rpm at 70.
• Second best: a T56 close-ratio box with 3.73-3.89 (about 2,050-2,130 rpm at 70 measured in 6th), if the owner truly wants a clutch pedal.
• Third: TH400 + Gear Vendors with 3.42 (about 2,320 at 70), but the cost and complexity (section 06) are not justified over the 4L80E.
• Worst: a plain TH400 at any ratio the 12-bolt or 9-inch offers. At 70 mph on the recommended tire it turns about 2,680 rpm with 3.08, about 2,980 with 3.42, and about 3,250 with 3.73, before 100-200 rpm of converter slip. Only 3.08 gets near "tolerable", and that ratio is slow off the line.
• Avoid: 4.10 or 4.56 with any gearbox (the 0.50 sixth is discontinued, so no shipping T56 gets that low), and 3.08 with the T56 0.63 (about 1,690 at 70, below the cam's useful range).

Columns are rear axle ratios. Rows are transmission top gear by speed. No marks here; marks appear in section 5. The 0.50 rows (*) are reference only: Tremec discontinued the .74/.50 set (tremec-blog.com ?p=697), so they apply only if a vendor confirms a surviving .50 box.

#4A. 28.0 in tire (275/60R15 class)

#4B. 29.0 in tire

#4C. 30.0 in tire

#4D. 31.0 in tire (31x10.50R15 class)

#5A. Spec diameter 27.87 in (comparable with sections 4A-4D)

#5B. Measured form, 746 revs/mile (rolling diameter 27.03 in). This is the one to plan around.

Bold = 70 mph value (the commute number). ★ = sweet spot at 70 mph. ✗ = out of bounds at 70 mph. Bounds used: overdrive automatic 2,000-2,450 rpm is the sweet spot; manual 1,900-2,200 is the sweet spot (below about 1,700 a lope cam and a manual lug; the cam makers' listed floors are 1,900-2,500, section 03); anything over 2,800 rpm at 70 is "spun" (the owner's own limit is "no spun to the moon", about 3,000 is the bad zone per section 06). A direct-drive TH400 cannot earn a ★ at any ratio here.

#5C. Ford 9-inch ratios, measured form (746 revs/mile), because section 09 picks the Currie 9-inch

For the same Ford ratios at spec diameter 27.87 in, multiply any cell by 0.969. Example, 70 mph, 4L80E: 3.70 gives 2,415 measured and 2,343 spec; 3.89 gives 2,539 measured and 2,463 spec.

All figures use the measured tire (746 revs/mile, rolling 27.03 in). Overall ratio = gear ratio x axle ratio. Launch (1st) overall ratio is shown first for every transmission and axle; an automatic's converter multiplies this by about 1.8-2.2 at stall (my range, depends on the converter), so a manual's 1st is not directly comparable.

The "rpm at 60 mph" column is simply the arithmetic for that gear; values above the 5,500-6,000 rpm powerband (1st gear, and 2nd in some combos) mean the truck would be past redline, not that anyone would drive 60 mph in that gear. The "mph at 5,500 / 6,000 rpm" columns give the usable speed range of each gear.

#6A. Overall 1st-gear ratio

Reference points from earlier sections: 3.73 with the 2.48 first is 9.25:1 (section 07; matches 9.25), 4.10 with 2.48 is 10.17:1 (section 07 says 10.2), and the T56 close box with 3.73 is 9.92:1 (section 08 says 9.9). All three check out.

#6B. 4L80E / 4L85E + 3.73 (12-bolt) [sweet spot A]

#6C. 4L80E / 4L85E + 3.42 (12-bolt) [relaxed alternative]

#6D. 4L80E / 4L85E + 3.70 (Ford 9-inch) [sweet spot A on the 9-inch]

#6E. TH400 + 3.42 (12-bolt) [the section 06 pick]

#6F. TH400 + Gear Vendors + 3.42

#6G. T56 close (2.66 box) + 3.73 [sweet spot B]

#6H. T56 wide (2.97 box, 0.74/0.50) + 4.10 (* availability uncertain)

#6I. Speed at which top gear reaches the cam's floor (measured tire)

"On the cam" is taken as 2,000 rpm (the lowest listed floor of a Path B cam, COMP 11-432-8 at 1,900-2,200, section 03) and 2,500 rpm (the stall/floor figure COMP quotes for the same grind and for the Magnum 232). Top gear speed = rpm x 60 / (746 x ratio).

Cam bands (section 03, makers' listed operating ranges, not dyno data): Path B candidates COMP 11-432-8 (230/236 @ .050, 110 LSA) 1,900-5,600 rpm (one source says 2,200-5,800; COMP's same-grind listing says 2,500+ stall); COMP Magnum 232 (11-450-8) 2,500-6,200; Clay Smith HR-294-0 max 6,000; Path A Howards CL120265-12 2,600-6,500. The owner's powerband target is 5,500-6,000. Section 03 also says to keep to about 238 degrees or less with a 0.75 overdrive and a 3.42-3.73 gear. The torque peak is likely somewhere around 3,500-4,500 rpm for the stroker; that is my estimate, not a source figure.

What the tables say for each path

Reading the TH400 result in section 06's terms. Section 06's table (28 in, spec diameter, no slip) shows 2,874 rpm for 3.42 at 70 mph and calls 3.42 with 30 in "acceptable." Those numbers are right for the spec diameter. On the recommended 275/50R17 (27.87 in spec, 27.03 in rolling) the same gear is 2,887 spec and 2,977 measured, plus slip, so about 3,100-3,200 in practice. That is a harder conclusion than section 06's "tolerable but loud."

The 2-hour Osceola trip. The owner puts it at about 2 hours; I estimate roughly 90-100 miles but did not check the route. Most of that is sustained 65-70 mph cruise. At 4L80E + 3.73 the engine turns about 2,260-2,435 rpm the whole way, which is a happy place for a cam with a 5,500-6,000 rpm top end, and the locked converter avoids the extra heat a non-lockup TH400 puts into the fluid. A TH400 at about 3,100+ for 90 minutes raises heat and noise but is not mechanically dangerous.

Converter stall vs cruise rpm. Section 03 (reconciled with sections 06, 07 and 90) now asks for an actual converter flash of 2,300-2,600 rpm for a 230-class cam. Stall sets launch and throttle flash only; once the lockup clutch applies (controller-set, independent of the stall rating) the engine is tied to the wheels and cruise rpm is the table value. With 3.73 that is 2,260 at 65 and 2,435 at 70, close to the converter's stall figure, so a loose converter that is not locked would flare in that range. Choose the converter and the lockup schedule after the final cam card is known (section 07).

• Tire choice moves everything about 3% from the spec numbers. Going from the 27.03 in loaded 275/50R17 to a 30 in tire lowers 70 mph rpm by about 10% (4L80E + 3.73: 2,435 to about 2,190, matching section 07's 2,193 for a 30 in tire). Section 07 and section 08 assumed 29-30 in tires; section 10 recommends about 27.9 in. These recommendations are consistent only if the gear is chosen after the tire. With the 27.9 in tire, section 07's "3.73 with a 29-30 in tire" should become 3.42-3.73.
• Gear ratio and axle type are coupled. A Ford 9-inch cannot be had in 3.42, 3.55 or 3.73. Stay on the 12-bolt for those, or pick 3.50 / 3.70 / 3.89 / 4.11 for the 9-inch (section 09).
• Speedometer and controllers. Every tire/gear change needs a speedometer recalibration. The 4L80E TCM, the T56 VSS, and the Gear Vendors module all need the final tire size and ratio (sections 06, 07, 08, 10). Finalize both before tuning.
• Cam vs brake vacuum. Gearing does not fix brake vacuum; the cam does, so the hydroboost or vacuum-pump decision in sections 03 and 12 stands regardless of gear.
• Exhaust drone. Section 15 notes drone peaks near 2,000 rpm at 65-70 mph. The sweet-spot combos (about 2,000-2,450 rpm) land in or just above that band, which is a reason to use the H-pipe default and resonators.
• Wheel and tire fitment. The 27.9 in rear is a 275/50R17 summer tire; if an all-season is substituted for winter (section 10 recommends planning for one), recompute with its revs/mile.
• Cooling. A non-lockup TH400 turning 3,000+ rpm makes more converter heat; the 4L80E locks up but section 14 still wants a true 40-row trans cooler for it.

1. 70 mph, 3.73, 4L80E 0.75, 28.0 in: 70 x 3.73 x 0.75 x 336.13 / 28.0 = 2,351 (sections 09 and 10 print 2,350).
2. 70 mph, 3.42, TH400, 28 in: 70 x 3.42 x 1.00 x 336.13 / 28 = 2,874 (matches section 06's 2,874).
3. 70 mph, 3.73, T56 0.63, 28 in: 70 x 3.73 x 0.63 x 336.13 / 28 = 1,975 (section 08 prints 1,974; the difference is rounding or 336 vs 336.13).
4. 70 mph, 4.10, T56 0.50, 28 in: 1,723 (section 08 prints 1,722).
5. Measured: 70 mph, 3.73, 0.75, 746 rev/mi: 70 x 746 x 3.73 x 0.75 / 60 = 2,435.
6. 4L80E 1st x 3.73 = 9.25; x 4.10 = 10.17; T56 2.66 x 3.73 = 9.92 (all match sections 07 and 08).
7. Section 10's printed grids (28 in, 29, 30, 31 in) were regenerated; all differences from this file are within 10 rpm rounding. Section 10 has no 55 mph or 4.56 columns, so those are new here.
8. Section 06's GV values (e.g. 3.42, 28 in, 70 mph: 2,241) match this file's formula (70 x 3.42 x 0.78 x 336.13 / 28 = 2,242, a 1 rpm rounding difference).
9. Section 07's 4L80E table (3.73, 29 in, 70 mph: 2,269; 30 in: 2,193; 4.10, 30 in: 2,411) matches this formula within 2 rpm.

1. Tire diameter basis. Sections 06-10 use spec diameters; real rolling diameters are about 3% smaller (746 and 743 rev/mi on the two tires that publish revs/mi). Treat all earlier cruise-rpm numbers as about 3% low.
2. Section 07 vs section 10 tires. Section 07 recommends 3.73 with 29-30 in tires; section 10 recommends 275/50R17 at about 27.9 in. The gear answer depends on the tire (see section 8).
3. TH400 verdict. Section 06 calls TH400 + 3.42 acceptable with 30 in tires. With the recommended 27.9 in tire it is not (about 2,980 plus slip).
4. Which axle. The requested ratios are GM 12-bolt ratios; section 09 picks a Ford 9-inch, whose ratios differ. The answer for 4L80E is 3.42-3.73 on the 12-bolt or 3.50-3.70 on the 9-inch.
5. T56 0.50 sixth availability (resolved in the integration pass). Mal Wood, AP, SST and Bowler still list 2.97/0.74/0.50, but Tremec's own blog states that the .74/.50 set was eliminated on the wide-ratio Magnum and Magnum XL and replaced by .80/.63. Only the 0.63 sixth is a verified shipping ratio; the 0.50 rows in sections 08-10 and here are reference only and the project spec is 0.63 with a 3.70 Ford ratio.
6. Cam floor and stall figures disagree across listings for the 11-432-8 (1,900 vs 2,200 vs 2,500+). Final converter and gear choice should wait for the cam card.
7. Actual rear gear and tire. The truck's real ratio (SPID code or tooth count) and the final tire are still unknown (section 09 and 10 questions). If the stock ratio is 3.08 or 3.42, the 12-bolt may already be in the sweet-spot window for a 4L80E.
8. Osceola route length is my estimate, not checked.