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Manual Gear Ratio's


Its a fact that in 1/4 mile and general acceleration terms the Muncie close ratio transmission, its M22 heavy duty cousin and perhaps FoMoCo's Top Loader 4 speed close ratio gearboxes have been the 'ultimate' aid in putting down high performance on the road for the last 50+ years. OK so an auto transmission and torque converter can be optimised to give better 1/4 mile performance but there is a price to pay with low axle ratio's and 'loose' converters that make any out of town driving a potentially unpleasant, noisy, tiring and heavy fuel usage experience. The close ratio M21 is probably best suited to use in a fairly torquey medium cubic inch lighter weight car. The closely spaced ratio's however may be problematic for heavier lower powered cars with low numerical axle ratio's and tuned engines with limited torque. Matched to a higher revving torquey small block or big block powered car with a low numerical axle 70 mph speeds in first gear are perfectly possible. Hence why many cars would have wide ration M20 transmissions.
With so many late model cars featuring 5 and 6 speed manual transmissions with one or two overdrive ratios quiet, relatively economical 1200 - 1500 rpm 70 mph cruising is now the norm. Compare that with the 2500 rpm cruise of a Vette with a M21 (or M22) gearbox. Not saying that its a problem - but we can (for a price) do it better. It is now commonplace for people to swap-in a late model transmission in to their early car to take advantage of the overdrive ratios. There is a downside however. Most of the 5 and 6 speed gearboxes are simply the same ones that are seen in late model cars. That means in many cases 1st through to 4th ratios are optimised for economy not ultimate performance. This is due to CAFE (Combined Average Fuel Economy) regulations meaning that the lower gear ratios are specifically designed to get the car away from the lights in the most economical way - and not really the best ratio's needed for maximum acceleration. So you'll see something with ratio's like this.
Magnum T56 XL ratio's
2.97 / 2.10 / 1.46 / 1.00 / 0.80 / 0.63

'Old School' Muncie M20 'wide' ratio's
2.52 / 1.88 / 1.46 / 1.00

'Old School' Muncie M21 close ratio's
2.20 / 1.64 / 1.28 / 1.00

'Close Ratio' Tremec TR 6060 (as fitted to late model Camaro ZL1)
2.29 / 1.61 / 1.21 / 1.00 / 0.85 / 0.54

Note how close the TR 6060 lower ratios compare with the old Muncie M21. On my '68 2500 rpm currently would mean 70 mph in the Muncie's 4th gear.
Using a Tr 6060 70 mph would equate equate to 2000 rpm in o/d 5th gear and 1350 rpm in o/d 6th. 80 mph in o/d 6th would be 1550 rpm.
Anyone got a spare £3.5K+ for one of these please? (presuming it'll fit.......)

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CCCUK Member
If you are organising a group buy, then I'm up for a 6 speed gearbox for my 68 327. Like yours, it revs around 2500rpm at 70mph. It doesn't bother me, but when it is legally permitted to do more (honest officer), then it does up the sound a little bit more and I feel guilty for punishing it so. On the plus side - more engine noise = less squeaks and rattles I hear. :)