Compared to Cessnas 1978 piston heyday, the 172R didnt set the world afire sales wise, but it did we’ll enough. Whats axiomatic in cars is also true in airplanes: The basic model begets a higher-price, tarted-up version that yields more margin for the manufacturer and perceived greater value for the buyer-enter the Cessna 172S/Skyhawk SP.

Cessna had always had the full 180-HP model on a back burner, but who would buy a 160-HP airplane if they knew that the same engine would produce more horsepower in the same airframe? In essence, the reason for two models of the same airplane is pure marketing. When the initial excitement of the new 172 wore off, the model needed a nudge to keep the line moving. In 1998, the 172 SP appeared and is now Cessnas top-selling single in volume, if not dollars. (The Turbo Skylane is second, and the 172R and straight 182 tie for third.) The SP made full use of the Lycoming engine by allowing increased RPM from 2400 to 2700. Cessna accomplished this rather simply by using the McCauley 1A170E/JHA 7660 (76-inch length/60 pitch) rather than the R models 1C235/LFA7570 (75-inch/70 pitch) and adjusting the mags. Twenty more horses allowed Cessna to increase the gross weight to 2550 pound and change. (The model we flew had a ramp weight of 2557 pounds.) The R-model 172 has a gross-weight limit of 2450 pounds. The SPs advertised maximum useful load is 837 pounds versus 758 pounds for the R-model.

Other differences are few and minor. An autopilot is available in the SP, first the Bendix/King KAP140 and now the fully integrated Garmin GFC 700. The SP is (generally) distinguishable with its polished rather than painted spinner and most roll out the door with rearseat airbag shoulder harnesses, a $1625 option.

The base price for the 172R was recently raised from $219,500 to $234,500 and it will be available only through mid-2008 and only with the Garmin G1000. Going forward after that, only 180-HP gasoline models will be sold in two variants: The S-GA selling for $254,500 and the SP at $283,500.

With its 180-HP engine, the S is technically a different model than the R, but the two S models remaining in the line are essentially differentiated by options. The S is the old SP and is fully equipped with the GFC 700, XM-based datalink weather and leather seats. The S-GA is the stripped down model-G1000, but no autopilot and fabric seats. Both have air conditioning as a $28,100 option, along with TAWS, airbag seatbelts and other options.

To gain a sense of how the airplanes compare, we flew all three models. In doing that, we noticed that the R-model prop kept the engine from turning to rated power and this is noticeable on takeoff and climb. A 172R, loaded with 758 pounds of people and stuff, will use 945 feet of runway and 1685 feet total to clear the standard 50-foot obstacle. It will climb to an optimal altitude of 8000 feet with an initial rate of 720 FPM and once there, it will cruise at about 122 knots at 80 percent power. It can climb to 13,500 feet but wont have much left at that altitude.

With a flatter pitch prop and additional 300 RPM, the S manages a 960-foot ground run and 1630 feet total to clear the 50-foot obstacle. It will climb to 8500 feet initially at 730 FPM and will cruise there at 124 knots on 75 percent power. Service ceiling is higher at 14,000 feet.

These are clearly marginal performance differences, considering the difference in price. We noted that the slower turning R engine burns considerably less fuel. Ranges are 580 miles for the R and 518 miles for the S. This extra 62 miles might make a difference at the end of a flight, considering the cruise speed difference is only 2 knots. The 35-minute difference wont mean much on a 550-mile trip if you have to stop for fuel.

Just for grins, we flew an older Hawk for a comparison. A 1967 172 with a 145-HP

engine carried 1275 pounds of useful load and cruised at 113 knots, with a range of 478 miles. That airplane was on the market at $30,000, or only about 13 percent of a new R-model and a mere 10 percent of the diesels price. Invest another $30,000 in the older airplanes paint, interior and avionics and you’ll have a nice, capable and clean airframe, but not one thats remotely like a new glass-panel Skyhawk.

This, more than anything, explains why used aircraft prices are now stable or declining. The older airplanes, while their performance offers an excellent value, are showing their age. Cosmetically and to a lesser degree mechanically, they show heavy wear. No amount of air freshener will make tired Royalite and years of neglect smell like new. And lets face it, the ability to work in the system is more important these days than performance and increasingly, that means a glass panel.

Comparing the old to the new, flying the old 172 and the factory fresh model revealed what we expected: The old airplane was sloppy and drafty, the new one was tight, aerodynamically and mechanically. The cabins in the new airplanes are dramatically quieter, almost night and day. Control response is typical Cessna conservative, but immediate and predictable-just whats wanted in a Cessna 172.

The starkest difference is in the interior. The new Cessna seats are better made, more comfortable and safer, so much so that for reasonable money, youre not going to duplicate that interior in an older airplane.

Why would anyone buy a new Skyhawk, anyway? It will still cost most of a quarter of a million bucks. Wouldnt it make sense to spend a little more and get a faster Diamond DA40 or perhaps an Archer or even a 182? Maybe. Or maybe not.

Flight schools buy 172s for the same reason they always have-flexibility, economy, ease-of-operation and maintainability. Schools make money on 172s, new or used. Private owners weve spoken to who have bought Skyhawks have done so because their performance needs are modest, they don’t fly every day or even every week and the Skyhawk has no bad habits to bite the pilot who falls to the dark side of the proficiency curve. Its a comfort-level thing and Cessna has that figured out, too. Cessna also has its name and reputation-70 years of experience, a worldwide comprehensive service network and one of the best safety records ever, although Diamond probably bests it, in our view.

The Cirrus SR20 is a strong contender in this market, but the Skyhawk outsells the SR20 by more than two to one, this despite the fact that the SR20s least expensive variant sells for $214,900 with a glass panel, compared to the 172R at $234,500. The SR20 is also nearly 30 knots faster than the Skyhawk.

But it isn’t a Skyhawk and thats exactly the point. So if you want a 172, which is the best value? If youre choosing between the -R, S-GA and the -S, a look at the performance numbers shows that the performance is so close as to hardly justify the considerable price difference. The S doesnt carry much more, its not much faster, it climbs only a little higher.

Digging deeper, the fact that an autopilot isn’t available on the soon-to-be-discontinued R, regardless of the avionics package, may be the dealmaker/breaker for many buyers. Or maybe its the leather seats that explain why the SP model has outsold the R model by about four to one.

The sleeper here is the diesel-powered 172 TD. Diamond has clearly shown that there’s worldwide interest in diesel powerplants for aircraft and despite serious complaints about maintenance and longevity, Diamonds diesel program is a qualified success. If Diamond can, Cessna can. Round two of diesel development will be powered by the new Centurion 2.0 from Thielert and we predict that if Cessna encounters any maintenance issues with this engine, it will bludgeon them to a pink pulp with its established service network. Cessna is more than big enough to hold Thielerts feet to the fire on this and we think they will.

That said, were reserving judgment on the TD until the production version is available and in the hands of owners so we can collect some feedback. We also plan to fly it again.

Meanwhile, if you want a new Skyhawk and an autopilot is a must, the 172S is the better choice, despite its higher invoice number. But if youre a 60-hour a year recreational pilot, you can save a bunch of money and still have a glass panel by foregoing the autopilot and the leather seats in favor of the S-GA model-the new version of the R. In our view, thats not a bad choice.