18%???

For a modern drivetrain, something closer to 11 to 11.5% is about right. That still makes your dyno figures impressive.
But I use 11.5% personally and here is why. These are my personal cars most recent, on a calibrated Mustang (constant load) chassis dyno, all on 93 octane:
1) '02 ZO6 M6 (stock, 5K mi): rated 405 SAE, 359 RWHP for 11.3% losses
(this engine was later bench dynoed out of the car to SAE specs and put out 406 HP). This car eventually made 740 RWHP on this same dyno with a sC'd C5R, and why I claimed it made 834 HP, reversing the ratio.
2) '04 Porsche Carrera S2 M6 (355 HP, 318 RWHP for 10.4%) I've always maintained Porsche horses are a bit bigger than others.
3) '07 Aston Martin Vantage Prodrive M6 (420 rated HP, 371 rwhp for 11.7% losses).
4) 08 Lincoln 3.5L A6 2wd (263 rated HP on 93 oct/255 on 89oct), 224 FWHP on 93 octane, 218 on 89 octane - (tested one month apart not the same night) for right at 14.8% losses -- but remember it is an automatic trans.
5) '08 Ferrari F430 stock, 3800 mi, 429 rwhp. This car is not my car but a friend's, and stone stock and not even tuned -- this gives 12.9% losses if you use the 493 HP rating you sometimes see quoted but 11.9% if you use the Us-specific figure of 485 which some dealers conveinently forget to mention. I have yet to have my car (which had a "Scuderia engine upgrade" fitted to it before I picked it up) tested - it has only 1500 miles. I will test it at 3500 miles and be very disappointed if it does not produce something just slightly over 450, and I admit even that increase over my buddy's car is poor return on the $53K for the upgrade, but I wanted 500+ factory HP.
6) Anyway, then last night, watched my son test a new (90 miles) 5.0 Mustang M6 (the first I'd seen, it had just come into his dealership). Rated 412 HP. Put down 359 for 12.8% losses, but then its is a new and very tight engine and trans.

 

I guess it would depend on the dyno.. but from what I've seen for regular 430's and Scud's on a dynoject, 15-18% would be typical.. we did some runs on a Mustang dyno and had similar results.

 

I've worked with enough modern automotive firms to know none would be pleased with engineering that left their cars with 15-18% lflywheel to road losses. 15% was about average in the late 80s, but the push for greater economy and efficinecy has led to some cars getting down close to 10% today: low friction face gears and bearings, better constant velocity joints, and very think lubricants all contribute about a percent of so each. 12% of a bit lower is about right for cars with clutches (manual or F1 type) and around 15% for auto trans cars now.

Curiousity made me have my F430 dynoed today (only 2000 miles so engine and drivetrain are both a bit tight still - I'll have it done again in another 2K) but it put down 448 rwhp, which means right at 12% (the upgraded engine kit was rated at 512). In another 2K miles I expect 451-452 of so.