No problem Dave. Thought more folks might chime in... guess no one cares, haha. Either that or I just nerded out hardcore.

Traffic on this site has been a bit slow since sub $2 gas came back, sadly. (gas tax NOW!)

Well I said I would post up my gear swap calculations, so here they are.

Available options for my TDI are 0.681 and 0.658 5th gear ratio sets. (Stock is 0.755). I have also been considering a set of 205/60/16 tires (3% more circumference vs the stock size of 205/55/16).

The combination of the .681 gear and the 205/60/16 tires is almost exactly equal to the effective overall gear ratio as the .658 gear set, btw.

So whereas we could place a mph scale atop the former graph since our gear ratio was unchanging, we can no longer do this. Instead, trusting that our aerodynamic and rolling resistance is constant at a given speed, despite the vehicle gearing situation, we can interpret vehicle speed as a constant power requirement. (e.g. the blue lines from before). So below I plotted 72.5 mph = 24.3 HP, and for a Cd of 0.27, 72.5 mph = 22.2 HP.

The green dot is our starting point. Changing the gear ratio with a .681 ratio 5th, and the 205/60/16 tires, we move from 2496 rpm at 72.5 mph in stock form down to 2189 rpm, along the 24.3 HP line. Finally I again looked at a coefficient of drag reduction down to 0.27, sticking with the new overall gear ratio. Estimating the BSFC for each point, these are my results:

mph rpm load(kW) BSFC est gal/h mpg

stock gear & tire 72.5 2496 18.11 263.0 1.480 48.98

0.681 gear 72.5 2251 18.11 248.5 1.399 51.84

0.658 gear 72.5 2175 18.11 243.0 1.368 53.01

205/60/16 tire only 72.5 2427 18.11 260.0 1.463 49.54

.681 and 205/60/16 72.5 2189 18.11 244.0 1.373 52.79

Cd=0.27 .681, 205/60 72.5 2189 16.54 248.0 1.275 56.87

So, I'm sold. I think I'll actually stick with the stock tire size and grab a .658 5th gear! I may not ever recoup the entire cost of the gear swap ($350 - sale of stock gear = ~$200). But the reduction in engine noise, wear, and fuel consumption are all worth money to me.

edit: stupid table formats never hold up during posting.