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.
It's quite in depth and covers many effciency topics including EGR dilution, NoX reduction, Lean Burn and Hydrogen boosted Lean burn. Quite in depth.
