Chris:
Others may differ (especially mathematicians and engineers that see my math
) and this is only one solution but, after years of doing this I think it is a good one. Especially if you add up the costs for the R&R of the R&P +, + + and you are left with only one box. Think about buying a re-mfg box with the 8:31.
What you have:
7:31 = 31/7 = 4.43
8:31 = 31/8 = 3.88
3.88 *1.14 = 4.4232 (close to 4.43) = ~14% difference.
6,200 (max rpm?) *14% = 868 rpm (roughly b/c I don't think this correlates directly)
Sooo.... if the gear stack is the same in the re-mfg. 915 box then you should be able to gain@ least something close to the ~ 868. I don't think it works out exactly like this but you get the idea. In our track boxes (not street 915s) we are geared for 800-1,100 rpm drops per gear (3rd, 4th 5th). This works for us b/c the motor is built to run there. You need to look at rpm drops between the gears you use the most (3rd, 4th, 5th ?), fat spot on your torque curve, and tire circumference.
This might create another problem b/c you already know final drive ratio isn't the only component to top end speed and doesn't correlate directly. You have less effective torque in each gear as each one is made taller by the 8:31 R&P stack. So, you may have less power in 5th to push against the air @ 120, 130, 140 than you did with the 7:31 R&P. Will the 3.6L make up for this...? I don't know. It probably has enough torque to overcome the aero resistance and the taller R&P.
How do you figure it out? I don't know. Some mathematician here can figure it out or you can get the gear charts from Bruce Anderson's book (?) and calculate hp req'd, available torque in rpm operating range, frontal area presented (drag coefficient) rolling resistance of tires, etc...
E.g when we swap from 10.5" and 14" F and R wheels to 10" and 12" wheels and switch the wide PCA CR bodywork (big air dam / splitter and 3.8 RSR double tail) back to the '73/'74 look RSR narrower bodywork (traditional '74 RSR nose and IROC tail) on our IROC we pick up a few hundred rpm and top end speed. However, in the process, we loose cornering speed as evidenced by doing segment timing. A good friend of ours runs a similar 3.4L RSR with narrow bodywork (no side pods, double tail) R-compounds (not slicks), '73 RSR nose. At the club race this past weekend at Sebring we were qualifying / running 2.19 and he was qualifying 2.20 ... on a 17 turn loooong track
. During the sprint race we were 2:20.2 and he was 2:20.7. The two cars are very similar (even the drivers / style). I know there are a lot of variables in this comparison but I ran against him last year (same cars) and he was quicker on the straights, we were quicker in the infield / corners. Cat and mouse, speed v. handling. Wide-er body tires v. narrower.
Your other solution is swapping rear tires for different total circumference. Known as the "poor-man's R&P swap."
This is what I did the last few years on the tub. Note: changes ride height as well and s-bars have to be adjusted - maybe).
E.g. I run 195/60/15 (1932mm Circ.) on a 6" rim on my tub. I get to about 7,200 rpm F an B straight of VIR. At SP I get to 7,000 rpm on F straight. Last CR @ SP I swapped to 205/50/15 (1841mm Circ) on 6s and picked up 1.5 - 2 .0 seconds
1932 - 1841 = 91mm
91 / 1932 = 4.7% (smaller)
7,000 * 1.047 = 7,329 (~7,400 rpm)
So, I was running up to 7,400 rpm in most gears b/c I shortened the tire circumference (too much). I ran out of rpms on the F straight.
Tradeoff: wider rubber was better everywhere, higher rpm limited the advantages of wider rubber. I could have gone faster if I could have found a tire (not wheel b/c of rules) size in the middle of the two. This only works
in this direction if you aren't running out of RPM after the swap.
At daytona I had the opposit problem. I would hit 7,400 rpm with the 195 / 60s and watch eveyone go by, or most everyone.
I could count 11 or 12 "mississippis" sitting there at 7,400 on the banking. Absolute motor killer.
Your numbers (based on RA-1s - assuming not shaved):
225/50/16 = 1983.6mm Circ.
255/50/16 = 2077.85mm Circ.
2077.85 - 1983.6 = 94.25
94.25 / 1983.6 = ~4.75% (larger)
4.75% of 6,200 = 295 rpm
Gets you ~300rpm (assuming 3.6L has the torque to get you there AND you can fit & get 255/50/16 rubber).
So ...
8:31 R&P will get you ~14%
255/50/16s will get you ~5%
Play around with it and have fun.
Jase