I'm not trying to insult you but it takes a fairly healthy engine to pull 120 mph @ 4,800 rpm. A 3.80 (8" I assume you have) would be good for Pacific raceway's long straight. Your mechanic is suggesting a four speed for better ratio selection in lower speed corners. It would make you faster that you are with the C4. If you are going to a T5, make sure you get one with the 2.95 low as the standard 3.35 low might be too much rpm in a lower gear and not enough in a higher gear due to the "wider" ratios. That is a technical track for sure.
No insult taken - I'm spinning closer to 5k to get to 120mph. I just used the 4k as a comparison with the different gears.
Gearing will depend on a few factors. You gave speed, tire height and possible transmission, what motor are you running and where will it make the best torque and hp and what rpm range does it make it? When I was running 289/302 engines, gearing was 456, 411, and 370. Went to a 351 and gearing was 370 and 350. Later went to a stroker and gearing is 350 or 325 all depending on the track.
Warmed over 289. Power band is about 2600-3400.
With the spec's in your post, you will need a 300 gear to give you 120mph @ 4800. fast, cheap, durable ... you can only pick 2
Funny you should mention a 3.00...I'm "borrowing" a friend's 3.00 3rd member this weekend - not to keep permanently, but just to narrow down the cause of the noise, to see how bad of shape the current gears are, and to buy me time to order new ones and get the 3rd rebuilt.
While I would LOVE to get lower gears, the T5 install is a ways off, and I need to run this car with the C4 for the foreseeable future. It sucks, as I would rather do it all at once and be done with it, but I'm still about $1300 short of the T5 conversion. Grrrr. Thanks all for the advice!!
1966 Restomod/track car, 1973 Q-code Mach 1, 1997 F250 diesel, 2000 VW Beetle Rallycross car, 2010 Crown Vic (ex-City of Lakewood PD), 2015 Focus ST-2