Not saying it didn't happen
( estimated figures below according to
www.wallaceracing.com/calculators )
did someone change the speedo pinion in the trans to match the new gears
if not "the speedo would be wrong", faster than reality
not knowing all the spec.s of said worn out old 440
maybe more was done to that motor than you knew ?
or maybe
1st time you actually opened up, wide open, the secondaries
(Vacuum Sec. maybe)
It'd be about a 3.44:1 gear (your case maybe a 3.55:1) with a 27" tall tire
at 6,000 rpm to go 140mph+
most even slightly modified 440s won't do 6,000 rpm for any time (more like 5,800)
unless some lighter valves & much better valve springs, you said you had 3.91:1 gear
same thing with 6,000 rpm 27" tall tire 3.91:1 rear gear, would only be 123.26mph
be 133.53 mph at 6,500rpm a stock-ish 440 won't go 6,500 rpm,
not without serious valve float, hydraulic flattappet, even Rhoads anti pup up lifters
back in the day
even with that 3.91:1 gear ratio, camshaft & headers, Holley carb
most would top out at maybe 5,800 with great air/cool sea level air even
be 140.81 mph at 7,000 rpm a stock-ish 440 even with a purple shaft cam
won't go 7,000, not live anyway, not without many other parts
unless you had taller tires, with that 3.91:1 gear ratio
& most taller street tires weren't available back then, N50/15 was like 27"
an L60/15 was about 27" too
maybe 28" tall at best
not trying to burst your bubble
sounds like fun
I got my stock 68 Charger R/T 440 727tf,
I think I only had 1.3/4" headers at the time, 2.5" exhaust turbo trust Hemi mufflers
nice low-ish miles like 50k org. miles, otherwise stock, with wheels & tires
with a 3.55:1 gears & like 26" tall tires Big-O L60 15s
to appr. 135-136-ish (according to my 150 mph stock speedo)
held on the floor for miles, would not go faster, nice cool fresh night air
going down hwy-50 from Placerville, back down to Shingle Springs (Calif foothills)
at my Highschool dance after a Basketball game, with our rival HS in Placerville
no way with a 3.91:1 gear, it wouldn't go over 5,600 rpm max,
at speed & at that wieght of the car & wind resistance
not until I changed a crapload of parts
292*/0.509" Purple Shaft HFT camshaft, true 1.5 Crane ductile Iron adj. rockers
matching valve springs, for that cam, with Rhoads Anti-pump-up lifters
w/titanium/retainers, keepers & Titanium intake valves,
aluminum TM 6 Edelbrock single plane Intake & well-tuned 850cfm Holley,
with the 3.55:1 yeah it'd go 150, still only like 6,500 rpm at best at speeds
stock heads are a real choke point on these B/RBs
unless you have MW or some extensive porting etc.
I was more into the stop-lite to stop-lite challengers
& off the line acceleration,
so I had steep gears like 4.30:1, 111-ish mph @ 6,500 at that point
it was spent
it could maybe wind higher, not for long with stock rods/stock cast pistons
especially with stock port cylinder heads 906s std on all 383-440 1968-70
or not spin a bearing or break it