I'm wondering what kind of MPH (trap speed) increase one would get from changing gears (2.73s to 3.73s). I'm perticularly talkin about a small inch small block that makes its power from 4000-6000, and nothing below 3500. The 3-4 shift happens just a little before the 1320' mark, so its trapping at like 3300-3400 rpm. So if any of you could offer some opinions, or experiences, i'd really appreciate it.

I am no expert, but I thought more gear (numerically higher) actually will DECREASE your trap speed. :confused: I am sure someone else will chime in soon..........

I don't think your MPH will change from swapping gears..you'll cross the stripe at the same MPH,but at a higher RPM

You'll just accelerate faster up to that mph, but you might get going faster

ok, so teach a young student here..... his TOP speed will decrease though, right? In other words, if two cars were identical, with the same transmission, a car with a 2.73 gear would have a higher top speed than a 4.11 geared car.

Originally posted by 71boo:
ok, so teach a young student here..... his TOP speed will decrease though, right? In other words, if two cars were identical, with the same transmission, a car with a 2.73 gear would have a higher top speed than a 4.11 geared car. correct
With 2.73's,I cross the 1/4 stripe at 4100 rpms @ 103 MPH with 26" tall tires..with 4.10's,it would be closer to 6000 or so for the same 103 MPH

I predict that you will increase mph by 5, decrease et by .6 .... how about that...

I switched from a 3.08 to a 3.55 rear gear. My trap speed went from 117.xx to 120.xx and my E.T. went from 12.0x to 11.7x. Same track and same conditions. This wasn't a Chevy vehicle so your "mileage" may vary. smile.gif

When I used to have a stick and 3.55 gears in my car the best trap speed at SDR was 116.79. I switched to a 4.11 gear and the best trap speed at the same track (similar weather conditions) was 120.46. Average trap speeds were up ~3mph all the time with the 4.11 gears.

Originally posted by 71boo:
ok, so teach a young student here..... his TOP speed will decrease though, right? In other words, if two cars were identical, with the same transmission, a car with a 2.73 gear would have a higher top speed than a 4.11 geared car. not necessarily...you would think so but only if the car is making enough power to go to the redline in both gears... with 273's you may only be able to go to 4000 rpms because the car isnt making enough torque to overcome air resistance and hence will have the same top speed...

Thanks for the responses guys. Yeah, I'm not talking about top speed. Just the trap speed. I believe it would increase because that 3-4 shift is so close to the end of the run, you lose any top end acceleration from the shift and the rpm's dropping right out of where it needs to be. Going from 2.73s to 3.73s would bring the trap RPM from about ~3400 to ~4800, which would be right around the cars peak torque. I'm sure any MPH increase would be insignificant in a mid to high torque automatic car. Could easily loose MPH if u put too much gear. But in this situation its a low torque small block with a 5 speed.

Kjett, how far down the track were you making your 3-4 shift, before and after the gear swap? Also how much did you trap RPM change? Thanks

Originally posted by -SS454-:
Kjett, how far down the track were you making your 3-4 shift, before and after the gear swap? Also how much did you trap RPM change? Thanks SS454,
With the 3.55 I was shifting 3-4 at approximate 1,100-1,200'. With the 4.11 I was shifting at around 700' (going from memory). Trap RPM with the 3.55s was ~4,900 and with the 4.11s it was right at 6,000. That engine combo made peak power at 6,200 so I could have prolly ran faster with a 4.33/4.56.

Originally posted by doggy69:
not necessarily...you would think so but only if the car is making enough power to go to the redline in both gears... with 273's you may only be able to go to 4000 rpms because the car isnt making enough torque to overcome air resistance and hence will have the same top speed... ..not the case in my car :D 4000 rpms I'm just getting started on the highway..I cruised at about 5000-5200 once (not for too long)..pretty wild with a built big block and 2.73's :D

Mr 4 speed 5k rpms is what 140mph?....I havent passed 120 on the street...in the chevelle. My mothers corvette is a different story :D

I looked back in my time slips and my car picked up one (1) mph going from 3.36 to 4.56 gears. ET picked up 3-4 tenths. Weather could have been a factor but this was during the summer (1999) so conditions were probably very similar. I don't have weather records.

Pat, is your car an automatic? Also what was your powerband with the engine?

Kjett, thanks for the additional information

Yes, a TH350. 3000-6500 at that time. Different engine now.

even if you end up with the same trap speed, the TIME it takes to get to that speed should decrease- say you ran 12.5 before the switch, you might see 12.2 or so after the switch, or maybe 12.1 or lower depending on how hard the engine was struggling to pul lthe gear..

i remember my very first gear swap i went from 2.56 single spinner with a bbc to a 4.56 posi.........well darn it i slowed down and my mph fell way back i think i lost about 5 mph...............with the 2.56 gears i was shifting at 5500 rpms on the 1-2 shift and crossing the line as well at 5500 rpms just at 100 mph..........not bad for a 14.40 pass..........the 4.56 and taller 30 inch slicks slowed me down my 60 foot time suffered and the car just didnt seem pull as well....the end result was a slower car......so then i built the engine to the gears......lol oh the younger days..........