Was cruizing home today from hot rod lunch today with the windows down enjoying the 70 degree day when I started thinking about the RPM the car was turning at highway speed. At 60 mph my wagon is turning 1900 rpm (327/350 combo) So what do you turn, who is the highest and lowest here.
in my burb i use to do a little over 3k rpm @ 60mph... probably less in the germchero can be a pain during L.A. freeway driving... semi's fully loaded do 65-70 and pass me up
I used the tach for lack of a speedo in my old Pontiac... ...determined that 2600rpm was 60mph, 305 Chevy, TH-350 trans, 12 bolt axle with 3.31:1 gears, 15" wheels with wide Formula One tires!
Boones. I'm 2184 @ 60. 2912 @ my usuall hwy speed of 80. MPH X gear ratio X 336 divided by tire dia. = rpm. You can also swap RPW in the formula to get MPH. See ya, buddy, -Abone.
[ QUOTE ] MPH X gear ratio X 336 divided by tire dia. = rpm. You can also swap RPW in the formula to get MPH. [/ QUOTE ]do you mean swap RPM for mph? i get a really high number.my speedo is off (tire diameter too small) so i'd like to figure out my MPH from the tachometer-george
Tire diameter X rpm, divided by 336 X diff ratio. To figure rpm at a particular mph, use 1000 rpm to develop an rpm constant. That will give you a constant - EG: 24 mph (in my case) you can use to figure rpm at mph. EG: 60 mph divided by 24 = 2500. Keep in mind this formula is exact only for stick shifts. Automatic transmission converters slip so you have to take that into account. In most cases, stock or low stall street converters will lock up fairly solid at highway cruising speeds with light throttle. In my 32 with advertised 2400 rpm stall (it actually flash stalls around 2800 rpm with the big engine at 3/4 throttle or so.) Stall speeds are related to torque input. IE: little engines lock up at a lower rpm and bigger engines at a higher rpm. Only a problem at full throttle settings. Highway speeds in my car are above the stall speed and even though not solidly locked up - there's always a little bit of slippage - the formula works out pretty well when comparing the tach with the on the money speedo.
3200 at 60 if the wind isn't pushing into me. I'm running 28.5" rear tires. Ford 302/C4/8" rear.. I'm fairly sure that's 4.11's or so. If it's worth anything, the C4 has about a 2000 rpm stall on acceleration, and it feels pretty locked past that.
[ QUOTE ] Speaking of RPMs, is there any place that sells a simple tach for less than $20? [/ QUOTE ] a swap meet
Lots. Stock 235, stock 3 speed, stock 3.70's... not so stock smaller tires. The tires will be changed soon though. Derrick
350/700r, 3:89 gear ratio, 30" tall tires, 2500lb. coupe =1800 rpm @ 60mph & 3000 rpm @ 100mph Later, Mark
Rocky goodtime van with 350/350, 27 inch tire, unknown gear ratio [supposed to be 3.08] turns 2700 @ 60 mph.
My speedo (picture that guys.... me in a speedo.... HOT!!! ) doesn't work, but I'd guess about 3300 - 3500 rpm... th400, 4000 stall, 4.11 gear, 30" tires.
About 2400 rpm w/ a 239 cid flatty (3:78 banjo & 31.5" tires). Great formula.....at least I know my 6V Sun tach is close.
approx 1750 at 60.......425hp-383 ci. sbc with T5 stick, 4:56 with 31inch tire but, usually drive around 75-80 mph ........or more EEEEHAAA !
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