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Speed/RPM

F

fasterthanu

Guest
I think I have a big problem. I got a 700R4 trans installed several months ago. My tach use to jump around, until I replaced coil. Seems to be fine now. I have checked RPM vs Speed of mine vs websites that have me enter in tire size, trans ratio, and rear end. 1st, and 2nd gear for sure match dead on. Can't remember if 3rd does, but no reason it shouldn't. Most 4 speed transmissions are 1:0 ratio in 3rd. In 4th gear though, I was taching 2500@74mph with a 3.08 gear, 255/60/R15 *27.05inches*, and supposedly a .70 4th gear. Well, http://www.angelfire.com/fl/procrastination/rear.html shows I shoould be running at 94mph. I KNOW my speedometer is off by 3mph, I checked it with a GPS, even at high speeds... Well, that then shows I have a .90 4th gear. Tested two other rpms, with same result. can someone run their tiresize, gear, and trans ratio on this site, and let me know if it is accurate? I have tested it against other sites, and the one under tools, and the results are identical. I do know I have run at 3000 in 4th, and definently have was not going 112mph. I am going to run two wires in the car with me next week and check that tach vs another tach.... Do these numbers match what you are doing on that site??
 
Joined
Jan 1, 2002
Messages
7,246
Location
Washington, Michigan
Corvette
'67 Marina Blue Convertible
My calculator shows you should tach 1982 rpm at 74 mph, and 2500 rpm should be 93 mph; if your tach is right, something else isn't - if it shows 2500 rpm at 74 mph, that would be a 3.90 axle with your 700R4.
 
F

fasterthanu

Guest
Yep, I would agree 100% with you on that. But, I raised both wheels off the ground, and turned them 1 full turn. The drive shaft moved just a hair more than 3 full turns. Will check it with another tach. The only rpms vs mph i tested i voice recorded on my phone yesterday. Shows I have .90 4th gear instead of .70. Will take my baby out tomorrow, and test rpm range in OD from 60mph - 90 mph... Also, have you checked the calculator vs your car? They are accurate?
 

mmvette80

Well-known member
Joined
May 12, 2002
Messages
273
Location
Central NY
Try www.700R4.com. They have a calculator for what your looking for. I installed a 700R4 about month ago and im running about 200rpms higher than the calculator. 75mph comes in at about 2100rpms. At 3000rpms im doin about 105ish. I also think that the rear ratio is actually 3.07 but dont quote me on that.
What kind of top speed can you get out of the tranny in OD. I got up to 120 then she would go in and out of OD
I have 255/60/15 BFG's on stock rims and stock rear.
 
F

fasterthanu

Guest
I have only had her up to about 120, no problems then. Wish I would have checked the rpm then =P
 

mmvette80

Well-known member
Joined
May 12, 2002
Messages
273
Location
Central NY
When I had it up to 120mph the rpms were at 3500. I dont have the WOT valve so it wouldnt go any faster.
 
R

rpounds

Guest
I have created an engine speed calculator in Microsoft Excel. It takes into account such things as tire diameter, transmission drive ratios, differential ratio, engine red line, etc. With this information, it will calculate theoretical engine speed for any given MPH or vice versa.

What it does not take into account is torque converter slippage. TC slippage is too variable, depending upon stall speed, etc., so I left it out.

I'll be glad to send it as an attachment to an e-mail. Anyone interested, drop me a line at Rpounds@hotpop.com and I'll forward it to you.

Ron
 

mmvette80

Well-known member
Joined
May 12, 2002
Messages
273
Location
Central NY
Wide Open Throtle Valve. If you hold the pedal to the floor OD will kick in over 120mph.
 

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