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Overdrive conversion site?
- Thread starter Coleman
- Start date
66benchcoupe
Member
rpm= (mph x gear ratio x 336)/(tire diameter in inches)
mph = (rpm x tire diameter)/(gear ratio x 336)
mph = (rpm x tire diameter)/(gear ratio x 336)
The AOD has a .67 :1 overdrive gear ratio, so your effective final drive ratio in overdrive is 3.40 * .67 = 2.278:1. Your tire is 24.7" tall / 77.5" circumference, so it should work out to about 2150 rpm at 70 mph."Coleman" said:
Might play with the calculators here: http://www.ramblerman.com/blazer/gearcalc.htm or http://www.clarks-garage.com/shop-manual/trans-01.htm
I've found this one helpful. http://www.car-videos.net/tools/speedrpm.asp Enter each transmission gear ratio, rear axle ratio, and tire size and it will give you the speed and RPMs for each transmission gear. You can change the RPM interval too, to go more in depth.
http://www.angelfire.com/fl/procrastina ... html That one is nice if you have information and want to know what speed a different rear axle ratio would give you for a certain RPM.
http://www.angelfire.com/fl/procrastina ... html That one is nice if you have information and want to know what speed a different rear axle ratio would give you for a certain RPM.
Jonk67
Well-Known Member
Here's one I use lot:
http://www.ringpinion.com/Calculators/Calc_RPM.aspx
Transmission Ratio RPM Section:
1. for 'R&P ratio' put your choices - 2.50 > 3.40. I made a spreadsheet with each ratio and the corresponding RPM's at 60, 70, 80, 90mph so that I could compare what effect each step up/down in R&P would make then chose the one that matched the RPM I wanted for my most common speed in OD (80).
2. for 'Tire height' put 24.7 (rounded from 24.69" as it only allows 3 numbers)
3. for 'Speed' I put 60, 70, 80, 90 as these were the speeds I would see, 80mph being most often on the interstate.
4. for 'Trans Ratio' put 067 (your AOD is 0.67:1 in OD, the box puts the decimal)
With your tire ht. and 3.40:1 in OD at 70mph your RPM's would be 2,169,
with a stock 2.89:1(lowest I've seen in a mustang, never heard of them coming with a 2.50:1) your RPM's at 70mph would be 1,844, which would be too low for me as I believe the engine would be almost lugging at that low RPM. Plus you would lose the off the line snap you gain by going OD and upping the rear gears. I settled on 3.50:1 for my AOD but also considered 3.73 as I plan to hit the track every few months, with 90% DD I settled on 3.50 as a compromise for now. My newer cars are at 2300-2500 at 80mph in OD.
Jon
http://www.ringpinion.com/Calculators/Calc_RPM.aspx
Transmission Ratio RPM Section:
1. for 'R&P ratio' put your choices - 2.50 > 3.40. I made a spreadsheet with each ratio and the corresponding RPM's at 60, 70, 80, 90mph so that I could compare what effect each step up/down in R&P would make then chose the one that matched the RPM I wanted for my most common speed in OD (80).
2. for 'Tire height' put 24.7 (rounded from 24.69" as it only allows 3 numbers)
3. for 'Speed' I put 60, 70, 80, 90 as these were the speeds I would see, 80mph being most often on the interstate.
4. for 'Trans Ratio' put 067 (your AOD is 0.67:1 in OD, the box puts the decimal)
With your tire ht. and 3.40:1 in OD at 70mph your RPM's would be 2,169,
with a stock 2.89:1(lowest I've seen in a mustang, never heard of them coming with a 2.50:1) your RPM's at 70mph would be 1,844, which would be too low for me as I believe the engine would be almost lugging at that low RPM. Plus you would lose the off the line snap you gain by going OD and upping the rear gears. I settled on 3.50:1 for my AOD but also considered 3.73 as I plan to hit the track every few months, with 90% DD I settled on 3.50 as a compromise for now. My newer cars are at 2300-2500 at 80mph in OD.
Jon