speed vs tire size vs rear ratio vs trans ratio

mywhite89

Registered User
http://www.f-body.org/gears/

Go to that website and plug all your numbers in. According to my 89 5-speed with stock gears my car should be going about 77 at 2,000 rpms. I am really going about 70mph at 2000 in any sc 5-speed that I've ever driven. What do you guys think about that? 3.08 rear gears would make 70mph at 2k about right, and speeds that you reach in the other gears about right.

Chris

I'll have to test driveshaft vs tire rotation to see what it is. bad calculator probably is my guess. Or maybe my rpm gauge is way off.

Double check with this website also: http://www.rearendparts.com/pages.php?pageid=16
 
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Or maybe my rpm gauge is way off.

The calculations need to assume various values barring any measurements. I.e tire diameter is assumed, not something that you can really assume since the same size tire can have many different outside diameters across manufacturers.

And yes, the stock tach is not exactly dead on, i've seen variations of easly 5-700 rpm on the upper range of the scale on indicated RPM vs RPM that the engine computer sees.
 
And yes, the stock tach is not exactly dead on, i've seen variations of easly 5-700 rpm on the upper range of the scale on indicated RPM vs RPM that the engine computer sees.

Not only that, but the speedometer could be off too.
 
Theres no way for it to compute the drag. At 70 mph the resistance on your car is huge! Like goin up hill requires more power (rpm) so does just cuttin through the air. Theres an article about top speed and hp. Try and find it.
 
Drag has nothing to do with speed, but on 95% of all SC's both the speedometer and the tachometer are wrong. Use a GPS speed and a QH to check speed and you'll find that the calculators are dead nuts right.
 
It does if your comparing rpms and vehicle speed. Put ur car in neutral and coast down a big hill til you get 70 mph. The rpms will be at idle, say 900 rpms. Turn around and go back up the hill. Your not gonna be at 900 rpms.
 
http://www.f-body.org/gears/

Go to that website and plug all your numbers in. According to my 89 5-speed with stock gears my car should be going about 77 at 2,000 rpms. I am really going about 70mph at 2000 in any sc 5-speed that I've ever driven. What do you guys think about that? 3.08 rear gears would make 70mph at 2k about right, and speeds that you reach in the other gears about right.

Chris

I'll have to test driveshaft vs tire rotation to see what it is. bad calculator probably is my guess. Or maybe my rpm gauge is way off.

Double check with this website also: http://www.rearendparts.com/pages.php?pageid=16

well i checked mine and it is almost the same as the calculator. but you said 3.08 gears when in fact all 5 speeds had 2.73 gears. on another note i used 5k as my redline so if i can get my car at 5k in 5th gear i will be going 193mph i think i'm gonna try it. plus you have to put in every gear ratio in the tranny.
 
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It does if your comparing rpms and vehicle speed. Put ur car in neutral and coast down a big hill til you get 70 mph. The rpms will be at idle, say 900 rpms. Turn around and go back up the hill. Your not gonna be at 900 rpms.

He is not talking about neutral. The fact is, the gear you are in compared to how fast you are driving is a direct math equation for how fast your engine is turning considering your rear axle ratio and gear ratio in your transmission for whatever gear you are in. Neutral has nothing to do with what we are talking about. Shut your engine off at 120mph and you have 0 rpms!

Chris
 
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well i checked mine and it is almost the same as the calculator. but you said 3.08 gears when in fact all 5 speeds had 2.73 gears. on another note i used 5k as my redline so if i can get my car at 5k in 5th gear i will be going 193mph i think i'm gonna try it. plus you have to put in every gear ratio in the tranny.

Yup, I was just stating then when I plugged in 3.08 instead of 2.73 as my rear axle ratio, my speed vs rpm seemed closer to what I actually see. My car does have stock 2.73's though.

Chris
 
Theres no way for it to compute the drag. At 70 mph the resistance on your car is huge! Like goin up hill requires more power (rpm) so does just cuttin through the air. Theres an article about top speed and hp. Try and find it.

If you maintain constant speed going uphill, RPM will not change unless you have an automatic & the converter unlocks, or the tranny downshifts... on manual tranny, RPM will not change unless your clutch is slipping...
 
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