The Stovebolt Site Forums The Shop Area The HiPo Shop RPM's with certain tire size/gear ratio and Trans

Hey all,
Could anyone tell me what rpm's I should have with a 3:54 gear ratio, a 327 engine with a mild cam in it (not sure exactly; just that it is mild),31X10R15 tires, and a T5 WC trans with 2.95 1st gear at a speed around 70-75 MPH?

THis is in a 1963 GMC 1/2 ton 4x4
I was concerned that 5th gear might bring the RPM's too low in 5th gear to be running at optimum performance which I believe to be around 2200 to 2500 RPM?

Also, if you can, could you tell me how you got this info so that I may know how to do so?

Thank you Thank you Thank you!!!!
Shane
Warrensburg, MO

Here's the formula; RPM = (MPH x Gear Ratio x 336) divided by Tire Diameter.
So in your case (assuming a .70:1 fifth gear) would be 2014.38 rpm at 75 mph.

Do you happen to know what RPM 327 likes to run at cruising speed? Thanks again!

2,000 to 2,800 with a mild cam .That's a educated guess you need the cam info or just drive it to get the feel for the torque from the engine.To many variables for a positive answer to the question.

TRY THIS SITE. THE MORE info you got the closer it will be to your rpm.

http://www.csgnetwork.com/multirpmcalc.html

Thanks everyone! I appreciate it very much!
Shane

I guess it will all boil down to the tires then. So, once I find out where the peak torque is in the 2000-28000 range, then I would just need to find a tire height that corresponds with that RPM at that speed with that gear ratio in fifth gear right?

Thanks again!
Shane

I'm running a 2.95 1st gear T5 "Z" hybrid behind my built 355 cid small block with a 3.73 R&P and a 28" tall rear tire in my streetrod and my 5th gear is a .63. Using your numbers and assuming your 2.95 1st gear T5 is also a .63 you would be turning 1692 rpms at 70 mph in 5th gear.

Based on my experience I'm afraid that your 5th gear would be of little use. (once again, assuming that your 5th gear ratio is .63) The 31" tire diameter and the 3.54 would have you coming into the most efficient section of your power curve at speeds well above the legal speed limit and would have you lugging down your engine and requiring downshifts at or below 60 mph.

My combo (.63,28"3.73,2850 lb) lets me drop down to 50 mph without a downshift but a 4.10-4.11 would let my engine operate deeper into the power curve more efficiently. With the heavier 4X4 a 4.56 which would only give you 2180 at 70 mph....very shallow in the power/torque curve for the small block.

If you plug in your numbers using the CSG calculator mentioned by "Grandpastruck" and you confirm the OD ratio of your T5 you should be able to come up with some numbers that would put you in the power curve of the SBC. Being that both your front and rear R&P ratios in your differentials are the same, it would appear to me that the least offensive way to get your engine spinning efficiently is to get your transmission final drive ratio numerically higher or to drive the truck as a 4 speed with limited use on the highway in OD.

Once again.....this info is based on the 2.95 T5 with a .63 OD. If by chance you have a different OD ratio you can plug in the numbers and make your own call. I hope this helps.

Dave