Gear RPMs/Speed not correct?

Sonic04GT · Dec 25, 2009

I just had 3.73s installed on my '04 GT but RPMs seem to be lower than I thought they would be. I found these figures online...

2558 RPMs @ 80mph in 5th gear
2078 RPMs @ 65mph in 5th gear
2351 RPMs @ 50mph in 4th gear

50mph in 4th gear seems to be pretty straight but at 65mph in 5th gear I appear to be sitting more at 1800 RPMs, not 2078.

Haven't been on the highway since I drove the car home so I can't test out 80 MPH but I do believe at around 75mph I was below 2500rpms.

Does this sound correct? Are those bad numbers?

Car has the Diablo 87 tune including the 3.73 change and should be calibrated for 18" wheels. Rear wheels are 18x10 with 295/35/18.

Edit: Found these numbers on another forum...hmmm

80mph in 5th = 2421
60mph in 5th = 1815
40mph in 4th = 1952

Kinda makes more sense. I know there's probably along of things that factor in. Should only be a couple few hundred RPM difference from stock right?

wmburns · Dec 25, 2009

At 18" diameter only includes the RIM size. Need to include the entire diameter of the TIRE. Try using 26.13" for the 295/35/18 tire diameter.

Tanus · Dec 25, 2009

tire height affects it,65mph + 5th= 1800 rpms, sounds about right, with 4.10s mines at about 2k at 65. maybe a little higher, but still pretty low.

Sonic04GT · Dec 25, 2009

Thanks for the info guys.

cronin49 · Dec 26, 2009

I have 373's, and at 2000 rpm I am doing about 66 mph in 5th. Your numbers seem to be off.

ChillPhatCat · Dec 26, 2009

wmburns said:
At 18" diameter only includes the RIM size. Need to include the entire diameter of the TIRE. Try using 26.13" for the tire diameter.

There's no way he could get those numbers with a difference of 8" in diameter... it would be off by almost 50%.

wmburns · Dec 26, 2009

I agree. But the OP stated that 18" was used as the tire size in the programmer which is clearly not correct. Perhaps the OP can tell us what numbers were used when the PCM was reprogrammed. Or the OP can re-run with the correct tire size and re-post the results.

Stan Weiss · Dec 26, 2009

Stock 245-45-17 tires (Diameter = 25.68) 3.73:1 rear 4th gear 1:1 2440 = 50 MPH

On my web site you will find these calculators
Automotive Performance Software / Interactive JavaScripts Calculators

Calculate Speed (MPH - Km/H) given RPM, Rear Gear Ratio and Trans. Gear Ratio

Calculate RPM given Speed (MPH), Rear Gear Ratio and Trans. Gear Ratio

Sonic04GT · Dec 27, 2009

These are the numbers I got with a 26.13" tire diameter.

4th Gear
40mph = 1919
50mph = 2398
60mph = 2877

5th Gear
70mph = 2249
80mph = 2571

We shall see if these make more sense. I gotta get back into the tuner and see what I have it set for. I remember doing the calculations online to get a number in the 200s I believe and then setting it in the tuner.

Thanks for the link Stan. Used your calculator to get those figures.

Used 1:1 for 4th and .670:1 for 5th.

patman0911 · Dec 27, 2009

This would probably get swallowed up by the margin of error anyway but 5th gear in a 3650 is .68 - T45 are .67

Sonic04GT · Dec 27, 2009

Wikipedia failed me then.

Tremec TR-3650 transmission - Wikipedia, the free encyclopedia

So in that case...

70mph = 2283
80mph = 2609

Gear RPMs/Speed not correct?

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