2005/2006 G35 Sedan owners with the Sport Package
and AUTO transmission...got a favor to ask:

With the STOCK Sport Package wheels and tires on
how fast are you going when the tach is pegged
right at 3000 rpm's??

If you could do me a favor and set your cruise so the tach needle
is right on 3000 rpms and check your speed for me that would be great.

Reason for the request:
I've put aftermarket wheels/tires on the car that are a bit larger
in O.D. than the stock Sport Package wheels/tires and want to
see exactly how much my speedo is off.
I know there are "tire calculators" but they are "estimates" at best since
every tires exact O.D. is a little different and the tire calculators don't
take wheel WIDTH into factor.

Thanks in advance for the help.

 

lol and how does the width of a tire affect the odometer exactly? haha

anyway at 3k rpms, i'm around 75mph

 

For the 18" sport wheel/tire (235/45R18)

MPH=(engine RPM X Tire Radius) / (Final Gear Ratio X 168)

engine RPM= 3,000

Tire radius: Aspect ratio (235*.45) x .0394(mm to inches conversion)+(rim size/2)= 13.17

Final gear: 3.36 x .83 (5th gear ratio 5AT)= 2.79

MPH= (3000 x 13.17)/(2.79 x168)

MPH= 39510/468.72

MPH= 84.29@3,000rpm

 

I took the G on the road today and noticed that at 3,000RPM the car is at about 75MPH. At 80MPH the RPMs are just over 3k. Car specs are listed in sig.

 

I have the 6Mt and at 3k, I'm at 80mph.. I know you said Auto, but I had to share

 

The width of the tire doesn't affect the odometer....the width of the WHEEL
will affect the O.D. of the tire. The wider the wheel the more "stretched" out the tire will/should be.
Ie: a 245/45/18 tire theoretically would be taller on a 7 inch wide wheel than it would be if stuck on an 8.5 inch wide wheel.


Anyway, thanks for the answers so far.
At 3000 rpm's I'm showing 77-78 mph with the slightly larger aftermarket
wheels/tires.

I knew my speedo was a bit off, so now at least I know that
it appears it's off about 2 or so mph.
On my 25.2 mile trip to work with my other cars, the G35 registers
it at only 24.6 miles. So even that way for every 100 miles I really drive the G35's odometer registers 98 miles driven.

 

I'm right at 78-79 at 3000rpm.

 

I suppose my equation above doesn't work very well for automatic cars, since it doesn't take into effect of a locking torque convertor which effectively will lower the overall ratio.

 

To begin with the accuracy of your speed odometer is less than perfect, start messing with tires and accuracy drops even farther.

If you really want to know your speed at 3000RPM's you either have to use a, Dyno, a hand held GPS that has a speed read out, or use the mile markers along the side of the HWY.

 

I have found that, if you can get a hold of a GPS, that they seem to work well in calculating speed. Drive a steady speed, and look on the info page of the GPS where at least 1 (my old one) displays the speed you are moving. It indicates that my car is within 1mpg of agreement (as well as my truck). My old car, which was off for some reason, allowed me to keep within the speed limit.

Another way, if your state takes care with mileage markers, is to drive an even 60mpg for a mile, and time it. You should be able to get a ratio if it takes more/less than a minute to drive it.

 

MPG or MPH?

 

Im sure he meant 60mph

 

At 3000 rpm, I'm going around 77 mph.

 

The tachometer on my 2006 G35 reads 3000 rpm at 75 mph and 3200 rpm at 80 mph. This is at steady speed with the cruise control set in 5th gear. I upgraded from a 2005 g35x to a 2006 g35, both being 5AT. In so doing I was expecting the rpms in 5th gear to be different at speeds, but that has not been the case. It very much appears to me that the final drives are identical (i.e., 3.538 .vs. 3.357).

If you calculate what the speed should be for 5th gear at 3000 rpm for either of these two final drive numbers and a 0.834 5th gear, it is higher than achieved by the 2006 g35 (calculate 84 mph with a 3.357 final drive and 80 mph with a 3.538 final drive .vs. 75 mph actual). One possibility for this discrepancy is if the tachometer is reading high. Another possibility is if the torque converter is never fully locked. I've not figured this out yet and have finally given up after the dealership technicians claim there is not final drive change from 2005 to 2006 for the g35.

Go figure.....

 

Unless final drives changed after '04....
The X and RWD AT both have the same final gear ratio according to the '04 service manual.

RWD 5AT = 3.357
AWD 5AT = 3.357
RWD 6MT = 3.538