How fast am I really going?
- Thread starter badass
- Start date
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According to the calculator if you are doing 70 you will be turning almost 3300 RPMs. What rpms are you reading? You will need to enter 1 for trans ratio and 1 for tranfer case ratio for the stock gearing.
badass said:
Axle ratio is 4.88
Transmission ratio is 1 (as in 1:1)
Transfer case ratio is also 1
Your advertised tire diameter is 35”. In actuality if you measure them, you will likely find them to be shorter, which will affect your engine RPM’s and speed.
swilson
lurker
Why calculate it? Grab your GPS (or borrow a friend's) and find out what it really is.
rusty_tlc
Dain Bramaged Member
X3 for the GPS. They are more accurate than most speedometers with a stock set-up.
Once you know your actual vs your spedo reading you can caculate the percent error. That can be applied to any speed your speedo registers.
Once you know your actual vs your spedo reading you can caculate the percent error. That can be applied to any speed your speedo registers.
You can make a reasonable estimate. The OEM speedo is calibrated for 30 inch tires and 4:11 diffs.
35 inch tires would be a 16.7 % overdrive. (35/30)
4.88 ratio would be an 18.7% underdrive (4.88/4.11)
The difference would be a 2% underdrive and 2% of 70 MPH is 1.4 MPH, so when your speedo reads 70, you are actually going about 68.6.
A GPS would be more accurate.
35 inch tires would be a 16.7 % overdrive. (35/30)
4.88 ratio would be an 18.7% underdrive (4.88/4.11)
The difference would be a 2% underdrive and 2% of 70 MPH is 1.4 MPH, so when your speedo reads 70, you are actually going about 68.6.
A GPS would be more accurate.
I just use those radars the cops leave out in your neighborhood to get you to slow down. I drive at it and get my reading versus their reading and go around the block and do it again. I found my speedo is 5 mph off.
mooker82 said:
so what does it mean if - in real life - in 4th gear I am at about 3500 RPM doing 65 on 33's and I have 4.88 and 1:1... seems like my RPMs run high then? maybe the gearing is not what the PO told me it was as why else would you run higher RPMs? IF I were to hit 70, I think IIRC I would be closer to 4000 RPM, not 3500 as these calculators claim.
Here is a poor mans way to calc. speed. Using the mile markers on the highway which are actually quite close to exactly a mile apart drive at 60 MPH (speedometer reading). 1 mile in 1 minute is 60 MPH. If you do it in 56 seconds you are actually going 65 MPH. If you do it in 52 seconds you are actually going 70 MPH. It is not perfect but if you don't have a GPS it's not bad.
Go here.
www.4lo.com
This website has a great calculator for tire size gear ration etc.
I love it.
JR
www.4lo.com
This website has a great calculator for tire size gear ration etc.
I love it.
JR
best calc ever is grimms ratio calc.
