Tires and Axle Ratios

[Peace2Peep]

Hey guys,

Been here and searched awhile and never got this figured out:

I've got 255/45/18 tires in front

I've got 295/45/18 tires in back

I've got a 4.10 tune from Aric at Injected Engineering

My speedo is 4-6 mph slower than actual

What is going on? Do I not have 4.10's?

How do I check and or fix this?

[red]

You will need your tire revs/mile set to 730 (the revs/mile of your rear tires) and you will need the gear ratio set to 4.10. Double check with them that these values are correct. Otherwise, you can confirm the gear ratio by jacking up the car, marking the tires and drive shaft for reference, and then rotating the passenger tire one revolution, counting the number of revolutions for the drive shaft.

[Peace2Peep]

I think I've got 3.73's...I tried the revs per mile you just gave me and punched in 3.73's and the navigation is about 1-1.2 miles per hour slower than speedo...is that about right? If that's true I'm ticked at the former owner for telling me they were 4.10's. I got a dyno tune based on 4.10s and I cant adjust it for 3.73s!

[red]

I think I've got 3.73's...I tried the revs per mile you just gave me and punched in 3.73's and the navigation is about 1-1.2 miles per hour slower than speedo...is that about right? If that's true I'm ticked at the former owner for telling me they were 4.10's. I got a dyno tune based on 4.10s and I cant adjust it for 3.73s!
Are you sure about your tire size too? What brand and model are they? You can try the manufacturer revs/mile, which should be very close to the calculated revs/mile based on static loaded radius. Otherwise, I'd recommend doing the drive shaft test (make sure the car is jacked up safely before attempting).

[red]

Also, are your tires inflated properly? What PSI?

[Peace2Peep]

I know the tires are correct...Nitto NT-555 295 45 18 in the back 255 45 18 in the front...I bought em. I just had that one tune out of three that was a dyno and isnt adjustable. The Lidio tune original is adjustable. Not sure my PSI but its prob a little low as its 64 degrees outside and originally when aired up it was 80+ degrees. That would make the 1-1.2 more accurate?

[red]

I know the tires are correct...Nitto NT-555 295 45 18 in the back 255 45 18 in the front...I bought em. I just had that one tune out of three that was a dyno and isnt adjustable. The Lidio tune original is adjustable. Not sure my PSI but its prob a little low as its 64 degrees outside and originally when aired up it was 80+ degrees. That would make the 1-1.2 more accurate?
Nitto states 734 revs/mile, so you can adjust to 735 for your trials. If the pressure is low, then yes. At 60 MPH, just a half inch shorter in diameter with your rear tires will result in a ~1.1 MPH faster reading on the speedometer (i.e., you'll be going ~1.1 MPH slower).

[red]

Also, keep in mind that as you wear off the tread, this number will change slightly. So, take into consideration your current treadwear.

[Peace2Peep]

ahhh...I am going to go try it now! Be right back!

[Peace2Peep]

OK just put it up to 735 revs and it was within .75/mile an hour My guess is that will shore up with increased amb temps or a couple pounds of air in the tires to get them to 35 psi(is that what I'm supposed to run them at?) So I guess it's resolved...I have 3.73's! Thank you soo much Red...you just dont know how you helped me sleep tonight. I just got to a point where I've had enough! I hate owning an awesome car and feeling like something isnt right(guessing my actual speed while driving this cool car is just plai WRONG) and you helped me know something ISNT RIGHT but at least I know for sure!

[Peace2Peep]

BTW Maginthat called this situation out to me today in Daytona Beach, but I just had to run the numbers...I had to KNOW...you know? I'm still gonna have the mechanics check it out! Ha!

[red]

Thank you soo much Red
Glad to help!

[red]

I'm still gonna have the mechanics check it out!
You can determine the ratio on your own if you have a jack, jack stands, and wheel chocks. If not, I'm sure Rich would help with that! :)