I plan on running 315/70/17's on my truck when these 265/70/17's are worn out.

I currently have 3.55 gears and I'm wondering what gears would keep my speedo most accurate with the 315/70/17's.

With stock gears my speedo will read 65 mph when I'm actually going 70.6mph.

Anyone know?

Basically going from a 31.5 overall diameter tire to a 34.5 overall diameter tire. What gears would keep speedo accurate?

 

Just remember gearing is all relative.

You are increasing diameter by 8.7%, so you want at least that much change in rear gear ratio, which means a theoretical 3.86:1 gear set. However, you'll have heavier tires that hurt performance, so rounding up to the next geat set would be a good idea. 4.10 if you drive a lot of miles or 4:56 would be my choice.

 

Yeah, I was thinking 4.10's also.

 

gears are the best way to go when going with bigger tires but if you are just looking to recalibrate your speedo buy any new power programmer for instance the diablo predator and you could recalibrate it with that. also going with gears doesn't mean it will make your speedo right i had 37's with 4.56's on my old truck and my speedo showed me going faster than i was really going.if it said 70 i was only going 65,and i was clocked more than once by cops that i know. i had 315's with 3.55's before i went with my 37's and the truck did not have problems pulling them. think about it your truck is rated to tow 9000 pounds with those gears. 315's should screw it up.

 

Some posted here a while back that they were able to go to the dealer and the dealer would do the computer/speedometer adjustment for you for a fee. I think there was a maximum tire size they would do this for.

The better choice may be to go with a tuner of choice and let the tuner adjust your speedo. Two for one benefit.

FYI I have the tire you are planning to purchase w/Troyer tuned excalibrator w/3.73 RP. My speedo is fine. My truck is a (heavy) dog. I don't think the 4.10 RP is enough gear. I happen to like gears as a reliable modification. I would go with a 4.56 RP minimum; I have 4.88s on order.

 

Here's a tire comparison calculator:

http://www.csgnetwork.com/tireinfo4calc.html

And here's a gear ratio calculator:

http://www.4x4wire.com/tech/applications/gearandtire/

You have to know your transmission ratios to get the last to work right, but it's pretty slick.

 

This is a calculator I found. I use it quite abit. Tire size Cal.