my truck sits on 35s with 3.73 gears. i was just wondering what my RPMs should be at 65...and yes, im getting a smarty soon!

 

borrow a buddy's GPS and know for sure

 

You should be off by 9-10% from stock if the tires actually measure 35".

You don't say auto or manual but my 6 sp is 1700 at 65 with 34,9" diameter tires. Calculated and GPS checked.

Cya

 

Mine was off by 7 mph when I switched to 35's. So if the speedo read 70 I was doing 77mph. The slower the speed the less its off by.

 

Find your self a stop watch or digital clock. Then find yourself a straight and clear stretch of highway that has a few miles posted. Get up to speed and set your Odometer and cruise control to what should be 60MPH. Then as you pass the first mile post note the time or start the stop watch. Cruise the mile to the next mile post with out adjusting the speed and note the time or stop the stop watch. You should complete a mile in 60 seconds at stock tires and every second above or below that will give you how fast or slow the Odometer is now. No need for a GPS or
Flashy Radar Machine, just some simple math problems.

For Example: I set my cruise to 60MPH. As I pass the 1st mile post I click my stop watch on. Then as I passed the 2nd Mile post I click the stop watch off. I noticed that I completed the mile with the cruise control set to 60MPH in under 55 seconds. So that means I am 5 miles faster and that means at 60MPH on the Odometer I am actually going 65MPH. Or you could use the math problem (1 mile / 60 seconds = 60 miles / 3600 seconds).

Shawn

 

wow...7mph is more than i thought...explains alot

 

Mine is also off by 10% according to gps.

 

with 35" tires and 3.73 at 65mph I was running around 1750rpm, was to pay $60 at service shop to recalibrate speedo, I had to explain to the service guy how to do it. Service manager, said no charge and thanked me for showing them how to do it! So much for "chrysler trained technicians"!!

 

Never say Speedo unless your wearing one , and being that yours is off please go cover up, this is a family forum!

 

Triplenickel is right; if your oem tire size was LT265/70R17 and you went to LT35x12.50R18, your speedometer (and odometer) are only reading about 91.77%. (31.9 OD on the 265's divided by 34.76 on the 35's).

Greg

 

I found that the faster you go...the more the speedo is off.

I have 37's and the dealer attempted to calibrate the speedo for me but couldnt.

My speedo reads Im going 100km/ my gps says 114km. But at lower speeds like 50km the gps reads 55km.

I dont think you are cute and blonde haha so Id get it fixed before you end up with a ticket. Fortunately for me, a smile and a dumbfound look to the cop got me away with no ticket lol

 

That's correct; as Triplenickel stated, using his 10% error, if you're showing 35 mph on your speedometer, you're really going about 38.5 mph. If you're showing 70 mph on your speedometer, you're really going about 77 mph. If you're showing 90 mph on your speedometer, you're really going about 99 mph.

Greg

 

Seems many people are having this problem. I just responded to a few threads about speedometer with gear changes, tires changes...

It needs to be set in the ABS brake section of the scan tool. Best bet is get the # of revolutions per mile from the tire manufacturer for the size and model of tire you have. This is the MOST accurate. Try not to use the default tire sizes DC gives you to choose from in the scan tool.

And yes it goes by percentage not a MPH number. So as speed increase so does the number.