speedometer off by 10-15mph

[4x4runner91]

I was following my girlfriend to her house the other day on the interstate and noticed that my speedometer said I was just going 65mph...I called her (on the phone in the car in front of me) and she said she was cruising at about 75-80mph (i was right behind her and she wasn't pulling away). Now I've noticed that the 4Runner I just bought has 31x10.5 tires on it. Does that involve any speedometer changes? Do I need to adjust it somehow....if so how would I do that? Thanks in advance!

 

[SixxiA]

My 95 has 31" tires and the speedo is just fine. I'd recommend driving by a digital sign that tells you the speed limit and how fast "you're going" and clock it by that, too, before any modifications. Those are calibrated pretty well.

If it's off, you may take it to a Toyota dealership, they can do it there, or any other high-end mechanics shop. I don't know TOO much about it, but I'm fairly certain you'd be better off having someone else do it, keeping the receipt in your car, and presenting it in court if you ever get a speeding ticket. At that point, they might just let you off on technicality. Bonus!

 

[Big Norm]

here is a cool site......you type in what your factory recomend tire size is and what you currently have on it and it will you tell you how much it changes your mph on the dash

http://www.miata.net/garage/tirecalc.html

 

[the_dingman]

I've got that issue after replacing my transmission. After talking to Toyota, they sell multiple speedometer gears, with different numbers of teeth. Mine currently is 17, I'm thinking of dropping to 15 to see if it solves the problem. They suggested trial-and-error to find the right one.

 

[fourwd1]

Taller tires throw off the speedo.

You can either ignore it, change the speedo sending unit, or regear your diffs.

Regearing is the correct way, because it puts your engine operating RPMs back in the powerband. You'll get back the power and MPG lost when moving to taller tires, as well as making the speedo accurate again.

Regearing is also expensive, and since 31's are not that much taller over stock the power loss is minimal, so why bother.

 

[4x4runner91]

If i were to re-gear the truck....would I change the rear and the front gears...or just the rear? I can tell some definite power loss from a stop to about 45-50mph. Would gears solve that also?

 

[Philbert]

I have been wondering this forever!

I had 32 inch BFGs installed on my truck about 15,000 miles ago and had noticed both my odometer and speedometer were off by about 10%. I confirmed this today on the freeway with my GPS - which gives real time speed

I'd like to adjust it so both the speedometer and odometer are synced up correctly going forward (i guess it's been about 17,500 miles since the tires were installed...not 15,000). I'm going to pass on the new gears since I just had a new diff put in (DAMN!).

Is there a DIY someone is aware of and can link to, or is it easy for one of you to explain in bullet points (photos a bonus!)?

Thanks in advance!

 

[fourwd1]

If i were to re-gear the truck....would I change the rear and the front gears...or just the rear? I can tell some definite power loss from a stop to about 45-50mph. Would gears solve that also?

Think about it, if you just regeared the rear, and put it in 4WD, the front axle and the rear axle would be traveling at different speeds. What do you think would happen?


What ratio to change to would depend on your truck and the ratio you have now.

 

[the_dingman]

As for changing your speedometer gear (or sending unit), disconnect the speedometer cable from the transfer case, remove the nut next to the connection for the cable, and pull it out. It's a pretty easy process... Then just pop the new one in.

 

[the_dingman]

Toyota says that the changing the number of teeth on the gear changes the speedo by about 3MPH (assuming at 55). I've got an 18 tooth gear, I'm off by about 6 MPH at 55, so going to a 16 tooth gear would be the answer. Toyota can't seem to get me a 16 tooth gear, they only have 15, 18, 19 and 32. Any ideas where I might be able to find something custom?