Im considering upgrading to 265/75R16 tires from 265/70R16. I know the diameter difference will affect the odometer and speedometer accuracy.

I know most people just live with it, but I like to log my fuel economy and having the odometer be off will mess with that. I could make a conversion equation but that’s a hassle. I don’t want to use my phone for this.

How would you adjust the speedometer so that it is accurate to a new tire size?

Im pretty sure 1998 don’t use a cable anymore. Mine has no ABS and it uses an analog counter, but I didn’t see a cable when I removed my cluster to replace bulbs.

Not sure but here's a search results that may or maynot help, if you figure it out be sure to report back to possibly help others in the future.

https://www.google.com/search?q=Adju...hrome&ie=UTF-8

If you don’t have ABS you can swap in a different speedo gear. Do a little searching on that topic to find out what you need. I think I changed mine from 31 to 33 teeth or something… it was a while ago

Isn’t yours a 99? I thought the digital ones didn’t use gears. Was the gear inside the gauge cluster or the transmission on yours.

And where did you get this gear from

I went from 265/70/16 to 265/70/17, so it's a similar diameter difference. For whatever reason, my speedometer doesn't seem to be affected at all. I've been through multiple different speed radar signs since getting the new wheels and tires, and every time the speed matches exactly what is being shown on my speedometer. It doesn't make much sense, maybe my speedometer was off before

Non-ABS 3rd gens use a speed sensor at the rear of the transfer case. ABS-equipped 3rd gens use a signal from the ABS computer. For you, gearing changes AND tire changes will affect your speedo. ABS-equipped trucks will only be affected by tire size changes. Not cable in either case.

You can change the gear on the speedo sender in the transfer case OR use any of a number of digital speedo healer type products that will adjust the digital signal before it gets to the cluster.

Likely it was off (reading too high) before you changed the tires. That is a 3.3% increase in tire size, which is right around 2mph difference in indicated speed on the highway - which is coincidentally about what they read high with factory tires...

-Charlie

I have an UltraGauge that I thought I recalibrated when I went to 265/75/16, but a free MPH app on my phone thinks the cluster needle is more accurate than the UG. I drive by feel these days with a glance at a variety of data sources.

How would I determine the new gear size I need for the tires?

Im running 265/75/16 and as stated above , I pass a radar and all agree.

Load a speedo app on your phone then you'll know the difference. It's a waste of money to modify stuff like that.

I did the same tire size upgrade years ago and there is such a small difference that the amount of work, time, money into correcting it doesn't make sense. IE you'll spend more money on "correcting" than you will on gas over the life of the vehicle. Plug the tire sizes into Tire Size Comparison to see what I mean.

Other things to keep in mind too is that with a larger diameter tire you could potentially get better MPG if you're driving on flat roads (less revs/mile). On the flipside you'll lose MPGs on hillier terrain since lower revs is less torque meaning your engine has to work harder.

I would be sure to check your speedo needle vs gps speed on your phone. With 265/75/16's my speedo is perfect. There is a slight possibility that someone changed it before I bought it 12 years ago, but I doubt it.

They all read slightly high from the factory. A small bump in tire size corrects it...

I went from the small 15" wheel tires (225/75-15, 28.3" diameter) to 265/70-17's (31.6" diameter), which is nearly a 12% difference in size - about the same as going from 265/70-16 to 35's - so I really needed to adjust the speedo.

-Charlie

I dug around and found that I used part number 33403-39415. If memory serves, this is a 33 tooth gear, which is the highest # of teeth available. More teeth = slower rotation on the encoder that feeds to signal to the speedometer in non-ABS rigs, so it would reduce the displayed speed and made the odometer click up more slowly. (If someone reads this carefully and I have it all backward -- sorry!)

Had trouble finding the part page with the details but this might help: Speedometer Gear- Toyota 4Runner, Pickup, T100 & Tacoma w/MT OEM Speedometer Driven Gear 33 Tooth (1991-2004) 33403-39415

Also Speedometer correction by T-case gear change | IH8MUD Forum