The Stovebolt Site Forums The Shop Area General Truck Talk I feel kinda dumb asking this but.....

I'm going from 16" wheels to 15" with about the same tire profile so just wondering how much and in what direction my speedo will be off. ? Once done, I'll get CHP to measure it or just use one of the roadside radar stations maybe...I guess they're accurate ...

Depends on the overall size of the tire. If the 15s have the same overall outside diameter as the 16s then it shouldn't vary much.

Here is a calculator for that:

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

IF U have a gps or can borrow one, they are accurate whe set on the speed setting. A hole bunch easier

There are different diameter 15" and 16" tires. Many truck radial tires on our narrow wheels (with proper inflation for a balance between good tracking and easy low speed turning) end up to be a wider diameter than their stated specs.

I used to check (and calibrate) my speedometer by driving (at 60 mph) 20 miles away on an interstate and 20 miles back. I take my odometer readings at the start and stop of every 10 mile interval. I gave those readings to The Speedometer Shop and they send me an adapter. With the adapter, the speedometer readings are now less than 1% off.

Now that I have a GPS, I'll calibrate on that and send the information to The Speedometer Shop, for them to make an adapter.

I drive a pretty much stock 50 3100 with a broken speedometer. I figure that as long as I don't drive it like I stole it, I won't be speeding excessively. They just don't go that fast.

As stated above, rim diameter doesn't play much part in speedometer readings, its the outer tire diameter that matters. Use the calculators at TireRack.com or any of the other online ones. Is the speedometer accurate now? How about the odometer? Its better to set the odometer since it is gear driven right off the cable. The speedometer is magnetically driven and over time can loose magnetic strength thus showing slower then actual speeds and the faster you go, the farther off it gets.

Find a marked section of highway and drive a good 10 miles each direction and see what your odometer shows, then with the new tires do the same thing. Shorter tires will add miles, taller tires will be less. It works just like a short legged dog running with a tall dog, the short one will take a whole lot more steps!

Joe

Never thought about using my GPS. Duhhhh

I do realize its more about overall wheel dia. I'll compare when I receive my new set.

Yeah im the same way, i switched my speedo and it broke again this time killing the gauge. Ive been given tickets but not for speeding.

I think if you going from 16s to 15s, unless the 15 is the same height, which would be really tall, then your speedo will say your going as faster then you actually are. (reassuring you your not speeding when following the speed limit)

I agree, the GPS is really handy for checking your speedometer readings.

My wife has a driving app on her phone. It's speed readout is super accurate. Not to mention all the speed limit signs/zones are programmed into. It'll beep when you're speeding.

Jim Carter sells an adaptor - you tell them what your "real" speed is and what the speedo needle says. They send you a custom-made adaptor that screws into the tranny housing. Works like a charm!!

Of course the rim size matters. For the same size tire, it changes the diameter by one inch.

What would be an example of a same sized tire for a 15" and a 16" rim?

I think people might be referring to original size tires available on original rims. 1947-1955 1/2 ton trucks could be ordered with 15" wheels/tires or 16" wheels/tires.

In 1952 (for example), the optional 15" wheel/tire combination was a wider diameter than either/both of the 16" wheels/tires combinations (standard or optional).

Chime in if I'm wrong , first number is width like say 235. The second is the height aspect . 235/55/16 would be 235 wide and 55% of that tall.

a 235/60 or 65/15 is going to be as tall as a 235/55/16

and regardless of what size rim, be it 15 or 20, it's the over all diameter that affects road speed

A 235/60/15 is aprox 26.1 diameter
A 235/45/18 is 26.33
A 235/45/20is approx 26.75

If U are using a aftermarket electronic speedo U can use the switches on the back made for adjustment.

Drew, I think you have a typo here the 20" rims are going to be 28.32 in overall diameter. It should read 235/35/20 to be the same

Tim.

I went back and researched this with tire calculators, and I think I am gonna stick to my post. The 235 (or whatever) is a metric width value. Does not matter what the rim size is. The 85 (for example) is a percentage that produces a metric height value. When I plug these values into several online calculators, they all say that with the "same" sized tire, a one-inch greater rim diameter increases the overall diameter by one inch. Is this wrong?

Harry

Harry your reasoning is correct, but he's going to smaller rims so he will actually be losing an inch on the diameter "with about the same profile" which means his speedometer will read faster than he's actually going.

This was what I wanted to know. Really wont matter much cuz I won't exactly be speeding around in this truck anyway, but good to know I'll actually be going slower than speedo says. No plans on dropping a V8 in her.

This is a good chart for tire sizes.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=53

Joe