35" tires affects speedometer, odometer too?

[jordanblea]

Hey guys not sure if this is a stupid question or not. I have been keeping a log of how many miles I cover vs how much gas I buy to try and get a ballpark number on mpgs. My question is this, with 35" tires my speedometer is off, says I'm going 50 when im going 61-62 mph, which is ok I can deal with that, but will that also throw off what my odometer is reading? Like if it says I went 40 miles, but really went 50? That'd be nice....Anyways, thanks!

[bonnieclyde100]

yes it will mess up them readings also unless you set the speedo gear for them tires.

[jordanblea]

Originally Posted by bonnieclyde100

yes it will mess up them readings also unless you set the speedo gear for them tires.

Thanks. Do you know which way it'll affect the odometer? More or less distance traveled vs what it's reading? If I get the correct speedometer gear(?) will that fix the odometer issue as well?

['68OrangeSunshine]

LESS.
That is, with a "slow" speedometer, your odometer will also count more slowly. Good news is if your tires have a warranty, you're getting better warranty coverage.
Three possibilities:
1] At a speedo shop run on the dynomometer and clock your indicated speed against your revolutions of their rollers and get a graph to post on your dash. So you know that, let's say, 47 indicated is really 55 actual MPH. Callibrations from 10 thru 90 on the top scale, and actual reading numbers on the lower scale.
2] With the above knowledge, some shops can build a compensator box [they're about 1" x 2"x 3"] that goes between your speedo cable and the transmission. Borg-Warner used to make them. With this gizmo, you're cured. Your ODO will read good. [Within 1 - 2.5%, or so]. I happenned to get lucky, once. I had an old comp box - came with a junkyard SM465. It read all wrong, but I kept it anyway. Some years later when I switched from R8x17.5LTs to R235/85-16LTs, I tried it on for laffs. I was amazed, when it matched my GPS speeds within 2-3 MPH. So if you find the little box keep it. A speedo shop can change the internal gears or you.
3] Get a GPS, set it on MPH, and read that when driving. No odometer support however, but it's way accurate.
You could also get a helper to record your speeds, matching indicated marks with actual GPS readings, and make up your own calibration curve chart -- without going to a speedo shop -- like I had to do in the '80s.
Good luck.
Hope this helps.

[jordanblea]

Originally Posted by '68OrangeSunshine

LESS.
That is, with a "slow" speedometer, your odometer will also count more slowly. Good news is if your tires have a warranty, you're getting better warranty coverage.
Three possibilities:
1] At a speedo shop run on the dynomometer and clock your indicated speed against your revolutions of their rollers and get a graph to post on your dash. So you know that, let's say, 47 indicated is really 55 actual MPH. Callibrations from 10 thru 90 on the top scale, and actual reading numbers on the lower scale.
2] With the above knowledge, some shops can build a compensator box [they're about 1" x 2"x 3"] that goes between your speedo cable and the transmission. Borg-Warner used to make them. With this gizmo, you're cured. Your ODO will read good. [Within 1 - 2.5%, or so]. I happenned to get lucky, once. I had an old comp box - came with a junkyard SM465. It read all wrong, but I kept it anyway. Some years later when I switched from R8x17.5LTs to R235/85-16LTs, I tried it on for laffs. I was amazed, when it matched my GPS speeds within 2-3 MPH. So if you find the little box keep it. A speedo shop can change the internal gears or you.
3] Get a GPS, set it on MPH, and read that when driving. No odometer support however, but it's way accurate.
You could also get a helper to record your speeds, matching indicated marks with actual GPS readings, and make up your own calibration curve chart -- without going to a speedo shop -- like I had to do in the '80s.
Good luck.
Hope this helps.

Very much so thanks!! Didn't want to see "LESS" on the distance traveled though lol, stands to reason though. Thank you sir!

[geezer#99]

Once you establish your actual speed then install a ratio adaptor like in this link.
http://www.transmissioncenter.net/sp...n_______va.htm

[jordanblea]

Originally Posted by geezer#99

Once you establish your actual speed then install a ratio adaptor like in this link.
http://www.transmissioncenter.net/sp...n_______va.htm

Thanks!

[jordanblea]

So I started thinking.....lol. If the odometer is counting slowly, then wouldn't I be covering more distance, not less, than it reads?

['68OrangeSunshine]

Originally Posted by jordanblea

So I started thinking.....lol. If the odometer is counting slowly, then wouldn't I be covering more distance, not less, than it reads?

Well, yes. What I meant by 'LESS' is you'll have indicated fewer miles than you have actually gone.
Think of different units of measure. A Statute Mile is 5,280 feet. A Nautical Mile is ~6,076 feet. A League is 3 Miles [Naut.] A bigger tire has a longer circumference which will click the ODO fewer times.
I once got a speeding ticket because I had taped "KNOTS" over my speedometer. [My correction was close to 1:1.15.] The cop, also familiar with general aviation, said I had to have known I was going too fast if I knew I was indicating in Knots.
55 Knots = 63.25 MPH. Besides the speeding tic I also had a repair order, so I had to go to a state-certified speedo shop to get at least a correction card [graph].

[jordanblea]

Originally Posted by '68OrangeSunshine

Well, yes. What I meant by 'LESS' is you'll have indicated fewer miles than you have actually gone.
Think of different units of measure. A Statute Mile is 5,280 feet. A Nautical Mile is ~6,076 feet. A League is 3 Miles [Naut.] A bigger tire has a longer circumference which will click the ODO fewer times.
I once got a speeding ticket because I had taped "KNOTS" over my speedometer. [My correction was close to 1:1.15.] The cop, also familiar with general aviation, said I had to have known I was going too fast if I knew I was indicating in Knots.
55 Knots = 63.25 MPH. Besides the speeding tic I also had a repair order, so I had to go to a state-certified speedo shop to get at least a correction card [graph].

Awesome, thanks!! I was thinking you meant "less" as far as mpg's lol.

['68OrangeSunshine]

As far as MPG, I rely on the Odometer in my '71 GMC Jimmy [K/5]. [350V8/SM465/NP205/3.73 posi rear].
The gas gauge is out of order, and replacing the in-tank sender is more than I want to deal with right now, so I keep a logbook and record my fill-ups and trip-run miles. I figure on an arbitrary 10 MPG, so I try to top off the 20 gallon tank every 100 miles or so.
The fuel tank is covered by a steel skid plate with about 40 bolts holding it on, and additionally there's a big Class IV hitch bolted on under that. I've thought of cutting a patch out of the floor panel on the back deck and bolting a repair plate over it --for access-- but plunge-cutting a grinder thru the floor on top of a stock fuel tank is its own kinda thrill.
Also the speedo and ODO read slow with 33x12.50x15LT tires. I use a GPS for speedometer sometimes. Or I just drive 43 indicated in a 50 MPH posted zone.
I have spare Borg Warner compensator widget, but it needs a pro speedo shop to correct it.

[jordanblea]

Originally Posted by '68OrangeSunshine

As far as MPG, I rely on the Odometer in my '71 GMC Jimmy [K/5]. [350V8/SM465/NP205/3.73 posi rear].
The gas gauge is out of order, and replacing the in-tank sender is more than I want to deal with right now, so I keep a logbook and record my fill-ups and trip-run miles. I figure on an arbitrary 10 MPG, so I try to top off the 20 gallon tank every 100 miles or so.
The fuel tank is covered by a steel skid plate with about 40 bolts holding it on, and additionally there's a big Class IV hitch bolted on under that. I've thought of cutting a patch out of the floor panel on the back deck and bolting a repair plate over it --for access-- but plunge-cutting a grinder thru the floor on top of a stock fuel tank is its own kinda thrill.
Also the speedo and ODO read slow with 33x12.50x15LT tires. I use a GPS for speedometer sometimes. Or I just drive 43 indicated in a 50 MPH posted zone.
I have spare Borg Warner compensator widget, but it needs a pro speedo shop to correct it.

We have a very similar set up: 350V8/SM465/T221/3.73 posi rear. Lol. Tires 35x12.5. I was figuring around 9mpg using the odometer readings divided by how many gallons I bought. At least it's a little better than 9mpgs.

Check out my other thread (if you're bored)

http://67-72chevytrucks.com/vboard/s...d.php?t=624856

I learned a lot on this little issue.

['68OrangeSunshine]

You know the SM465/T221 lash up was only made in 1968? Before that it was SM420/T221 and after, it was SM465/NP205. I have a 465/221 I'm saving for a '72 Blazer future project: 'The Blue Phantom of the Junkyard.'

[jordanblea]

Originally Posted by '68OrangeSunshine

You know the SM465/T221 lash up was only made in 1968? Before that it was SM420/T221 and after, it was SM465/NP205. I have a 465/221 I'm saving for a '72 Blazer future project: 'The Blue Phantom of the Junkyard.'

I did not know that specifically, most I've seen on them isn't a whole lot. Timken used them in a 6x6 vehicle of some sort.