Gear ratio and tires size
 

Gear ratio and tire size question.....so my mechanic recommended I get a 28 inch tall tire to go with my automatic 700r4 tranny I'm planning. My rear is a 3.73 posi. But most you guys recommended a 215/70 tire which is only 26.9 inches. It was stated a 75 was too tall but it would put me at 27.7 inches.
I'm planning a highway/daily driver kind of truck. Just curious of thoughts we are currently in the mockup stage so trying to find tire size. According to this gear ratio chart I feel like I would still be within parameters with a 70 series tire. I do like seeing tire haha. Ppanning original type 15 rims. Curious of thoughts
Thanks!

Re: Gear ratio and tires size
 

Looking at the chart it’s easy to see that 1 inch difference is only 100 rpm. Not enough difference to be much of a concern.
The controlling factor in tire size is the height and width. You’ll need room for the tire to fit inside the wheel well without rubbing anywhere.
Normally you need 1 inch on each side of the tire for safe clearance.
It’s usually best to select tires after everything else is put togethor. You’ll need to do good measurements.
Another thing to consider is rim selection. What offset works and how wide.
Your 215 tire will work on a 6 inch wide rim. It’s not recommended to put a narrow tire on a wider rim.

Are you lowering the truck?

And what motor?

Re: Gear ratio and tires size
 

Thanks for the response. So it's a 350v8 going in. Yes I'm currently installing a mustang ii front end. I'm in mockup stage now that's why I'm trying to grab a logical tire to see how things fit before I final weld everything in. My rims are original/ford aftermarket and are 6inches wide currently. I have found quite a few lowered trucks running the 215/70 so seems logical I hope haha

Re: Gear ratio and tires size
 

I think the guys who suggest 215 and at times I may be one of them are talking more about a specific appearance rather than road performance. The "it looks just right in the wheel opening on a 15 inch wheel" thing.





You need to multiply the rpm on that chart by the over drive ration in the trans to get the true rpm. Meaning that you need to multiply the rpm they show by .70.

I'm wont to say that every engine has it's happy spot as far as cruising down the long roads at a specific speed. We don't all cruise at the same speeds either.

I found out with the ot car that I had then that even on the 80 MPH speed limit roads in the west that the difference between 74/76 MPH and 80 MPH was 6 miles per gallon. On that leg between here and Wichita that was over a tank of gas difference.

Re: Gear ratio and tires size
 

so I have 700r4 /4.10 gear /205/75/15 tire am I gonna be in trouble on the freeway

Re: Gear ratio and tires size
 

Find some cheap tires and rims to mock it up. Worn out tires are cheap.
Before you final weld that m II in place hang a fender on with a tire/rim mounted and step back to see how centered the wheel/tire are in the opening. Sometimes lowering the front will make the wheel/tire look like it’s tucked back towards the fender.
If you want to compare tire sizes just search on tire rack for 215 70 15, find a tire and look at the sizes chart. The chart will show different heights, widths, recommended rim width. All good info for tire selection.

https://www.tirerack.com/content/tir.../homepage.html

Is your 350 totally stock or a modified motor?

Re: Gear ratio and tires size
 

Likely not.
Use this calculator.

http://www.advanced-ev.com/Calculators/TireSize/

Re: Gear ratio and tires size
 

imo... trucks need a lot of tire to look right
lots o' tire also smooths out some of the pot holes in michigan roads
low profile tires/rims take a beating on mi roads, not a fan of low profile tires on much of anything

i run five P235/70R15 on 15x8 rims on truk if you want to see what a 28" tire looks like on tf trucks
i run a 700r4 with a 3.27 gear, so i'm off your chart, truk gets 21mpg on hwy
i set truk up for hwy cruising, it's long hauled 3 hotrod power tours that average 3000 miles each

Re: Gear ratio and tires size
 

So engine is stock. I'm building daily driver not a powerhouse haha

And yeah I did find a 215/75 today I'm gonna use for the mockup. We are hoping to put a fender on today to check my centerline. I much prefer forward than back in the fender from what I have seen

Re: Gear ratio and tires size
 

Nice truck!! And yes I like seeing tire too. What is your truck lowered with? Mustang? Dropped axle?

I would love 21mpg Haha

Re: Gear ratio and tires size
 

235/70R-15 here too. The trans is a TH400 with 1:1 3rd gear and a 3.55 diff, 50 mph is 2200 rpms, 70 is 3000 rpms.

Here's an online calculator that makes it easy to see the effects of changing tire size and/or gear ratio.

https://spicerparts.com/calculators/...rpm-calculator

Re: Gear ratio and tires size
 

With the advance EV calculator you have to multiply the over drive ration by the rear gear ration.

It figures out to be right at 2494 at 70 mph for Jackson.

4.11 gears x.70 = effective 2.877 gear rato with a 27.13 inch tall tire @ 70mph =2494 rpm

I've tried a bunch of other calculators including the Wallace racing one that I can't get to work on my laptop and the EV one is the simplest to do.

Re: Gear ratio and tires size
 

This is the Wallace racing calculator and someone with a faster computer and better internet might be able to make it work. http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

Re: Gear ratio and tires size
 

mustII up front. stock springs modified with axle flipped on top of springs in back

Re: Gear ratio and tires size
 

To me the TF trucks need a bit taller tire than the AD trucks do to look right. Ogre and Black Sheep's trucks are prime examples of getting it right when it comes to same size tires front and rear. The tires on both trucks fit the wheel opening just right and just flat look right for the look they went after.

Thinking back to 1975/76 when I put the 283 in my 48 it was geared out so that at 3000 rpm it ran 80 mph with N-50 14 tires on the back.
Playing with the EV chart says it had a 3.013 ratio. Early 70's Camaro rear end That sure wasn't much good for pulling a loaded 16 ft U haul trailer across the country.

Re: Gear ratio and tires size
 

Ok cool thanks......question for the rear flip did you just flip and leave original spring perch hanger location for it or I read some. guys flip the spring perch hanger to the top if the frame to bring it back up a little? I cant find the stinking article at the moment. But wasnt sure if it was to make the truck level or for someone wanting the lower front and higher rear end (the like nose down stance) sorry for the lamens terms haha

Re: Gear ratio and tires size
 

I do like more tire. Just trying to understand what will fit. When I originally asked for suggestions it seemed like the majority had the 215/70 in front and alot liked 235 in back. I am glad to know that I can maybe stuff a 235 under the front as these 2 have because yes I do like their look also

Re: Gear ratio and tires size
 

Thanks for the info. Your truck is a beauty!! Do you have a side profile picture of your truck? What did you do for lowering it?

Re: Gear ratio and tires size
 

For the rear I used the TCI flip kit, in front is a '75 Nova subframe with 2" dropped spindles. The front coils have settled a little so I have a set of Detroit Speed spacers that should get the front end up about an inch.

Re: Gear ratio and tires size
 

i left the spring hangers where they were. i personally wouldn't want the truk any lower. my truk is almost level, i like the stance as is

Re: Gear ratio and tires size
 

I found the article
http://67-72chevytrucks.com/vboard/s...d.php?t=442586

He was claiming it brings the rear up because it was too low as is. I'm sure it depends on alot of individual factors with front end also. But yes I wasnt wanting a truck on the ground either haha