Thread: gear ratio

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hey guys, here's a quick little question for all you engineering types. in my rod, I have a 350/350 with a 2:76 rear. last year I rean a 25" tall tire, now I have a 30" tall bias ply WW. what will that do for performance , milage etc? what is the effective gear ratio? thanks

Originally Posted by tudorkeith

hey guys, here's a quick little question for all you engineering types. in my rod, I have a 350/350 with a 2:76 rear. last year I rean a 25" tall tire, now I have a 30" tall bias ply WW. what will that do for performance , milage etc? what is the effective gear ratio? thanks

You can plug your numbers into this calculator and see what you get...http://www.ringpinion.com/Calc_RPM.aspx

Effective gear ratio will now be in the neighborhood of 2.3. That is pretty tall. It will be more doggy out of the hole, and performance throughout will be a little more sluggish. Gas milage may or may not improve depending on where the engine is in it's power band and how hard it will be working at the speeds you drive.

Only real way to know is to bolt them on and see.

Don

The calculator Randyr linked you to uses a formula where Cruise RPM = (MPH x gear ratio x 336 x OD ratio) / Tire Diameter in inches. Plugging in your 2.76 rear gear ratio and no OverDrive (1.0 for top gear) your 25" tires pushed you to 2597rpm at 70mph, and your new 30" tires will drop that to 2164rpm with no other changes. The 336 is just a factor to align units and give you RPM.

Originally Posted by Itoldyouso

Gas milage may or may not improve depending on where the engine is in it's power band and how hard it will be working at the speeds you drive.

Don has hit the nail squarely on the head with this observation.

thanks guys, luckily I'm looking to cruise. she seems to drive well and do'nt get much chance to open it up to 70 in the areaa that I live...especially being new to bias ply, totally different car than when I had wide 17" dia radials on it last year. I'll be posting pics of all the body work I did this winter. thanks again

A quick way to guesstimate the RPM change is to swap the numbers. 25" to 30", change to 30 and 25, so where you were turning 30 or 3000 RPM, now you'll turn 25 or 2500 RPM. No rear end ratios or OD ratios needed! It's a simple straight line graph, so 1250 RPM (half of 2500) was 1500 RPM (half of 3000).

Like Don and Tech point out, the rear gear ratio is only one piece of the puzzle. With your TH350 ratios of 2.52, 1.52 and 1.0 you are going to be putting a lot of strain on the tranny every time you take off, especially if you push it. If you use 3000rpm as your "predicted" shift point your speed for 1 to 2 is 39mph, 2 to 3 is 64mph and you will cruise at 97mph in third at 3000rpm. I don't think you will be happy with those tall tires and tall gears if you liked the way it performed last year with 25" tires, but like Don said, bolt it up and try it. Changing to 3.08's yields 2415rpm @ 70, 3.42's gives you 2681rpm, and 3.73's takes you to 2924rpm, just about where they ran back in the day - 3000 @ 70 and gulping gas. An OD gear would make things a lot easier to balance.

Kieth

Your mention of bias ply tires -----there is another thread going about bad crashes and I have been thinking about the differences of straight line performance and handling vs road performance and handling and had eventually thought about how on these cars with big n little tires how often they are mixed radial/bias---something very basic that we should all be aware of---

good point jerry but let me assure you I have the bias plys at all 4 corners. however I wouls like to get some radial wide white's for next year. the difference is staggering...but the look is priceless. I'll be posting pics of my cars rebuild soon.